ArrayList.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

/*============================================================
**
** Class:  ArrayList
**
** Purpose: Implements a dynamically sized List as an array,
**          and provides many convenience methods for treating
**          an array as an IList.
**
===========================================================*/

using System.Security;
using System.Diagnostics;
#if FEATURE_NETCORE
using System.Runtime.CompilerServices;
#endif
using System.Diagnostics.CodeAnalysis;
using System.Diagnostics.Contracts;

namespace System.Collections
{
    // Implements a variable-size List that uses an array of objects to store the
    // elements. A ArrayList has a capacity, which is the allocated length
    // of the internal array. As elements are added to a ArrayList, the capacity
    // of the ArrayList is automatically increased as required by reallocating the
    // internal array.
    // 
#if FEATURE_NETCORE
    [FriendAccessAllowed]
#endif
    [DebuggerTypeProxy(typeof(System.Collections.ArrayList.ArrayListDebugView))]
    [DebuggerDisplay("Count = {Count}")]
    public class ArrayList : IList
    {
        private Object[] _items;
        [ContractPublicPropertyName("Count")]
        private int _size;
        private int _version;
        private Object _syncRoot;

        private const int _defaultCapacity = 4;

        // Copy of Array.MaxArrayLength
        internal const int MaxArrayLength = 0X7FEFFFFF;

        // Note: this constructor is a bogus constructor that does nothing
        // and is for use only with SyncArrayList.
        internal ArrayList(bool trash)
        {
        }

        // Constructs a ArrayList. The list is initially empty and has a capacity
        // of zero. Upon adding the first element to the list the capacity is
        // increased to _defaultCapacity, and then increased in multiples of two as required.
        public ArrayList()
        {
            _items = Array.Empty<Object>();
        }

        // Constructs a ArrayList with a given initial capacity. The list is
        // initially empty, but will have room for the given number of elements
        // before any reallocations are required.
        // 
        public ArrayList(int capacity)
        {
            if (capacity < 0) throw new ArgumentOutOfRangeException(nameof(capacity), SR.Format(SR.ArgumentOutOfRange_MustBeNonNegNum, nameof(capacity)));
            Contract.EndContractBlock();

            if (capacity == 0)
                _items = Array.Empty<Object>();
            else
                _items = new Object[capacity];
        }

        // Constructs a ArrayList, copying the contents of the given collection. The
        // size and capacity of the new list will both be equal to the size of the
        // given collection.
        // 
        public ArrayList(ICollection c)
        {
            if (c == null)
                throw new ArgumentNullException(nameof(c), SR.ArgumentNull_Collection);
            Contract.EndContractBlock();

            int count = c.Count;
            if (count == 0)
            {
                _items = Array.Empty<Object>();
            }
            else
            {
                _items = new Object[count];
                AddRange(c);
            }
        }

        // Gets and sets the capacity of this list.  The capacity is the size of
        // the internal array used to hold items.  When set, the internal 
        // array of the list is reallocated to the given capacity.
        // 
        public virtual int Capacity
        {
            get
            {
                Contract.Ensures(Contract.Result<int>() >= Count);
                return _items.Length;
            }
            set
            {
                if (value < _size)
                {
                    throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_SmallCapacity);
                }
                Contract.Ensures(Capacity >= 0);
                Contract.EndContractBlock();
                // We don't want to update the version number when we change the capacity.
                // Some existing applications have dependency on this.
                if (value != _items.Length)
                {
                    if (value > 0)
                    {
                        Object[] newItems = new Object[value];
                        if (_size > 0)
                        {
                            Array.Copy(_items, 0, newItems, 0, _size);
                        }
                        _items = newItems;
                    }
                    else
                    {
                        _items = new Object[_defaultCapacity];
                    }
                }
            }
        }

        // Read-only property describing how many elements are in the List.
        public virtual int Count
        {
            get
            {
                Contract.Ensures(Contract.Result<int>() >= 0);
                return _size;
            }
        }

        public virtual bool IsFixedSize
        {
            get { return false; }
        }


        // Is this ArrayList read-only?
        public virtual bool IsReadOnly
        {
            get { return false; }
        }

        // Is this ArrayList synchronized (thread-safe)?
        public virtual bool IsSynchronized
        {
            get { return false; }
        }

        // Synchronization root for this object.
        public virtual Object SyncRoot
        {
            get
            {
                if (_syncRoot == null)
                {
                    System.Threading.Interlocked.CompareExchange<Object>(ref _syncRoot, new Object(), null);
                }
                return _syncRoot;
            }
        }

        // Sets or Gets the element at the given index.
        // 
        public virtual Object this[int index]
        {
            get
            {
                if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();
                return _items[index];
            }
            set
            {
                if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();
                _items[index] = value;
                _version++;
            }
        }

        // Creates a ArrayList wrapper for a particular IList.  This does not
        // copy the contents of the IList, but only wraps the IList.  So any
        // changes to the underlying list will affect the ArrayList.  This would
        // be useful if you want to Reverse a subrange of an IList, or want to
        // use a generic BinarySearch or Sort method without implementing one yourself.
        // However, since these methods are generic, the performance may not be
        // nearly as good for some operations as they would be on the IList itself.
        //
        public static ArrayList Adapter(IList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();
            return new IListWrapper(list);
        }

        // Adds the given object to the end of this list. The size of the list is
        // increased by one. If required, the capacity of the list is doubled
        // before adding the new element.
        //
        public virtual int Add(Object value)
        {
            Contract.Ensures(Contract.Result<int>() >= 0);
            if (_size == _items.Length) EnsureCapacity(_size + 1);
            _items[_size] = value;
            _version++;
            return _size++;
        }

        // Adds the elements of the given collection to the end of this list. If
        // required, the capacity of the list is increased to twice the previous
        // capacity or the new size, whichever is larger.
        //
        public virtual void AddRange(ICollection c)
        {
            InsertRange(_size, c);
        }

        // Searches a section of the list for a given element using a binary search
        // algorithm. Elements of the list are compared to the search value using
        // the given IComparer interface. If comparer is null, elements of
        // the list are compared to the search value using the IComparable
        // interface, which in that case must be implemented by all elements of the
        // list and the given search value. This method assumes that the given
        // section of the list is already sorted; if this is not the case, the
        // result will be incorrect.
        //
        // The method returns the index of the given value in the list. If the
        // list does not contain the given value, the method returns a negative
        // integer. The bitwise complement operator (~) can be applied to a
        // negative result to produce the index of the first element (if any) that
        // is larger than the given search value. This is also the index at which
        // the search value should be inserted into the list in order for the list
        // to remain sorted.
        // 
        // The method uses the Array.BinarySearch method to perform the
        // search.
        // 
        public virtual int BinarySearch(int index, int count, Object value, IComparer comparer)
        {
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.Ensures(Contract.Result<int>() < Count);
            Contract.Ensures(Contract.Result<int>() < index + count);
            Contract.EndContractBlock();

            return Array.BinarySearch((Array)_items, index, count, value, comparer);
        }

        public virtual int BinarySearch(Object value)
        {
            Contract.Ensures(Contract.Result<int>() < Count);
            return BinarySearch(0, Count, value, null);
        }

        public virtual int BinarySearch(Object value, IComparer comparer)
        {
            Contract.Ensures(Contract.Result<int>() < Count);
            return BinarySearch(0, Count, value, comparer);
        }


        // Clears the contents of ArrayList.
        public virtual void Clear()
        {
            if (_size > 0)
            {
                Array.Clear(_items, 0, _size); // Don't need to doc this but we clear the elements so that the gc can reclaim the references.
                _size = 0;
            }
            _version++;
        }

