Streamline collections

Makes our collections sealed instead of abstract.

Only the IntermediateNodeCollection needs to have a read only variant.

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/InjectIntermediateNode.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
 
         public string MemberName { get; set; }
 
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/DefaultCodeRenderingContext.cs

@@ -52,8 +52,8 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             _documentNode = documentNode;
             Options = options;
 
-            Diagnostics = new DefaultRazorDiagnosticCollection();
+            Diagnostics = new RazorDiagnosticCollection();
-            Items = new DefaultItemCollection();
+            Items = new ItemCollection();
             LineMappings = new List<LineMapping>();
 
             TagHelperRenderingContext = new TagHelperRenderingContext();

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

@@ -10,7 +10,7 @@ namespace Microsoft.AspNetCore.Razor.Language
     internal class DefaultAllowedChildTagDescriptorBuilder : AllowedChildTagDescriptorBuilder
     {
         private readonly DefaultTagHelperDescriptorBuilder _parent;
-        private DefaultRazorDiagnosticCollection _diagnostics;
+        private RazorDiagnosticCollection _diagnostics;
 
         public DefaultAllowedChildTagDescriptorBuilder(DefaultTagHelperDescriptorBuilder parent)
         {
@@ -27,7 +27,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

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

@@ -32,7 +32,7 @@ namespace Microsoft.AspNetCore.Razor.Language
         private readonly string _kind;
         private readonly Dictionary<string, string> _metadata;
 
-        private DefaultRazorDiagnosticCollection _diagnostics;
+        private RazorDiagnosticCollection _diagnostics;
 
         public DefaultBoundAttributeDescriptorBuilder(DefaultTagHelperDescriptorBuilder parent, string kind)
         {
@@ -66,7 +66,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

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

@@ -1,116 +0,0 @@
-// 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;
-using System.Collections.Generic;
-
-namespace Microsoft.AspNetCore.Razor.Language
-{
-    internal class DefaultItemCollection : ItemCollection
-    {
-        private readonly Dictionary<object, object> _items;
-
-        public DefaultItemCollection()
-        {
-            _items = new Dictionary<object, object>();
-        }
-
-        public override object this[object key]
-        {
-            get
-            {
-                if (key == null)
-                {
-                    throw new ArgumentNullException(nameof(key));
-                }
-
-                object value;
-                _items.TryGetValue(key, out value);
-                return value;
-            }
-
-            set
-            {
-                if (key == null)
-                {
-                    throw new ArgumentNullException(nameof(key));
-                }
-
-                _items[key] = value;
-            }
-        }
-
-        public override int Count => _items.Count;
-
-        public override bool IsReadOnly => false;
-
-        public override void Add(object key, object value)
-        {
-            if (key == null)
-            {
-                throw new ArgumentNullException(nameof(key));
-            }
-
-            _items.Add(key, value);
-        }
-
-        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/DefaultRazorCodeDocument.cs

@@ -21,7 +21,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             Source = source;
             Imports = imports?.ToArray() ?? RazorSourceDocument.EmptyArray;
 
-            Items = new DefaultItemCollection();
+            Items = new ItemCollection();
         }
 
