Fixes #1296 - ItemCollection should be ICollection

A little bit more kindness for consumers.

I didn't add any tests because these are all just trivial forwards.

src/Microsoft.AspNetCore.Razor.Language/DefaultItemCollection.cs

@@ -39,5 +39,68 @@ public override object this[object key]
                 _items[key] = value;
             }
         }
+
+        public override int Count => _items.Count;
+
+        public override bool IsReadOnly => false;
+
+        public override void Add(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            ((ICollection<KeyValuePair<object, object>>)_items).Add(item);
+        }
+
+        public override void Clear()
+        {
+            _items.Clear();
+        }
+
+        public override bool Contains(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            return ((ICollection<KeyValuePair<object, object>>)_items).Contains(item);
+        }
+
+        public override void CopyTo(KeyValuePair<object, object>[] array, int arrayIndex)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException(nameof(array));
+            }
+
+            if (arrayIndex < 0 || arrayIndex > array.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(arrayIndex));
+            }
+            else if (array.Length - arrayIndex < Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(arrayIndex));
+            }
+
+            ((ICollection<KeyValuePair<object, object>>)_items).CopyTo(array, arrayIndex);
+        }
+
+        public override IEnumerator<KeyValuePair<object, object>> GetEnumerator()
+        {
+            return _items.GetEnumerator();
+        }
+
+        public override bool Remove(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            return ((ICollection<KeyValuePair<object, object>>)_items).Remove(item);
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/ItemCollection.cs

@@ -1,10 +1,34 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System.Collections;
+using System.Collections.Generic;
+
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    public abstract class ItemCollection
+    public abstract class ItemCollection : ICollection<KeyValuePair<object, object>>
     {
         public abstract object this[object key] { get; set; }
+
+        public abstract int Count { get; }
+
+        public abstract bool IsReadOnly { get; }
+
+        public abstract void Add(KeyValuePair<object, object> item);
+
+        public abstract void Clear();
+
+        public abstract bool Contains(KeyValuePair<object, object> item);
+
+        public abstract void CopyTo(KeyValuePair<object, object>[] array, int arrayIndex);
+
+        public abstract IEnumerator<KeyValuePair<object, object>> GetEnumerator();
+
+        public abstract bool Remove(KeyValuePair<object, object> item);
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/ReadonlyItemCollection.cs

@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
@@ -14,5 +16,68 @@ public override object this[object key]
             get => null;
             set => throw new NotSupportedException();
         }
+
+        public override int Count => 0;
+
+        public override bool IsReadOnly => true;
+
+        public override void Add(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            throw new NotSupportedException();
+        }
+
+        public override void Clear()
+        {
+            throw new NotSupportedException();
+        }
+
+        public override bool Contains(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            return false;
+        }
+
+        public override void CopyTo(KeyValuePair<object, object>[] array, int arrayIndex)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException(nameof(array));
+            }
+
+            if (arrayIndex < 0 || arrayIndex > array.Length)
+            {
+                throw new ArgumentOutOfRangeException(nameof(arrayIndex));
+            }
+            else if (array.Length - arrayIndex < Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(arrayIndex));
+            }
+
+            // Do nothing.
+        }
+
+        public override IEnumerator<KeyValuePair<object, object>> GetEnumerator()
+        {
+            return Enumerable.Empty<KeyValuePair<object, object>>().GetEnumerator();
+        }
+
+        public override bool Remove(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            throw new NotSupportedException();
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/Resources.resx

@@ -210,4 +210,7 @@
   <data name="DirectiveMustAppearAtStartOfLine" xml:space="preserve">
     <value>The '{0}` directive must appear at the start of the line.</value>
   </data>
+  <data name="KeyMustNotBeNull" xml:space="preserve">
+    <value>The key must not be null.</value>
+  </data>
 </root>