        // Clones this ArrayList, doing a shallow copy.  (A copy is made of all
        // Object references in the ArrayList, but the Objects pointed to 
        // are not cloned).
        public virtual Object Clone()
        {
            Contract.Ensures(Contract.Result<Object>() != null);
            ArrayList la = new ArrayList(_size);
            la._size = _size;
            la._version = _version;
            Array.Copy(_items, 0, la._items, 0, _size);
            return la;
        }


        // Contains returns true if the specified element is in the ArrayList.
        // It does a linear, O(n) search.  Equality is determined by calling
        // item.Equals().
        //
        public virtual bool Contains(Object item)
        {
            if (item == null)
            {
                for (int i = 0; i < _size; i++)
                    if (_items[i] == null)
                        return true;
                return false;
            }
            else
            {
                for (int i = 0; i < _size; i++)
                    if ((_items[i] != null) && (_items[i].Equals(item)))
                        return true;
                return false;
            }
        }

        // Copies this ArrayList into array, which must be of a 
        // compatible array type.  
        //
        public virtual void CopyTo(Array array)
        {
            CopyTo(array, 0);
        }

        // Copies this ArrayList into array, which must be of a 
        // compatible array type.  
        //
        public virtual void CopyTo(Array array, int arrayIndex)
        {
            if ((array != null) && (array.Rank != 1))
                throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
            Contract.EndContractBlock();
            // Delegate rest of error checking to Array.Copy.
            Array.Copy(_items, 0, array, arrayIndex, _size);
        }

        // Copies a section of this list to the given array at the given index.
        // 
        // The method uses the Array.Copy method to copy the elements.
        // 
        public virtual void CopyTo(int index, Array array, int arrayIndex, int count)
        {
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            if ((array != null) && (array.Rank != 1))
                throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
            Contract.EndContractBlock();
            // Delegate rest of error checking to Array.Copy.
            Array.Copy(_items, index, array, arrayIndex, count);
        }

        // Ensures that the capacity of this list is at least the given minimum
        // value. If the current capacity of the list is less than min, the
        // capacity is increased to twice the current capacity or to min,
        // whichever is larger.
        private void EnsureCapacity(int min)
        {
            if (_items.Length < min)
            {
                int newCapacity = _items.Length == 0 ? _defaultCapacity : _items.Length * 2;
                // Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
                // Note that this check works even when _items.Length overflowed thanks to the (uint) cast
                if ((uint)newCapacity > MaxArrayLength) newCapacity = MaxArrayLength;
                if (newCapacity < min) newCapacity = min;
                Capacity = newCapacity;
            }
        }

        // Returns a list wrapper that is fixed at the current size.  Operations
        // that add or remove items will fail, however, replacing items is allowed.
        //
        public static IList FixedSize(IList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<IList>() != null);
            Contract.EndContractBlock();
            return new FixedSizeList(list);
        }

        // Returns a list wrapper that is fixed at the current size.  Operations
        // that add or remove items will fail, however, replacing items is allowed.
        //
        public static ArrayList FixedSize(ArrayList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();
            return new FixedSizeArrayList(list);
        }

        // Returns an enumerator for this list with the given
        // permission for removal of elements. If modifications made to the list 
        // while an enumeration is in progress, the MoveNext and 
        // GetObject methods of the enumerator will throw an exception.
        //
        public virtual IEnumerator GetEnumerator()
        {
            Contract.Ensures(Contract.Result<IEnumerator>() != null);
            return new ArrayListEnumeratorSimple(this);
        }

        // Returns an enumerator for a section of this list with the given
        // permission for removal of elements. If modifications made to the list 
        // while an enumeration is in progress, the MoveNext and 
        // GetObject methods of the enumerator will throw an exception.
        //
        public virtual IEnumerator GetEnumerator(int index, int count)
        {
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.Ensures(Contract.Result<IEnumerator>() != null);
            Contract.EndContractBlock();

            return new ArrayListEnumerator(this, index, count);
        }

        // Returns the index of the first occurrence of a given value in a range of
        // this list. The list is searched forwards from beginning to end.
        // The elements of the list are compared to the given value using the
        // Object.Equals method.
        // 
        // This method uses the Array.IndexOf method to perform the
        // search.
        // 
        public virtual int IndexOf(Object value)
        {
            Contract.Ensures(Contract.Result<int>() < Count);
            return Array.IndexOf((Array)_items, value, 0, _size);
        }

        // Returns the index of the first occurrence of a given value in a range of
        // this list. The list is searched forwards, starting at index
        // startIndex and ending at count number of elements. The
        // elements of the list are compared to the given value using the
        // Object.Equals method.
        // 
        // This method uses the Array.IndexOf method to perform the
        // search.
        // 
        public virtual int IndexOf(Object value, int startIndex)
        {
            if (startIndex > _size)
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            Contract.Ensures(Contract.Result<int>() < Count);
            Contract.EndContractBlock();
            return Array.IndexOf((Array)_items, value, startIndex, _size - startIndex);
        }

        // Returns the index of the first occurrence of a given value in a range of
        // this list. The list is searched forwards, starting at index
        // startIndex and upto count number of elements. The
        // elements of the list are compared to the given value using the
        // Object.Equals method.
        // 
        // This method uses the Array.IndexOf method to perform the
        // search.
        // 
        public virtual int IndexOf(Object value, int startIndex, int count)
        {
            if (startIndex > _size)
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            if (count < 0 || startIndex > _size - count) throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Count);
            Contract.Ensures(Contract.Result<int>() < Count);
            Contract.EndContractBlock();
            return Array.IndexOf((Array)_items, value, startIndex, count);
        }

        // Inserts an element into this list at a given index. The size of the list
        // is increased by one. If required, the capacity of the list is doubled
        // before inserting the new element.
        // 
        public virtual void Insert(int index, Object value)
        {
            // Note that insertions at the end are legal.
            if (index < 0 || index > _size) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_ArrayListInsert);
            //Contract.Ensures(Count == Contract.OldValue(Count) + 1);
            Contract.EndContractBlock();

            if (_size == _items.Length) EnsureCapacity(_size + 1);
            if (index < _size)
            {
                Array.Copy(_items, index, _items, index + 1, _size - index);
            }
            _items[index] = value;
            _size++;
            _version++;
        }

        // Inserts the elements of the given collection at a given index. If
        // required, the capacity of the list is increased to twice the previous
        // capacity or the new size, whichever is larger.  Ranges may be added
        // to the end of the list by setting index to the ArrayList's size.
        //
        public virtual void InsertRange(int index, ICollection c)
        {
            if (c == null)
                throw new ArgumentNullException(nameof(c), SR.ArgumentNull_Collection);
            if (index < 0 || index > _size) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            //Contract.Ensures(Count == Contract.OldValue(Count) + c.Count);
            Contract.EndContractBlock();

            int count = c.Count;
            if (count > 0)
            {
                EnsureCapacity(_size + count);
                // shift existing items
                if (index < _size)
                {
                    Array.Copy(_items, index, _items, index + count, _size - index);
                }

                Object[] itemsToInsert = new Object[count];
                c.CopyTo(itemsToInsert, 0);
                itemsToInsert.CopyTo(_items, index);
                _size += count;
                _version++;
            }
        }

        // Returns the index of the last occurrence of a given value in a range of
        // this list. The list is searched backwards, starting at the end 
        // and ending at the first element in the list. The elements of the list 
        // are compared to the given value using the Object.Equals method.
        // 
        // This method uses the Array.LastIndexOf method to perform the
        // search.
        // 
        public virtual int LastIndexOf(Object value)
        {
            Contract.Ensures(Contract.Result<int>() < _size);
            return LastIndexOf(value, _size - 1, _size);
        }