         public override IReadOnlyList<RazorSourceDocument> Imports { get; }

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

@@ -1,128 +0,0 @@
-// 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;
-using System.Collections.Generic;
-
-namespace Microsoft.AspNetCore.Razor.Language
-{
-    public sealed class DefaultRazorDiagnosticCollection : RazorDiagnosticCollection
-    {
-        private readonly List<RazorDiagnostic> _inner = new List<RazorDiagnostic>();
-
-        public override RazorDiagnostic this[int index]
-        {
-            get
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                return _inner[index];
-            }
-            set
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                _inner[index] = value;
-            }
-        }
-
-        public override int Count => _inner.Count;
-
-        public override bool IsReadOnly => false;
-
-        public override void Add(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            _inner.Add(item);
-        }
-
-        public override void Clear()
-        {
-            _inner.Clear();
-        }
-
-        public override bool Contains(RazorDiagnostic item)
-        {
-            return _inner.Contains(item);
-        }
-
-        public override void CopyTo(RazorDiagnostic[] 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));
-            }
-
-            _inner.CopyTo(array, arrayIndex);
-        }
-
-        public override IEnumerator<RazorDiagnostic> GetEnumerator()
-        {
-            return _inner.GetEnumerator();
-        }
-
-        public override int IndexOf(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return _inner.IndexOf(item);
-        }
-
-        public override void Insert(int index, RazorDiagnostic item)
-        {
-            if (index < 0 || index > Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            _inner.Insert(index, item);
-        }
-
-        public override bool Remove(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return _inner.Remove(item);
-        }
-
-        public override void RemoveAt(int index)
-        {
-            if (index < 0 || index >= Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            _inner.RemoveAt(index);
-        }
-    }
-}

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

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     internal class DefaultRequiredAttributeDescriptorBuilder : RequiredAttributeDescriptorBuilder
     {
-        private DefaultRazorDiagnosticCollection _diagnostics;
+        private RazorDiagnosticCollection _diagnostics;
 
         public override string Name { get; set; }
 
@@ -25,7 +25,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

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

@@ -15,7 +15,7 @@ namespace Microsoft.AspNetCore.Razor.Language
         private List<DefaultAllowedChildTagDescriptorBuilder> _allowedChildTags;
         private List<DefaultBoundAttributeDescriptorBuilder> _attributeBuilders;
         private List<DefaultTagMatchingRuleDescriptorBuilder> _tagMatchingRuleBuilders;
-        private DefaultRazorDiagnosticCollection _diagnostics;
+        private RazorDiagnosticCollection _diagnostics;
 
         public DefaultTagHelperDescriptorBuilder(string kind, string name, string assemblyName)
         {
@@ -50,7 +50,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

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

@@ -11,7 +11,7 @@ namespace Microsoft.AspNetCore.Razor.Language
     internal class DefaultTagMatchingRuleDescriptorBuilder : TagMatchingRuleDescriptorBuilder
     {
         private List<DefaultRequiredAttributeDescriptorBuilder> _requiredAttributeBuilders;
-        private DefaultRazorDiagnosticCollection _diagnostics;
+        private RazorDiagnosticCollection _diagnostics;
 
         internal DefaultTagMatchingRuleDescriptorBuilder()
         {
@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperBodyIntermediateNode.cs

@@ -33,7 +33,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             }
         }
 
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public TagMode TagMode { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperCreateIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     public sealed class DefaultTagHelperCreateIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children { get; } = IntermediateNodeCollection.ReadOnly;
 
         public string Field { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperExecuteIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     public sealed class DefaultTagHelperExecuteIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children { get; } = IntermediateNodeCollection.ReadOnly;
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperHtmlAttributeIntermediateNode.cs

@@ -39,7 +39,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 
         public AttributeStructure AttributeStructure { get; set; }
 
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperPropertyIntermediateNode.cs

@@ -38,7 +38,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             }
         }
 
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string AttributeName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/DefaultTagHelperRuntimeIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     public sealed class DefaultTagHelperRuntimeIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Extensions/DesignTimeDirectiveIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     internal sealed class DesignTimeDirectiveIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Extensions/PreallocatedTagHelperHtmlAttributeIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     internal sealed class PreallocatedTagHelperHtmlAttributeIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string VariableName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/PreallocatedTagHelperHtmlAttributeValueIntermediateNode.cs

@@ -33,7 +33,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             }
         }
 
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string VariableName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/PreallocatedTagHelperPropertyIntermediateNode.cs

@@ -30,7 +30,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
             TagHelper = propertyNode.TagHelper;
         }
 
