comparer.cs

// ==++==
// 
//   Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// ==--==
// <OWNER>Microsoft</OWNER>
// 

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics.Contracts;
//using System.Globalization;
using System.Runtime.CompilerServices;

namespace System.Collections.Generic
{    
    [Serializable]
    [TypeDependencyAttribute("System.Collections.Generic.ObjectComparer`1")] 
    public abstract class Comparer<T> : IComparer, IComparer<T>
    {
        static volatile Comparer<T> defaultComparer;    

        public static Comparer<T> Default {
            get {
                Contract.Ensures(Contract.Result<Comparer<T>>() != null);

                Comparer<T> comparer = defaultComparer;
                if (comparer == null) {
                    comparer = CreateComparer();
                    defaultComparer = comparer;
                }
                return comparer;
            }
        }

        public static Comparer<T> Create(Comparison<T> comparison)
        {
            Contract.Ensures(Contract.Result<Comparer<T>>() != null);

            if (comparison == null)
                throw new ArgumentNullException("comparison");

            return new ComparisonComparer<T>(comparison);
        }

        //
        // Note that logic in this method is replicated in vm\compile.cpp to ensure that NGen
        // saves the right instantiations
        //
        [System.Security.SecuritySafeCritical]  // auto-generated
        private static Comparer<T> CreateComparer() {
            RuntimeType t = (RuntimeType)typeof(T);

            // If T implements IComparable<T> return a GenericComparer<T>
#if FEATURE_LEGACYNETCF
            // Pre-Apollo Windows Phone call the overload that sorts the keys, not values this achieves the same result
            if (CompatibilitySwitches.IsAppEarlierThanWindowsPhone8) {
                if (t.ImplementInterface(typeof(IComparable<T>))) {
                    return (Comparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericComparer<int>), t);
                }
            }
            else
#endif
                if (typeof(IComparable<T>).IsAssignableFrom(t)) {
                    return (Comparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericComparer<int>), t);
                }

            // If T is a Nullable<U> where U implements IComparable<U> return a NullableComparer<U>
            if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>)) {
                RuntimeType u = (RuntimeType)t.GetGenericArguments()[0];
                if (typeof(IComparable<>).MakeGenericType(u).IsAssignableFrom(u)) {
                    return (Comparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(NullableComparer<int>), u);
                }
            }
            // Otherwise return an ObjectComparer<T>
          return new ObjectComparer<T>();
        }

        public abstract int Compare(T x, T y);

        int IComparer.Compare(object x, object y) {
            if (x == null) return y == null ? 0 : -1;
            if (y == null) return 1;
            if (x is T && y is T) return Compare((T)x, (T)y);
            ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArgumentForComparison);
            return 0;
        }
    }

    [Serializable]
    internal class GenericComparer<T> : Comparer<T> where T: IComparable<T>
    {    
        public override int Compare(T x, T y) {
            if (x != null) {
                if (y != null) return x.CompareTo(y);
                return 1;
            }
            if (y != null) return -1;
            return 0;
        }

        // Equals method for the comparer itself. 
        public override bool Equals(Object obj){
            GenericComparer<T> comparer = obj as GenericComparer<T>;
            return comparer != null;
        }        

        public override int GetHashCode() {
            return this.GetType().Name.GetHashCode();
        }
    }

    [Serializable]
    internal class NullableComparer<T> : Comparer<Nullable<T>> where T : struct, IComparable<T>
    {
        public override int Compare(Nullable<T> x, Nullable<T> y) {
            if (x.HasValue) {
                if (y.HasValue) return x.value.CompareTo(y.value);
                return 1;
            }
            if (y.HasValue) return -1;
            return 0;
        }

        // Equals method for the comparer itself. 
        public override bool Equals(Object obj){
            NullableComparer<T> comparer = obj as NullableComparer<T>;
            return comparer != null;
        }        


        public override int GetHashCode() {
            return this.GetType().Name.GetHashCode();
        }
    }

    [Serializable]
    internal class ObjectComparer<T> : Comparer<T>
    {
        public override int Compare(T x, T y) {
            return System.Collections.Comparer.Default.Compare(x, y);
        }

        // Equals method for the comparer itself. 
        public override bool Equals(Object obj){
            ObjectComparer<T> comparer = obj as ObjectComparer<T>;
            return comparer != null;
        }        

        public override int GetHashCode() {
            return this.GetType().Name.GetHashCode();
        }
    }

    [Serializable]
    internal class ComparisonComparer<T> : Comparer<T>
    {
        private readonly Comparison<T> _comparison;

        public ComparisonComparer(Comparison<T> comparison) {
            _comparison = comparison;
        }

        public override int Compare(T x, T y) {
            return _comparison(x, y);
        }
    }
}