        // Returns the index of the last occurrence of a given value in a range of
        // this list. The list is searched backwards, starting at index
        // startIndex and ending at the first element in the list. The 
        // elements of the list are compared to the given value using the 
        // Object.Equals method.
        // 
        // This method uses the Array.LastIndexOf method to perform the
        // search.
        // 
        public virtual int LastIndexOf(Object value, int startIndex)
        {
            if (startIndex >= _size)
                throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
            Contract.Ensures(Contract.Result<int>() < Count);
            Contract.EndContractBlock();
            return LastIndexOf(value, startIndex, startIndex + 1);
        }

        // Returns the index of the last occurrence of a given value in a range of
        // this list. The list is searched backwards, starting at index
        // startIndex and upto count elements. The elements of
        // the list are compared to the given value using the Object.Equals
        // method.
        // 
        // This method uses the Array.LastIndexOf method to perform the
        // search.
        // 
        public virtual int LastIndexOf(Object value, int startIndex, int count)
        {
            if (Count != 0 && (startIndex < 0 || count < 0))
                throw new ArgumentOutOfRangeException(startIndex < 0 ? nameof(startIndex) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            Contract.Ensures(Contract.Result<int>() < Count);
            Contract.EndContractBlock();

            if (_size == 0)  // Special case for an empty list
                return -1;

            if (startIndex >= _size || count > startIndex + 1)
                throw new ArgumentOutOfRangeException(startIndex >= _size ? nameof(startIndex) : nameof(count), SR.ArgumentOutOfRange_BiggerThanCollection);

            return Array.LastIndexOf((Array)_items, value, startIndex, count);
        }

        // Returns a read-only IList wrapper for the given IList.
        //
#if FEATURE_NETCORE
        [FriendAccessAllowed]
#endif
        public static IList ReadOnly(IList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<IList>() != null);
            Contract.EndContractBlock();
            return new ReadOnlyList(list);
        }

        // Returns a read-only ArrayList wrapper for the given ArrayList.
        //
        public static ArrayList ReadOnly(ArrayList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();
            return new ReadOnlyArrayList(list);
        }

        // Removes the element at the given index. The size of the list is
        // decreased by one.
        // 
        public virtual void Remove(Object obj)
        {
            Contract.Ensures(Count >= 0);

            int index = IndexOf(obj);
            Debug.Assert(index >= 0 || !(obj is Int32), "You passed an Int32 to Remove that wasn't in the ArrayList." + Environment.NewLine + "Did you mean RemoveAt?  int: " + obj + "  Count: " + Count);
            if (index >= 0)
                RemoveAt(index);
        }

        // Removes the element at the given index. The size of the list is
        // decreased by one.
        // 
        public virtual void RemoveAt(int index)
        {
            if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
            Contract.Ensures(Count >= 0);
            //Contract.Ensures(Count == Contract.OldValue(Count) - 1);
            Contract.EndContractBlock();

            _size--;
            if (index < _size)
            {
                Array.Copy(_items, index + 1, _items, index, _size - index);
            }
            _items[_size] = null;
            _version++;
        }

        // Removes a range of elements from this list.
        // 
        public virtual void RemoveRange(int index, int count)
        {
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.Ensures(Count >= 0);
            //Contract.Ensures(Count == Contract.OldValue(Count) - count);
            Contract.EndContractBlock();

            if (count > 0)
            {
                int i = _size;
                _size -= count;
                if (index < _size)
                {
                    Array.Copy(_items, index + count, _items, index, _size - index);
                }
                while (i > _size) _items[--i] = null;
                _version++;
            }
        }

        // Returns an IList that contains count copies of value.
        //
        public static ArrayList Repeat(Object value, int count)
        {
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();

            ArrayList list = new ArrayList((count > _defaultCapacity) ? count : _defaultCapacity);
            for (int i = 0; i < count; i++)
                list.Add(value);
            return list;
        }

        // Reverses the elements in this list.
        public virtual void Reverse()
        {
            Reverse(0, Count);
        }

        // Reverses the elements in a range of this list. Following a call to this
        // method, an element in the range given by index and count
        // which was previously located at index i will now be located at
        // index index + (index + count - i - 1).
        // 
        // This method uses the Array.Reverse method to reverse the
        // elements.
        // 
        public virtual void Reverse(int index, int count)
        {
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.EndContractBlock();
            Array.Reverse(_items, index, count);
            _version++;
        }

        // Sets the elements starting at the given index to the elements of the
        // given collection.
        //
        public virtual void SetRange(int index, ICollection c)
        {
            if (c == null) throw new ArgumentNullException(nameof(c), SR.ArgumentNull_Collection);
            Contract.EndContractBlock();
            int count = c.Count;
            if (index < 0 || index > _size - count) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);

            if (count > 0)
            {
                c.CopyTo(_items, index);
                _version++;
            }
        }

        public virtual ArrayList GetRange(int index, int count)
        {
            if (index < 0 || count < 0)
                throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();
            return new Range(this, index, count);
        }

        // Sorts the elements in this list.  Uses the default comparer and 
        // Array.Sort.
        public virtual void Sort()
        {
            Sort(0, Count, Comparer.Default);
        }

        // Sorts the elements in this list.  Uses Array.Sort with the
        // provided comparer.
        public virtual void Sort(IComparer comparer)
        {
            Sort(0, Count, comparer);
        }

        // Sorts the elements in a section of this list. The sort compares the
        // elements to each other using the given IComparer interface. If
        // comparer is null, the elements are compared to each other using
        // the IComparable interface, which in that case must be implemented by all
        // elements of the list.
        // 
        // This method uses the Array.Sort method to sort the elements.
        // 
        public virtual void Sort(int index, int count, IComparer comparer)
        {
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
            if (_size - index < count)
                throw new ArgumentException(SR.Argument_InvalidOffLen);
            Contract.EndContractBlock();

            Array.Sort(_items, index, count, comparer);
            _version++;
        }

        // Returns a thread-safe wrapper around an IList.
        //
        public static IList Synchronized(IList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<IList>() != null);
            Contract.EndContractBlock();
            return new SyncIList(list);
        }

        // Returns a thread-safe wrapper around a ArrayList.
        //
        public static ArrayList Synchronized(ArrayList list)
        {
            if (list == null)
                throw new ArgumentNullException(nameof(list));
            Contract.Ensures(Contract.Result<ArrayList>() != null);
            Contract.EndContractBlock();
            return new SyncArrayList(list);
        }

        // ToArray returns a new Object array containing the contents of the ArrayList.
        // This requires copying the ArrayList, which is an O(n) operation.
        public virtual Object[] ToArray()
        {
            Contract.Ensures(Contract.Result<Object[]>() != null);

            if (_size == 0)
                return Array.Empty<Object>();

            Object[] array = new Object[_size];
            Array.Copy(_items, 0, array, 0, _size);
            return array;
        }

        // ToArray returns a new array of a particular type containing the contents 
        // of the ArrayList.  This requires copying the ArrayList and potentially
        // downcasting all elements.  This copy may fail and is an O(n) operation.
        // Internally, this implementation calls Array.Copy.
        //
        [SecuritySafeCritical]
        public virtual Array ToArray(Type type)
        {
            if (type == null)
                throw new ArgumentNullException(nameof(type));
            Contract.Ensures(Contract.Result<Array>() != null);
            Contract.EndContractBlock();
            Array array = Array.CreateInstance(type, _size);
            Array.Copy(_items, 0, array, 0, _size);
            return array;
        }

        // Sets the capacity of this list to the size of the list. This method can
        // be used to minimize a list's memory overhead once it is known that no
        // new elements will be added to the list. To completely clear a list and
        // release all memory referenced by the list, execute the following
        // statements:
        // 
        // list.Clear();
        // list.TrimToSize();
        // 
        public virtual void TrimToSize()
        {
            Capacity = _size;
        }