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string AttributeName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/PreallocatedTagHelperPropertyValueIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     internal sealed class PreallocatedTagHelperPropertyValueIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string VariableName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/SectionIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     public sealed class SectionIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Name { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Extensions/TemplateIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Extensions
 {
     public sealed class TemplateIntermediateNode : ExtensionIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Intermediate/CSharpCodeAttributeValueIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class CSharpCodeAttributeValueIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Prefix { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/CSharpCodeIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class CSharpCodeIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Intermediate/CSharpExpressionAttributeValueIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class CSharpExpressionAttributeValueIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Prefix { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/CSharpExpressionIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class CSharpExpressionIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Intermediate/ClassDeclarationIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class ClassDeclarationIntermediateNode : MemberDeclarationIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public IList<string> Modifiers { get; } = new List<string>();
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/DefaultIntermediateNodeCollection.cs

@@ -1,128 +0,0 @@
-// 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;
-using System.Collections.Generic;
-
-namespace Microsoft.AspNetCore.Razor.Language.Intermediate
-{
-    public sealed class DefaultIntermediateNodeCollection : IntermediateNodeCollection
-    {
-        private readonly List<IntermediateNode> _inner = new List<IntermediateNode>();
-
-        public override IntermediateNode this[int index]
-        {
-            get
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                return _inner[index];
-            }
-            set
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                _inner[index] = value;
-            }
-        }
-
-        public override int Count => _inner.Count;
-
-        public override bool IsReadOnly => false;
-
-        public override void Add(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            _inner.Add(item);
-        }
-
-        public override void Clear()
-        {
-            _inner.Clear();
-        }
-
-        public override bool Contains(IntermediateNode item)
-        {
-            return _inner.Contains(item);
-        }
-
-        public override void CopyTo(IntermediateNode[] 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));
-            }
-
-            _inner.CopyTo(array, arrayIndex);
-        }
-
-        public override IEnumerator<IntermediateNode> GetEnumerator()
-        {
-            return _inner.GetEnumerator();
-        }
-
-        public override int IndexOf(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return _inner.IndexOf(item);
-        }
-
-        public override void Insert(int index, IntermediateNode item)
-        {
-            if (index < 0 || index > Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            _inner.Insert(index, item);
-        }
-
-        public override bool Remove(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return _inner.Remove(item);
-        }
-
-        public override void RemoveAt(int index)
-        {
-            if (index < 0 || index >= Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            _inner.RemoveAt(index);
-        }
-    }
-}

src/Microsoft.AspNetCore.Razor.Language/Intermediate/DirectiveIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class DirectiveIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Name { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/DirectiveTokenIntermediateNode.cs

@@ -5,7 +5,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class DirectiveTokenIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string Content { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/DocumentIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class DocumentIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string DocumentKind { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/FieldDeclarationIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class FieldDeclarationIntermediateNode : MemberDeclarationIntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public IList<string> Modifiers { get; } = new List<string>();
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/HtmlAttributeIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class HtmlAttributeIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string AttributeName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/HtmlAttributeValueIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class HtmlAttributeValueIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Prefix { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/HtmlContentIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class HtmlContentIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Intermediate/IntermediateNode.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
             {
                 if (_annotations == null)
                 {
-                    _annotations = new DefaultItemCollection();
+                    _annotations = new ItemCollection();
                 }
 
                 return _annotations;
@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
             {
                 if (_diagnostics == null)
                 {
-                    _diagnostics = new DefaultRazorDiagnosticCollection();
+                    _diagnostics = new RazorDiagnosticCollection();
                 }
 
                 return _diagnostics;

src/Microsoft.AspNetCore.Razor.Language/Intermediate/IntermediateNodeCollection.cs

@@ -1,40 +1,198 @@
 // 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;
 using System.Collections;
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
-    public abstract class IntermediateNodeCollection : IList<IntermediateNode>
+    public sealed class IntermediateNodeCollection : IList<IntermediateNode>
     {
-        public abstract IntermediateNode this[int index] { get; set; }
+        public static readonly IntermediateNodeCollection ReadOnly = new IntermediateNodeCollection(new List<IntermediateNode>().AsReadOnly());
 
-        public abstract int Count { get; }
+        private readonly IList<IntermediateNode> _inner;
 