        // This class wraps an IList, exposing it as a ArrayList
        // Note this requires reimplementing half of ArrayList...
        private class IListWrapper : ArrayList
        {
            private IList _list;

            internal IListWrapper(IList list)
            {
                _list = list;
                _version = 0; // list doesn't not contain a version number
            }

            public override int Capacity
            {
                get { return _list.Count; }
                set
                {
                    if (value < Count) throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_SmallCapacity);
                    Contract.EndContractBlock();
                }
            }

            public override int Count
            {
                get { return _list.Count; }
            }

            public override bool IsReadOnly
            {
                get { return _list.IsReadOnly; }
            }

            public override bool IsFixedSize
            {
                get { return _list.IsFixedSize; }
            }


            public override bool IsSynchronized
            {
                get { return _list.IsSynchronized; }
            }

            public override Object this[int index]
            {
                get
                {
                    return _list[index];
                }
                set
                {
                    _list[index] = value;
                    _version++;
                }
            }

            public override Object SyncRoot
            {
                get { return _list.SyncRoot; }
            }

            public override int Add(Object obj)
            {
                int i = _list.Add(obj);
                _version++;
                return i;
            }

            public override void AddRange(ICollection c)
            {
                InsertRange(Count, c);
            }

            // Other overloads with automatically work
            public override int BinarySearch(int index, int count, Object value, IComparer comparer)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();
                if (comparer == null)
                    comparer = Comparer.Default;

                int lo = index;
                int hi = index + count - 1;
                int mid;
                while (lo <= hi)
                {
                    mid = (lo + hi) / 2;
                    int r = comparer.Compare(value, _list[mid]);
                    if (r == 0)
                        return mid;
                    if (r < 0)
                        hi = mid - 1;
                    else
                        lo = mid + 1;
                }
                // return bitwise complement of the first element greater than value.
                // Since hi is less than lo now, ~lo is the correct item.
                return ~lo;
            }

            public override void Clear()
            {
                // If _list is an array, it will support Clear method.
                // We shouldn't allow clear operation on a FixedSized ArrayList
                if (_list.IsFixedSize)
                {
                    throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
                }

                _list.Clear();
                _version++;
            }

            public override Object Clone()
            {
                // This does not do a shallow copy of _list into a ArrayList!
                // This clones the IListWrapper, creating another wrapper class!
                return new IListWrapper(_list);
            }

            public override bool Contains(Object obj)
            {
                return _list.Contains(obj);
            }

            public override void CopyTo(Array array, int index)
            {
                _list.CopyTo(array, index);
            }

            public override void CopyTo(int index, Array array, int arrayIndex, int count)
            {
                if (array == null)
                    throw new ArgumentNullException(nameof(array));
                if (index < 0 || arrayIndex < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(arrayIndex), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (count < 0)
                    throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (array.Length - arrayIndex < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                if (array.Rank != 1)
                    throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
                Contract.EndContractBlock();

                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);

                for (int i = index; i < index + count; i++)
                    array.SetValue(_list[i], arrayIndex++);
            }

            public override IEnumerator GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            public override IEnumerator GetEnumerator(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                Contract.EndContractBlock();
                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);

                return new IListWrapperEnumWrapper(this, index, count);
            }

            public override int IndexOf(Object value)
            {
                return _list.IndexOf(value);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex)
            {
                return IndexOf(value, startIndex, _list.Count - startIndex);
            }

            public override int IndexOf(Object value, int startIndex, int count)
            {
                if (startIndex < 0 || startIndex > Count) throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
                if (count < 0 || startIndex > Count - count) throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Count);
                Contract.EndContractBlock();

                int endIndex = startIndex + count;
                if (value == null)
                {
                    for (int i = startIndex; i < endIndex; i++)
                        if (_list[i] == null)
                            return i;
                    return -1;
                }
                else
                {
                    for (int i = startIndex; i < endIndex; i++)
                        if (_list[i] != null && _list[i].Equals(value))
                            return i;
                    return -1;
                }
            }

            public override void Insert(int index, Object obj)
            {
                _list.Insert(index, obj);
                _version++;
            }

            public override void InsertRange(int index, ICollection c)
            {
                if (c == null)
                    throw new ArgumentNullException(nameof(c), SR.ArgumentNull_Collection);
                if (index < 0 || index > Count) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();

                if (c.Count > 0)
                {
                    ArrayList al = _list as ArrayList;
                    if (al != null)
                    {
                        // We need to special case ArrayList. 
                        // When c is a range of _list, we need to handle this in a special way.
                        // See ArrayList.InsertRange for details.
                        al.InsertRange(index, c);
                    }
                    else
                    {
                        IEnumerator en = c.GetEnumerator();
                        while (en.MoveNext())
                        {
                            _list.Insert(index++, en.Current);
                        }
                    }
                    _version++;
                }
            }

            public override int LastIndexOf(Object value)
            {
                return LastIndexOf(value, _list.Count - 1, _list.Count);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex)
            {
                return LastIndexOf(value, startIndex, startIndex + 1);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex, int count)
            {
                if (_list.Count == 0)
                    return -1;

                if (startIndex < 0 || startIndex >= _list.Count) throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
                if (count < 0 || count > startIndex + 1) throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Count);

                int endIndex = startIndex - count + 1;
                if (value == null)
                {
                    for (int i = startIndex; i >= endIndex; i--)
                        if (_list[i] == null)
                            return i;
                    return -1;
                }
                else
                {
                    for (int i = startIndex; i >= endIndex; i--)
                        if (_list[i] != null && _list[i].Equals(value))
                            return i;
                    return -1;
                }
            }

            public override void Remove(Object value)
            {
                int index = IndexOf(value);
                if (index >= 0)
                    RemoveAt(index);
            }

            public override void RemoveAt(int index)
            {
                _list.RemoveAt(index);
                _version++;
            }

            public override void RemoveRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                Contract.EndContractBlock();
                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);

                if (count > 0)    // be consistent with ArrayList
                    _version++;

                while (count > 0)
                {
                    _list.RemoveAt(index);
                    count--;
                }
            }

            public override void Reverse(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                Contract.EndContractBlock();
                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);

                int i = index;
                int j = index + count - 1;
                while (i < j)
                {
                    Object tmp = _list[i];
                    _list[i++] = _list[j];
                    _list[j--] = tmp;
                }
                _version++;
            }

            public override void SetRange(int index, ICollection c)
            {
                if (c == null)
                {
                    throw new ArgumentNullException(nameof(c), SR.ArgumentNull_Collection);
                }
                Contract.EndContractBlock();

                if (index < 0 || index > _list.Count - c.Count)
                {
                    throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                }

                if (c.Count > 0)
                {
                    IEnumerator en = c.GetEnumerator();
                    while (en.MoveNext())
                    {
                        _list[index++] = en.Current;
                    }
                    _version++;
                }
            }

            public override ArrayList GetRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                Contract.EndContractBlock();
                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                return new Range(this, index, count);
            }

            public override void Sort(int index, int count, IComparer comparer)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                Contract.EndContractBlock();
                if (_list.Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);

                Object[] array = new Object[count];
                CopyTo(index, array, 0, count);
                Array.Sort(array, 0, count, comparer);
                for (int i = 0; i < count; i++)
                    _list[i + index] = array[i];

                _version++;
            }


            public override Object[] ToArray()
            {
                if (Count == 0)
                    return Array.Empty<Object>();

                Object[] array = new Object[Count];
                _list.CopyTo(array, 0);
                return array;
            }