-        public abstract bool IsReadOnly { get; }
+        public IntermediateNodeCollection()
+            : this(new List<IntermediateNode>())
+        {
+        }
 
-        public abstract void Add(IntermediateNode item);
+        private IntermediateNodeCollection(IList<IntermediateNode> inner)
+        {
+            _inner = inner;
+        }
 
-        public abstract void Clear();
+        public IntermediateNode this[int index]
+        {
+            get
+            {
+                if (index < 0 || index >= Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(index));
+                }
 
-        public abstract bool Contains(IntermediateNode item);
+                return _inner[index];
+            }
+            set
+            {
+                if (index < 0 || index >= Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(index));
+                }
 
-        public abstract void CopyTo(IntermediateNode[] array, int arrayIndex);
+                _inner[index] = value;
+            }
+        }
 
-        public abstract IEnumerator<IntermediateNode> GetEnumerator();
+        public int Count => _inner.Count;
 
-        public abstract int IndexOf(IntermediateNode item);
+        public bool IsReadOnly => _inner.IsReadOnly;
 
-        public abstract void Insert(int index, IntermediateNode item);
+        public void Add(IntermediateNode item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
 
-        public abstract bool Remove(IntermediateNode item);
+            _inner.Add(item);
+        }
 
-        public abstract void RemoveAt(int index);
+        public void Clear()
+        {
+            _inner.Clear();
+        }
+
+        public bool Contains(IntermediateNode item)
+        {
+            return _inner.Contains(item);
+        }
+
+        public void CopyTo(IntermediateNode[] 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));
+            }
+
+            _inner.CopyTo(array, arrayIndex);
+        }
+
+        public Enumerator GetEnumerator()
+        {
+            return new Enumerator(this);
+        }
+
+        IEnumerator<IntermediateNode> IEnumerable<IntermediateNode>.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
 
         IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator();
         }
+
+        public int IndexOf(IntermediateNode item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            return _inner.IndexOf(item);
+        }
+
+        public void Insert(int index, IntermediateNode item)
+        {
+            if (index < 0 || index > Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            _inner.Insert(index, item);
+        }
+
+        public bool Remove(IntermediateNode item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            return _inner.Remove(item);
+        }
+
+        public void RemoveAt(int index)
+        {
+            if (index < 0 || index >= Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            _inner.RemoveAt(index);
+        }
+
+        public struct Enumerator : IEnumerator<IntermediateNode>
+        {
+            private readonly IList<IntermediateNode> _items;
+            private int _index;
+
+            public Enumerator(IntermediateNodeCollection collection)
+            {
+                if (collection == null)
+                {
+                    throw new ArgumentNullException(nameof(collection));
+                }
+
+                _items = collection._inner;
+                _index = -1;
+            }
+
+            public IntermediateNode Current
+            {
+                get
+                {
+                    if (_index < 0 || _index >= _items.Count)
+                    {
+                        return null;
+                    }
+
+                    return _items[_index];
+                }
+            }
+
+            object IEnumerator.Current => Current;
+
+            public void Dispose()
+            {
+            }
+
+            public bool MoveNext()
+            {
+                _index++;
+                return _index < _items.Count;
+            }
+
+            public void Reset()
+            {
+                _index = -1;
+            }
+        }
     }
 }

src/Microsoft.AspNetCore.Razor.Language/Intermediate/IntermediateToken.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class IntermediateToken : IntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string Content { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/MalformedDirectiveIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class MalformedDirectiveIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Name { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/MethodDeclarationIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class MethodDeclarationIntermediateNode : MemberDeclarationIntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public IList<string> Modifiers { get; } = new List<string>();
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/NamespaceDeclarationIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class NamespaceDeclarationIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string Content { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/PropertyDeclarationIntermediateNode.cs

@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class PropertyDeclarationIntermediateNode : MemberDeclarationIntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public IList<string> Modifiers { get; } = new List<string>();
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/ReadOnlyIntermediateNodeCollection.cs

@@ -1,133 +0,0 @@
-// 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;
-using System.Collections.Generic;
-using System.Linq;
-
-namespace Microsoft.AspNetCore.Razor.Language.Intermediate
-{
-    public sealed class ReadOnlyIntermediateNodeCollection : IntermediateNodeCollection
-    {
-        public static readonly ReadOnlyIntermediateNodeCollection Instance = new ReadOnlyIntermediateNodeCollection();
-
-        private ReadOnlyIntermediateNodeCollection()
-        {
-        }
-
-        public override IntermediateNode this[int index]
-        {
-            get
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                throw null; // Unreachable
-            }
-            set
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                throw null; // Unreachable
-            }
-        }
-
-        public override int Count => 0;
-
-        public override bool IsReadOnly => true;
-
-        public override void Add(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override void Clear()
-        {
-            throw new NotSupportedException();
-        }
-
-        public override bool Contains(IntermediateNode item)
-        {
-            return false;
-        }
-
-        public override void CopyTo(IntermediateNode[] 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));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override IEnumerator<IntermediateNode> GetEnumerator()
-        {
-            return Enumerable.Empty<IntermediateNode>().GetEnumerator();
-        }
-
-        public override int IndexOf(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return -1;
-        }
-
-        public override void Insert(int index, IntermediateNode item)
-        {
-            if (index < 0 || index > Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override bool Remove(IntermediateNode item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return false;
-        }
-
-        public override void RemoveAt(int index)
-        {
-            if (index < 0 || index >= Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            throw new NotSupportedException();
-        }
-    }
-}

src/Microsoft.AspNetCore.Razor.Language/Intermediate/TagHelperBodyIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class TagHelperBodyIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public override void Accept(IntermediateNodeVisitor visitor)
         {

src/Microsoft.AspNetCore.Razor.Language/Intermediate/TagHelperHtmlAttributeIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class TagHelperHtmlAttributeIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string AttributeName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/TagHelperIntermediateNode.cs

@@ -9,7 +9,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class TagHelperIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public TagMode TagMode { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/TagHelperPropertyIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class TagHelperPropertyIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+        public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
         public string AttributeName { get; set; }
 

src/Microsoft.AspNetCore.Razor.Language/Intermediate/UsingDirectiveIntermediateNode.cs

@@ -7,7 +7,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 {
     public sealed class UsingDirectiveIntermediateNode : IntermediateNode
     {
-        public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+        public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
         public string Content { get; set; }
 

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

@@ -1,36 +1,124 @@
 // 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;
 using System.Collections;
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    public abstract class ItemCollection : ICollection<KeyValuePair<object, object>>
+    public sealed class ItemCollection : ICollection<KeyValuePair<object, object>>
     {
-        public abstract object this[object key] { get; set; }
+        private readonly Dictionary<object, object> _inner;
 
-        public abstract int Count { get; }
+        public ItemCollection()
+        {
+            _inner = new Dictionary<object, object>();
+        }
 
-        public abstract bool IsReadOnly { get; }
+        public object this[object key]
+        {
+            get
+            {
+                if (key == null)
+                {
+                    throw new ArgumentNullException(nameof(key));
+                }
+                
+                _inner.TryGetValue(key, out var value);
+                return value;
+            }
+            set
+            {
+                if (key == null)
+                {
+                    throw new ArgumentNullException(nameof(key));
+                }
 
-        public abstract void Add(object key, object value);
+                _inner[key] = value;
+            }
+        }
 
-        public abstract void Add(KeyValuePair<object, object> item);
+        public int Count => _inner.Count;
 
-        public abstract void Clear();
+        public bool IsReadOnly => _inner != null;
 
-        public abstract bool Contains(KeyValuePair<object, object> item);
+        int ICollection<KeyValuePair<object, object>>.Count => throw new NotImplementedException();
 
-        public abstract void CopyTo(KeyValuePair<object, object>[] array, int arrayIndex);
+        bool ICollection<KeyValuePair<object, object>>.IsReadOnly => throw new NotImplementedException();
 