            [SecuritySafeCritical]
            public override Array ToArray(Type type)
            {
                if (type == null)
                    throw new ArgumentNullException(nameof(type));
                Contract.EndContractBlock();
                Array array = Array.CreateInstance(type, _list.Count);
                _list.CopyTo(array, 0);
                return array;
            }

            public override void TrimToSize()
            {
                // Can't really do much here...
            }

            // This is the enumerator for an IList that's been wrapped in another
            // class that implements all of ArrayList's methods.
            private sealed class IListWrapperEnumWrapper : IEnumerator
            {
                private IEnumerator _en;
                private int _remaining;
                private int _initialStartIndex; // for reset
                private int _initialCount;      // for reset
                private bool _firstCall;        // firstCall to MoveNext

                internal IListWrapperEnumWrapper(IListWrapper listWrapper, int startIndex, int count)
                {
                    _en = listWrapper.GetEnumerator();
                    _initialStartIndex = startIndex;
                    _initialCount = count;
                    while (startIndex-- > 0 && _en.MoveNext()) ;
                    _remaining = count;
                    _firstCall = true;
                }

                public bool MoveNext()
                {
                    if (_firstCall)
                    {
                        _firstCall = false;
                        return _remaining-- > 0 && _en.MoveNext();
                    }
                    if (_remaining < 0)
                        return false;
                    bool r = _en.MoveNext();
                    return r && _remaining-- > 0;
                }

                public Object Current
                {
                    get
                    {
                        if (_firstCall)
                            throw new InvalidOperationException(SR.InvalidOperation_EnumNotStarted);
                        if (_remaining < 0)
                            throw new InvalidOperationException(SR.InvalidOperation_EnumEnded);
                        return _en.Current;
                    }
                }

                public void Reset()
                {
                    _en.Reset();
                    int startIndex = _initialStartIndex;
                    while (startIndex-- > 0 && _en.MoveNext()) ;
                    _remaining = _initialCount;
                    _firstCall = true;
                }
            }
        }


        private class SyncArrayList : ArrayList
        {
            private ArrayList _list;
            private Object _root;

            internal SyncArrayList(ArrayList list)
                : base(false)
            {
                _list = list;
                _root = list.SyncRoot;
            }

            public override int Capacity
            {
                get
                {
                    lock (_root)
                    {
                        return _list.Capacity;
                    }
                }
                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
                set
                {
                    lock (_root)
                    {
                        _list.Capacity = value;
                    }
                }
            }

            public override int Count
            {
                get { lock (_root) { return _list.Count; } }
            }

            public override bool IsReadOnly
            {
                get { return _list.IsReadOnly; }
            }

            public override bool IsFixedSize
            {
                get { return _list.IsFixedSize; }
            }


            public override bool IsSynchronized
            {
                get { return true; }
            }

            public override Object this[int index]
            {
                get
                {
                    lock (_root)
                    {
                        return _list[index];
                    }
                }
                set
                {
                    lock (_root)
                    {
                        _list[index] = value;
                    }
                }
            }

            public override Object SyncRoot
            {
                get { return _root; }
            }

            public override int Add(Object value)
            {
                lock (_root)
                {
                    return _list.Add(value);
                }
            }

            public override void AddRange(ICollection c)
            {
                lock (_root)
                {
                    _list.AddRange(c);
                }
            }

            public override int BinarySearch(Object value)
            {
                lock (_root)
                {
                    return _list.BinarySearch(value);
                }
            }

            public override int BinarySearch(Object value, IComparer comparer)
            {
                lock (_root)
                {
                    return _list.BinarySearch(value, comparer);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int BinarySearch(int index, int count, Object value, IComparer comparer)
            {
                lock (_root)
                {
                    return _list.BinarySearch(index, count, value, comparer);
                }
            }

            public override void Clear()
            {
                lock (_root)
                {
                    _list.Clear();
                }
            }

            public override Object Clone()
            {
                lock (_root)
                {
                    return new SyncArrayList((ArrayList)_list.Clone());
                }
            }

            public override bool Contains(Object item)
            {
                lock (_root)
                {
                    return _list.Contains(item);
                }
            }

            public override void CopyTo(Array array)
            {
                lock (_root)
                {
                    _list.CopyTo(array);
                }
            }

            public override void CopyTo(Array array, int index)
            {
                lock (_root)
                {
                    _list.CopyTo(array, index);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void CopyTo(int index, Array array, int arrayIndex, int count)
            {
                lock (_root)
                {
                    _list.CopyTo(index, array, arrayIndex, count);
                }
            }

            public override IEnumerator GetEnumerator()
            {
                lock (_root)
                {
                    return _list.GetEnumerator();
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override IEnumerator GetEnumerator(int index, int count)
            {
                lock (_root)
                {
                    return _list.GetEnumerator(index, count);
                }
            }

            public override int IndexOf(Object value)
            {
                lock (_root)
                {
                    return _list.IndexOf(value);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex)
            {
                lock (_root)
                {
                    return _list.IndexOf(value, startIndex);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex, int count)
            {
                lock (_root)
                {
                    return _list.IndexOf(value, startIndex, count);
                }
            }

            public override void Insert(int index, Object value)
            {
                lock (_root)
                {
                    _list.Insert(index, value);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void InsertRange(int index, ICollection c)
            {
                lock (_root)
                {
                    _list.InsertRange(index, c);
                }
            }

            public override int LastIndexOf(Object value)
            {
                lock (_root)
                {
                    return _list.LastIndexOf(value);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex)
            {
                lock (_root)
                {
                    return _list.LastIndexOf(value, startIndex);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex, int count)
            {
                lock (_root)
                {
                    return _list.LastIndexOf(value, startIndex, count);
                }
            }

            public override void Remove(Object value)
            {
                lock (_root)
                {
                    _list.Remove(value);
                }
            }

            public override void RemoveAt(int index)
            {
                lock (_root)
                {
                    _list.RemoveAt(index);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void RemoveRange(int index, int count)
            {
                lock (_root)
                {
                    _list.RemoveRange(index, count);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Reverse(int index, int count)
            {
                lock (_root)
                {
                    _list.Reverse(index, count);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void SetRange(int index, ICollection c)
            {
                lock (_root)
                {
                    _list.SetRange(index, c);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override ArrayList GetRange(int index, int count)
            {
                lock (_root)
                {
                    return _list.GetRange(index, count);
                }
            }

            public override void Sort()
            {
                lock (_root)
                {
                    _list.Sort();
                }
            }

            public override void Sort(IComparer comparer)
            {
                lock (_root)
                {
                    _list.Sort(comparer);
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Sort(int index, int count, IComparer comparer)
            {
                lock (_root)
                {
                    _list.Sort(index, count, comparer);
                }
            }

            public override Object[] ToArray()
            {
                lock (_root)
                {
                    return _list.ToArray();
                }
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override Array ToArray(Type type)
            {
                lock (_root)
                {
                    return _list.ToArray(type);
                }
            }

            public override void TrimToSize()
            {
                lock (_root)
                {
                    _list.TrimToSize();
                }
            }
        }


        private class SyncIList : IList
        {
            private IList _list;
            private Object _root;

            internal SyncIList(IList list)
            {
                _list = list;
                _root = list.SyncRoot;
            }

            public virtual int Count
            {
                get { lock (_root) { return _list.Count; } }
            }

            public virtual bool IsReadOnly
            {
                get { return _list.IsReadOnly; }
            }

            public virtual bool IsFixedSize
            {
                get { return _list.IsFixedSize; }
            }


            public virtual bool IsSynchronized
            {
                get { return true; }
            }

            public virtual Object this[int index]
            {
                get
                {
                    lock (_root)
                    {
                        return _list[index];
                    }
                }
                set
                {
                    lock (_root)
                    {
                        _list[index] = value;
                    }
                }
            }