-        public abstract IEnumerator<KeyValuePair<object, object>> GetEnumerator();
+        public void Add(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
 
-        public abstract bool Remove(KeyValuePair<object, object> item);
+            ((ICollection<KeyValuePair<object, object>>)_inner).Add(item);
+        }
+
+        public void Add(object key, object value)
+        {
+            if (key == null)
+            {
+                throw new ArgumentNullException(nameof(key));
+            }
+
+            _inner.Add(key, value);
+        }
+
+        public void Clear()
+        {
+            _inner.Clear();
+        }
+
+        public bool Contains(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            return ((ICollection<KeyValuePair<object, object>>)_inner).Contains(item);
+        }
+
+        public 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>>)_inner).CopyTo(array, arrayIndex);
+        }
+
+        public IEnumerator<KeyValuePair<object, object>> GetEnumerator()
+        {
+            return _inner.GetEnumerator();
+        }
 
         IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator();
         }
+
+        public bool Remove(KeyValuePair<object, object> item)
+        {
+            if (item.Key == null)
+            {
+                throw new ArgumentException(Resources.KeyMustNotBeNull, nameof(item));
+            }
+
+            return ((ICollection<KeyValuePair<object, object>>)_inner).Remove(item);
+        }
     }
 }

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

@@ -1,40 +1,191 @@
 // 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;
 using System.Collections;
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    public abstract class RazorDiagnosticCollection : IList<RazorDiagnostic>
+    public sealed class RazorDiagnosticCollection : IList<RazorDiagnostic>
     {
-        public abstract RazorDiagnostic this[int index] { get; set; }
+        private readonly List<RazorDiagnostic> _inner;
 
-        public abstract int Count { get; }
+        public RazorDiagnosticCollection()
+        {
+            _inner = new List<RazorDiagnostic>();
+        }
 
-        public abstract bool IsReadOnly { get; }
+        public RazorDiagnostic this[int index]
+        {
+            get
+            {
+                if (index < 0 || index >= Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(index));
+                }
 
-        public abstract void Add(RazorDiagnostic item);
+                return _inner[index];
+            }
+            set
+            {
+                if (index < 0 || index >= Count)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(index));
+                }
 
-        public abstract void Clear();
+                _inner[index] = value;
+            }
+        }
 
-        public abstract bool Contains(RazorDiagnostic item);
+        public int Count => _inner.Count;
 
-        public abstract void CopyTo(RazorDiagnostic[] array, int arrayIndex);
+        public bool IsReadOnly => _inner != null;
 
-        public abstract IEnumerator<RazorDiagnostic> GetEnumerator();
+        public void Add(RazorDiagnostic item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
 
-        public abstract int IndexOf(RazorDiagnostic item);
+            _inner.Add(item);
+        }
 
-        public abstract void Insert(int index, RazorDiagnostic item);
+        public void Clear()
+        {
+            _inner.Clear();
+        }
 
-        public abstract bool Remove(RazorDiagnostic item);
+        public bool Contains(RazorDiagnostic item)
+        {
+            return _inner.Contains(item);
+        }
 
-        public abstract void RemoveAt(int index);
+        public void CopyTo(RazorDiagnostic[] 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));
+            }
+
+            _inner.CopyTo(array, arrayIndex);
+        }
+
+        public Enumerator GetEnumerator()
+        {
+            return new Enumerator(this);
+        }
+
+        IEnumerator<RazorDiagnostic> IEnumerable<RazorDiagnostic>.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
 
         IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator();
         }
+
+        public int IndexOf(RazorDiagnostic item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            return _inner.IndexOf(item);
+        }
+
+        public void Insert(int index, RazorDiagnostic item)
+        {
+            if (index < 0 || index > Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            _inner.Insert(index, item);
+        }
+
+        public bool Remove(RazorDiagnostic item)
+        {
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+
+            return _inner.Remove(item);
+        }
+
+        public void RemoveAt(int index)
+        {
+            if (index < 0 || index >= Count)
+            {
+                throw new ArgumentOutOfRangeException(nameof(index));
+            }
+
+            _inner.RemoveAt(index);
+        }
+
+        public struct Enumerator : IEnumerator<RazorDiagnostic>
+        {
+            private readonly IList<RazorDiagnostic> _items;
+            private int _index;
+
+            public Enumerator(RazorDiagnosticCollection collection)
+            {
+                if (collection == null)
+                {
+                    throw new ArgumentNullException(nameof(collection));
+                }
+
+                _items = collection._inner;
+                _index = -1;
+            }
+
+            public RazorDiagnostic Current
+            {
+                get
+                {
+                    if (_index < 0 || _index >= _items.Count)
+                    {
+                        return null;
+                    }
+
+                    return _items[_index];
+                }
+            }
+
+            object IEnumerator.Current => Current;
+
+            public void Dispose()
+            {
+            }
+
+            public bool MoveNext()
+            {
+                _index++;
+                return _index < _items.Count;
+            }
+
+            public void Reset()
+            {
+                _index = -1;
+            }
+        }
     }
 }

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

@@ -1,133 +0,0 @@
-// 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;
-using System.Collections.Generic;
-using System.Linq;
-
-namespace Microsoft.AspNetCore.Razor.Language
-{
-    public sealed class ReadOnlyDiagnosticCollection : RazorDiagnosticCollection
-    {
-        public static readonly ReadOnlyDiagnosticCollection Instance = new ReadOnlyDiagnosticCollection();
-
-        private ReadOnlyDiagnosticCollection()
-        {
-        }
-
-        public override RazorDiagnostic this[int index]
-        {
-            get
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                throw null; // Unreachable
-            }
-            set
-            {
-                if (index < 0 || index >= Count)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(index));
-                }
-
-                throw null; // Unreachable
-            }
-        }
-
-        public override int Count => 0;
-
-        public override bool IsReadOnly => true;
-
-        public override void Add(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override void Clear()
-        {
-            throw new NotSupportedException();
-        }
-
-        public override bool Contains(RazorDiagnostic item)
-        {
-            return false;
-        }
-
-        public override void CopyTo(RazorDiagnostic[] 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));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override IEnumerator<RazorDiagnostic> GetEnumerator()
-        {
-            return Enumerable.Empty<RazorDiagnostic>().GetEnumerator();
-        }
-
-        public override int IndexOf(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return -1;
-        }
-
-        public override void Insert(int index, RazorDiagnostic item)
-        {
-            if (index < 0 || index > Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        public override bool Remove(RazorDiagnostic item)
-        {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
-
-            return false;
-        }
-
-        public override void RemoveAt(int index)
-        {
-            if (index < 0 || index >= Count)
-            {
-                throw new ArgumentOutOfRangeException(nameof(index));
-            }
-
-            throw new NotSupportedException();
-        }
-    }
-}

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

@@ -1,93 +0,0 @@
-// 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;
-using System.Collections.Generic;
-using System.Linq;
-
-namespace Microsoft.AspNetCore.Razor.Language
-{
-    internal class ReadOnlyItemCollection : ItemCollection
-    {
-        public static readonly ItemCollection Empty = new ReadOnlyItemCollection();
-
-        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(object key, object value)
-        {
-            if (key == null)
-            {
-                throw new ArgumentNullException(nameof(key));
-            }
-
-            throw new NotSupportedException();
-        }
-
-        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/TagHelperDescriptorProviderContext.cs

@@ -33,7 +33,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             {
                 Results = results;
 
-                Items = new DefaultItemCollection();
+                Items = new ItemCollection();
             }
 
             public override ItemCollection Items { get; }

test/Microsoft.AspNetCore.Razor.Language.Test/CodeGeneration/DesignTimeNodeWriterTest.cs