            public virtual Object SyncRoot
            {
                get { return _root; }
            }

            public virtual int Add(Object value)
            {
                lock (_root)
                {
                    return _list.Add(value);
                }
            }


            public virtual void Clear()
            {
                lock (_root)
                {
                    _list.Clear();
                }
            }

            public virtual bool Contains(Object item)
            {
                lock (_root)
                {
                    return _list.Contains(item);
                }
            }

            public virtual void CopyTo(Array array, int index)
            {
                lock (_root)
                {
                    _list.CopyTo(array, index);
                }
            }

            public virtual IEnumerator GetEnumerator()
            {
                lock (_root)
                {
                    return _list.GetEnumerator();
                }
            }

            public virtual int IndexOf(Object value)
            {
                lock (_root)
                {
                    return _list.IndexOf(value);
                }
            }

            public virtual void Insert(int index, Object value)
            {
                lock (_root)
                {
                    _list.Insert(index, value);
                }
            }

            public virtual void Remove(Object value)
            {
                lock (_root)
                {
                    _list.Remove(value);
                }
            }

            public virtual void RemoveAt(int index)
            {
                lock (_root)
                {
                    _list.RemoveAt(index);
                }
            }
        }

        private class FixedSizeList : IList
        {
            private IList _list;

            internal FixedSizeList(IList l)
            {
                _list = l;
            }

            public virtual int Count
            {
                get { return _list.Count; }
            }

            public virtual bool IsReadOnly
            {
                get { return _list.IsReadOnly; }
            }

            public virtual bool IsFixedSize
            {
                get { return true; }
            }

            public virtual bool IsSynchronized
            {
                get { return _list.IsSynchronized; }
            }

            public virtual Object this[int index]
            {
                get
                {
                    return _list[index];
                }
                set
                {
                    _list[index] = value;
                }
            }

            public virtual Object SyncRoot
            {
                get { return _list.SyncRoot; }
            }

            public virtual int Add(Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public virtual void Clear()
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public virtual bool Contains(Object obj)
            {
                return _list.Contains(obj);
            }

            public virtual void CopyTo(Array array, int index)
            {
                _list.CopyTo(array, index);
            }

            public virtual IEnumerator GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            public virtual int IndexOf(Object value)
            {
                return _list.IndexOf(value);
            }

            public virtual void Insert(int index, Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public virtual void Remove(Object value)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public virtual void RemoveAt(int index)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }
        }

        private class FixedSizeArrayList : ArrayList
        {
            private ArrayList _list;

            internal FixedSizeArrayList(ArrayList l)
            {
                _list = l;
                _version = _list._version;
            }

            public override int Count
            {
                get { return _list.Count; }
            }

            public override bool IsReadOnly
            {
                get { return _list.IsReadOnly; }
            }

            public override bool IsFixedSize
            {
                get { return true; }
            }

            public override bool IsSynchronized
            {
                get { return _list.IsSynchronized; }
            }

            public override Object this[int index]
            {
                get
                {
                    return _list[index];
                }
                set
                {
                    _list[index] = value;
                    _version = _list._version;
                }
            }

            public override Object SyncRoot
            {
                get { return _list.SyncRoot; }
            }

            public override int Add(Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public override void AddRange(ICollection c)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int BinarySearch(int index, int count, Object value, IComparer comparer)
            {
                return _list.BinarySearch(index, count, value, comparer);
            }

            public override int Capacity
            {
                get { return _list.Capacity; }
                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
                set
                { throw new NotSupportedException(SR.NotSupported_FixedSizeCollection); }
            }

            public override void Clear()
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public override Object Clone()
            {
                FixedSizeArrayList arrayList = new FixedSizeArrayList(_list);
                arrayList._list = (ArrayList)_list.Clone();
                return arrayList;
            }

            public override bool Contains(Object obj)
            {
                return _list.Contains(obj);
            }

            public override void CopyTo(Array array, int index)
            {
                _list.CopyTo(array, index);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void CopyTo(int index, Array array, int arrayIndex, int count)
            {
                _list.CopyTo(index, array, arrayIndex, count);
            }

            public override IEnumerator GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override IEnumerator GetEnumerator(int index, int count)
            {
                return _list.GetEnumerator(index, count);
            }

            public override int IndexOf(Object value)
            {
                return _list.IndexOf(value);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex)
            {
                return _list.IndexOf(value, startIndex);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex, int count)
            {
                return _list.IndexOf(value, startIndex, count);
            }

            public override void Insert(int index, Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void InsertRange(int index, ICollection c)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public override int LastIndexOf(Object value)
            {
                return _list.LastIndexOf(value);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex)
            {
                return _list.LastIndexOf(value, startIndex);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex, int count)
            {
                return _list.LastIndexOf(value, startIndex, count);
            }

            public override void Remove(Object value)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            public override void RemoveAt(int index)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void RemoveRange(int index, int count)
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void SetRange(int index, ICollection c)
            {
                _list.SetRange(index, c);
                _version = _list._version;
            }

            public override ArrayList GetRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                return new Range(this, index, count);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Reverse(int index, int count)
            {
                _list.Reverse(index, count);
                _version = _list._version;
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Sort(int index, int count, IComparer comparer)
            {
                _list.Sort(index, count, comparer);
                _version = _list._version;
            }

            public override Object[] ToArray()
            {
                return _list.ToArray();
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override Array ToArray(Type type)
            {
                return _list.ToArray(type);
            }

            public override void TrimToSize()
            {
                throw new NotSupportedException(SR.NotSupported_FixedSizeCollection);
            }
        }

        private class ReadOnlyList : IList
        {
            private IList _list;

            internal ReadOnlyList(IList l)
            {
                _list = l;
            }

            public virtual int Count
            {
                get { return _list.Count; }
            }

            public virtual bool IsReadOnly
            {
                get { return true; }
            }

            public virtual bool IsFixedSize
            {
                get { return true; }
            }

            public virtual bool IsSynchronized
            {
                get { return _list.IsSynchronized; }
            }

            public virtual Object this[int index]
            {
                get
                {
                    return _list[index];
                }
                set
                {
                    throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
                }
            }

            public virtual Object SyncRoot
            {
                get { return _list.SyncRoot; }
            }

            public virtual int Add(Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public virtual void Clear()
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public virtual bool Contains(Object obj)
            {
                return _list.Contains(obj);
            }

            public virtual void CopyTo(Array array, int index)
            {
                _list.CopyTo(array, index);
            }

            public virtual IEnumerator GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            public virtual int IndexOf(Object value)
            {
                return _list.IndexOf(value);
            }

            public virtual void Insert(int index, Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public virtual void Remove(Object value)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public virtual void RemoveAt(int index)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }
        }

        private class ReadOnlyArrayList : ArrayList
        {
            private ArrayList _list;

            internal ReadOnlyArrayList(ArrayList l)
            {
                _list = l;
            }

            public override int Count
            {
                get { return _list.Count; }
            }

            public override bool IsReadOnly
            {
                get { return true; }
            }

            public override bool IsFixedSize
            {
                get { return true; }
            }

            public override bool IsSynchronized
            {
                get { return _list.IsSynchronized; }
            }

            public override Object this[int index]
            {
                get
                {
                    return _list[index];
                }
                set
                {
                    throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
                }
            }

            public override Object SyncRoot
            {
                get { return _list.SyncRoot; }
            }

            public override int Add(Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override void AddRange(ICollection c)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int BinarySearch(int index, int count, Object value, IComparer comparer)
            {
                return _list.BinarySearch(index, count, value, comparer);
            }


            public override int Capacity
            {
                get { return _list.Capacity; }
                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
                set
                { throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection); }
            }

            public override void Clear()
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override Object Clone()
            {
                ReadOnlyArrayList arrayList = new ReadOnlyArrayList(_list);
                arrayList._list = (ArrayList)_list.Clone();
                return arrayList;
            }

            public override bool Contains(Object obj)
            {
                return _list.Contains(obj);
            }

            public override void CopyTo(Array array, int index)
            {
                _list.CopyTo(array, index);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void CopyTo(int index, Array array, int arrayIndex, int count)
            {
                _list.CopyTo(index, array, arrayIndex, count);
            }

            public override IEnumerator GetEnumerator()
            {
                return _list.GetEnumerator();
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override IEnumerator GetEnumerator(int index, int count)
            {
                return _list.GetEnumerator(index, count);
            }

            public override int IndexOf(Object value)
            {
                return _list.IndexOf(value);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex)
            {
                return _list.IndexOf(value, startIndex);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int IndexOf(Object value, int startIndex, int count)
            {
                return _list.IndexOf(value, startIndex, count);
            }

            public override void Insert(int index, Object obj)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void InsertRange(int index, ICollection c)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override int LastIndexOf(Object value)
            {
                return _list.LastIndexOf(value);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex)
            {
                return _list.LastIndexOf(value, startIndex);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex, int count)
            {
                return _list.LastIndexOf(value, startIndex, count);
            }

            public override void Remove(Object value)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override void RemoveAt(int index)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void RemoveRange(int index, int count)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void SetRange(int index, ICollection c)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override ArrayList GetRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (Count - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                return new Range(this, index, count);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Reverse(int index, int count)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void Sort(int index, int count, IComparer comparer)
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }

            public override Object[] ToArray()
            {
                return _list.ToArray();
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override Array ToArray(Type type)
            {
                return _list.ToArray(type);
            }

            public override void TrimToSize()
            {
                throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
            }
        }


        // Implements an enumerator for a ArrayList. The enumerator uses the
        // internal version number of the list to ensure that no modifications are
        // made to the list while an enumeration is in progress.
        private sealed class ArrayListEnumerator : IEnumerator
        {
            private ArrayList _list;
            private int _index;
            private int _endIndex;       // Where to stop.
            private int _version;
            private Object _currentElement;
            private int _startIndex;     // Save this for Reset.

            internal ArrayListEnumerator(ArrayList list, int index, int count)
            {
                _list = list;
                _startIndex = index;
                _index = index - 1;
                _endIndex = _index + count;  // last valid index
                _version = list._version;
                _currentElement = null;
            }

            public bool MoveNext()
            {
                if (_version != _list._version) throw new InvalidOperationException(SR.InvalidOperation_EnumFailedVersion);
                if (_index < _endIndex)
                {
                    _currentElement = _list[++_index];
                    return true;
                }
                else
                {
                    _index = _endIndex + 1;
                }

                return false;
            }

            public Object Current
            {
                get
                {
                    if (_index < _startIndex)
                        throw new InvalidOperationException(SR.InvalidOperation_EnumNotStarted);
                    else if (_index > _endIndex)
                    {
                        throw new InvalidOperationException(SR.InvalidOperation_EnumEnded);
                    }
                    return _currentElement;
                }
            }

            public void Reset()
            {
                if (_version != _list._version) throw new InvalidOperationException(SR.InvalidOperation_EnumFailedVersion);
                _index = _startIndex - 1;
            }
        }

        // Implementation of a generic list subrange. An instance of this class
        // is returned by the default implementation of List.GetRange.
        private class Range : ArrayList
        {
            private ArrayList _baseList;
            private int _baseIndex;
            [ContractPublicPropertyName("Count")]
            private int _baseSize;
            private int _baseVersion;

            internal Range(ArrayList list, int index, int count) : base(false)
            {
                _baseList = list;
                _baseIndex = index;
                _baseSize = count;
                _baseVersion = list._version;
                // we also need to update _version field to make Range of Range work
                _version = list._version;
            }

            private void InternalUpdateRange()
            {
                if (_baseVersion != _baseList._version)
                    throw new InvalidOperationException(SR.InvalidOperation_UnderlyingArrayListChanged);
            }

            private void InternalUpdateVersion()
            {
                _baseVersion++;
                _version++;
            }

            public override int Add(Object value)
            {
                InternalUpdateRange();
                _baseList.Insert(_baseIndex + _baseSize, value);
                InternalUpdateVersion();
                return _baseSize++;
            }

            public override void AddRange(ICollection c)
            {
                if (c == null)
                {
                    throw new ArgumentNullException(nameof(c));
                }
                Contract.EndContractBlock();

                InternalUpdateRange();
                int count = c.Count;
                if (count > 0)
                {
                    _baseList.InsertRange(_baseIndex + _baseSize, c);
                    InternalUpdateVersion();
                    _baseSize += count;
                }
            }

            public override int BinarySearch(int index, int count, Object value, IComparer comparer)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();
                InternalUpdateRange();

                int i = _baseList.BinarySearch(_baseIndex + index, count, value, comparer);
                if (i >= 0) return i - _baseIndex;
                return i + _baseIndex;
            }

            public override int Capacity
            {
                get
                {
                    return _baseList.Capacity;
                }

                set
                {
                    if (value < Count) throw new ArgumentOutOfRangeException(nameof(value), SR.ArgumentOutOfRange_SmallCapacity);
                    Contract.EndContractBlock();
                }
            }


            public override void Clear()
            {
                InternalUpdateRange();
                if (_baseSize != 0)
                {
                    _baseList.RemoveRange(_baseIndex, _baseSize);
                    InternalUpdateVersion();
                    _baseSize = 0;
                }
            }

            public override Object Clone()
            {
                InternalUpdateRange();
                Range arrayList = new Range(_baseList, _baseIndex, _baseSize);
                arrayList._baseList = (ArrayList)_baseList.Clone();
                return arrayList;
            }

            public override bool Contains(Object item)
            {
                InternalUpdateRange();
                if (item == null)
                {
                    for (int i = 0; i < _baseSize; i++)
                        if (_baseList[_baseIndex + i] == null)
                            return true;
                    return false;
                }
                else
                {
                    for (int i = 0; i < _baseSize; i++)
                        if (_baseList[_baseIndex + i] != null && _baseList[_baseIndex + i].Equals(item))
                            return true;
                    return false;
                }
            }

            public override void CopyTo(Array array, int index)
            {
                if (array == null)
                    throw new ArgumentNullException(nameof(array));
                if (array.Rank != 1)
                    throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
                if (index < 0)
                    throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (array.Length - index < _baseSize)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.CopyTo(_baseIndex, array, index, _baseSize);
            }

            public override void CopyTo(int index, Array array, int arrayIndex, int count)
            {
                if (array == null)
                    throw new ArgumentNullException(nameof(array));
                if (array.Rank != 1)
                    throw new ArgumentException(SR.Arg_RankMultiDimNotSupported, nameof(array));
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (array.Length - arrayIndex < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.CopyTo(_baseIndex + index, array, arrayIndex, count);
            }

            public override int Count
            {
                get
                {
                    InternalUpdateRange();
                    return _baseSize;
                }
            }

            public override bool IsReadOnly
            {
                get { return _baseList.IsReadOnly; }
            }

            public override bool IsFixedSize
            {
                get { return _baseList.IsFixedSize; }
            }

            public override bool IsSynchronized
            {
                get { return _baseList.IsSynchronized; }
            }

            public override IEnumerator GetEnumerator()
            {
                return GetEnumerator(0, _baseSize);
            }

            public override IEnumerator GetEnumerator(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                return _baseList.GetEnumerator(_baseIndex + index, count);
            }

            public override ArrayList GetRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                return new Range(this, index, count);
            }

            public override Object SyncRoot
            {
                get
                {
                    return _baseList.SyncRoot;
                }
            }


            public override int IndexOf(Object value)
            {
                InternalUpdateRange();
                int i = _baseList.IndexOf(value, _baseIndex, _baseSize);
                if (i >= 0) return i - _baseIndex;
                return -1;
            }

            public override int IndexOf(Object value, int startIndex)
            {
                if (startIndex < 0)
                    throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (startIndex > _baseSize)
                    throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();