@@ -472,7 +472,7 @@ Render Children
 
         private class MyExtensionIntermediateNode : ExtensionIntermediateNode
         {
-            public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+            public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
             public override void Accept(IntermediateNodeVisitor visitor)
             {

test/Microsoft.AspNetCore.Razor.Language.Test/CodeGeneration/RuntimeNodeWriterTest.cs

@@ -583,7 +583,7 @@ WriteAttributeValue("" "", 27, false, 28, 6, false);
 
         private class MyExtensionIntermediateNode : ExtensionIntermediateNode
         {
-            public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+            public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
             public override void Accept(IntermediateNodeVisitor visitor)
             {

test/Microsoft.AspNetCore.Razor.Language.Test/DefaultItemCollectionTest.cs

@@ -5,13 +5,13 @@ using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language
 {
-    public class DefaultItemCollectionTest
+    public class ItemCollectionTest
     {
         [Fact]
         public void Get_MissingValueReturnsNull()
         {
             // Arrange
-            var items = new DefaultItemCollection();
+            var items = new ItemCollection();
 
             // Act
             var value = items["foo"];
@@ -24,7 +24,7 @@ namespace Microsoft.AspNetCore.Razor.Language
         public void GetAndSet_ReturnsValue()
         {
             // Arrange
-            var items = new DefaultItemCollection();
+            var items = new ItemCollection();
 
             var expected = "bar";
             items["foo"] = expected;
@@ -40,7 +40,7 @@ namespace Microsoft.AspNetCore.Razor.Language
         public void Set_CanSetValueToNull()
         {
             // Arrange
-            var items = new DefaultItemCollection();
+            var items = new ItemCollection();
             
             items["foo"] = "bar";
 

test/Microsoft.AspNetCore.Razor.Language.Test/Intermediate/DefaultRazorIntermediateNodeBuilderTest.cs

@@ -206,7 +206,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 
         private class BasicIntermediateNode : IntermediateNode
         {
-            public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+            public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
             public override void Accept(IntermediateNodeVisitor visitor)
             {

test/Microsoft.AspNetCore.Razor.Language.Test/Intermediate/ExtensionIntermediateNodeTest.cs

@@ -43,7 +43,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 
         private class TestExtensionIntermediateNode : ExtensionIntermediateNode
         {
-            public override IntermediateNodeCollection Children => ReadOnlyIntermediateNodeCollection.Instance;
+            public override IntermediateNodeCollection Children => IntermediateNodeCollection.ReadOnly;
 
             public override void Accept(IntermediateNodeVisitor visitor)
             {

test/Microsoft.AspNetCore.Razor.Language.Test/Intermediate/IntermediateNodeReferenceTest.cs

@@ -305,7 +305,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void InsertAfter_SingleNode_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -320,7 +320,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void InsertAfter_MulipleNodes_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -335,7 +335,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void InsertBefore_SingleNode_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -350,7 +350,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void InsertBefore_MulipleNodes_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -365,7 +365,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void Remove_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -380,7 +380,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         public void Replace_ThrowsForReadOnlyCollection()
         {
             // Arrange
-            var parent = new BasicIntermediateNode("Parent", ReadOnlyIntermediateNodeCollection.Instance);
+            var parent = new BasicIntermediateNode("Parent", IntermediateNodeCollection.ReadOnly);
 
             var node1 = new BasicIntermediateNode("Node1");
 
@@ -484,7 +484,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
         private class BasicIntermediateNode : IntermediateNode
         {
             public BasicIntermediateNode(string name)
-                : this(name, new DefaultIntermediateNodeCollection())
+                : this(name, new IntermediateNodeCollection())
             {
                 Name = name;
             }

test/Microsoft.AspNetCore.Razor.Language.Test/Intermediate/IntermediateNodeWalkerTest.cs

@@ -124,7 +124,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Intermediate
 
             public string Name { get; }
 
-            public override IntermediateNodeCollection Children { get; } = new DefaultIntermediateNodeCollection();
+            public override IntermediateNodeCollection Children { get; } = new IntermediateNodeCollection();
 
             public override void Accept(IntermediateNodeVisitor visitor)
             {