                InternalUpdateRange();
                int i = _baseList.IndexOf(value, _baseIndex + startIndex, _baseSize - startIndex);
                if (i >= 0) return i - _baseIndex;
                return -1;
            }

            public override int IndexOf(Object value, int startIndex, int count)
            {
                if (startIndex < 0 || startIndex > _baseSize)
                    throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);

                if (count < 0 || (startIndex > _baseSize - count))
                    throw new ArgumentOutOfRangeException(nameof(count), SR.ArgumentOutOfRange_Count);
                Contract.EndContractBlock();

                InternalUpdateRange();
                int i = _baseList.IndexOf(value, _baseIndex + startIndex, count);
                if (i >= 0) return i - _baseIndex;
                return -1;
            }

            public override void Insert(int index, Object value)
            {
                if (index < 0 || index > _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.Insert(_baseIndex + index, value);
                InternalUpdateVersion();
                _baseSize++;
            }

            public override void InsertRange(int index, ICollection c)
            {
                if (index < 0 || index > _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                if (c == null)
                {
                    throw new ArgumentNullException(nameof(c));
                }
                Contract.EndContractBlock();

                InternalUpdateRange();
                int count = c.Count;
                if (count > 0)
                {
                    _baseList.InsertRange(_baseIndex + index, c);
                    _baseSize += count;
                    InternalUpdateVersion();
                }
            }

            public override int LastIndexOf(Object value)
            {
                InternalUpdateRange();
                int i = _baseList.LastIndexOf(value, _baseIndex + _baseSize - 1, _baseSize);
                if (i >= 0) return i - _baseIndex;
                return -1;
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex)
            {
                return LastIndexOf(value, startIndex, startIndex + 1);
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override int LastIndexOf(Object value, int startIndex, int count)
            {
                InternalUpdateRange();
                if (_baseSize == 0)
                    return -1;

                if (startIndex >= _baseSize)
                    throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_Index);
                if (startIndex < 0)
                    throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ArgumentOutOfRange_NeedNonNegNum);

                int i = _baseList.LastIndexOf(value, _baseIndex + startIndex, count);
                if (i >= 0) return i - _baseIndex;
                return -1;
            }

            // Don't need to override Remove

            public override void RemoveAt(int index)
            {
                if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.RemoveAt(_baseIndex + index);
                InternalUpdateVersion();
                _baseSize--;
            }

            public override void RemoveRange(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                // No need to call _bastList.RemoveRange if count is 0.
                // In addition, _baseList won't change the version number if count is 0. 
                if (count > 0)
                {
                    _baseList.RemoveRange(_baseIndex + index, count);
                    InternalUpdateVersion();
                    _baseSize -= count;
                }
            }

            public override void Reverse(int index, int count)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.Reverse(_baseIndex + index, count);
                InternalUpdateVersion();
            }

            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
            public override void SetRange(int index, ICollection c)
            {
                InternalUpdateRange();
                if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                _baseList.SetRange(_baseIndex + index, c);
                if (c.Count > 0)
                {
                    InternalUpdateVersion();
                }
            }

            public override void Sort(int index, int count, IComparer comparer)
            {
                if (index < 0 || count < 0)
                    throw new ArgumentOutOfRangeException(index < 0 ? nameof(index) : nameof(count), SR.ArgumentOutOfRange_NeedNonNegNum);
                if (_baseSize - index < count)
                    throw new ArgumentException(SR.Argument_InvalidOffLen);
                Contract.EndContractBlock();

                InternalUpdateRange();
                _baseList.Sort(_baseIndex + index, count, comparer);
                InternalUpdateVersion();
            }

            public override Object this[int index]
            {
                get
                {
                    InternalUpdateRange();
                    if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                    return _baseList[_baseIndex + index];
                }
                set
                {
                    InternalUpdateRange();
                    if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException(nameof(index), SR.ArgumentOutOfRange_Index);
                    _baseList[_baseIndex + index] = value;
                    InternalUpdateVersion();
                }
            }

            public override Object[] ToArray()
            {
                InternalUpdateRange();
                if (_baseSize == 0)
                    return Array.Empty<Object>();
                Object[] array = new Object[_baseSize];
                Array.Copy(_baseList._items, _baseIndex, array, 0, _baseSize);
                return array;
            }

            [SecuritySafeCritical]
            public override Array ToArray(Type type)
            {
                if (type == null)
                    throw new ArgumentNullException(nameof(type));
                Contract.EndContractBlock();

                InternalUpdateRange();
                Array array = Array.CreateInstance(type, _baseSize);
                _baseList.CopyTo(_baseIndex, array, 0, _baseSize);
                return array;
            }

            public override void TrimToSize()
            {
                throw new NotSupportedException(SR.NotSupported_RangeCollection);
            }
        }

        private sealed class ArrayListEnumeratorSimple : IEnumerator
        {
            private ArrayList _list;
            private int _index;
            private int _version;
            private Object _currentElement;
            private bool _isArrayList;
            // this object is used to indicate enumeration has not started or has terminated
            private static Object s_dummyObject = new Object();

            internal ArrayListEnumeratorSimple(ArrayList list)
            {
                _list = list;
                _index = -1;
                _version = list._version;
                _isArrayList = (list.GetType() == typeof(ArrayList));
                _currentElement = s_dummyObject;
            }

            public bool MoveNext()
            {
                if (_version != _list._version)
                {
                    throw new InvalidOperationException(SR.InvalidOperation_EnumFailedVersion);
                }

                if (_isArrayList)
                {  // avoid calling virtual methods if we are operating on ArrayList to improve performance
                    if (_index < _list._size - 1)
                    {
                        _currentElement = _list._items[++_index];
                        return true;
                    }
                    else
                    {
                        _currentElement = s_dummyObject;
                        _index = _list._size;
                        return false;
                    }
                }
                else
                {
                    if (_index < _list.Count - 1)
                    {
                        _currentElement = _list[++_index];
                        return true;
                    }
                    else
                    {
                        _index = _list.Count;
                        _currentElement = s_dummyObject;
                        return false;
                    }
                }
            }

            public Object Current
            {
                get
                {
                    object temp = _currentElement;
                    if (s_dummyObject == temp)
                    { // check if enumeration has not started or has terminated
                        if (_index == -1)
                        {
                            throw new InvalidOperationException(SR.InvalidOperation_EnumNotStarted);
                        }
                        else
                        {
                            throw new InvalidOperationException(SR.InvalidOperation_EnumEnded);
                        }
                    }

                    return temp;
                }
            }

            public void Reset()
            {
                if (_version != _list._version)
                {
                    throw new InvalidOperationException(SR.InvalidOperation_EnumFailedVersion);
                }

                _currentElement = s_dummyObject;
                _index = -1;
            }
        }

        internal class ArrayListDebugView
        {
            private ArrayList _arrayList;

            public ArrayListDebugView(ArrayList arrayList)
            {
                if (arrayList == null)
                    throw new ArgumentNullException(nameof(arrayList));

                _arrayList = arrayList;
            }

            [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
            public Object[] Items
            {
                get
                {
                    return _arrayList.ToArray();
                }
            }
        }
    }
}