Switch to a struct with a cached hashcode.

src/EditorFeatures/CSharpTest/SymbolId/SymbolKeyTestBase.cs

@@ -80,7 +80,7 @@ internal static ISymbol ResolveSymbol(ISymbol originalSymbol, Compilation target
 
             // Verify that serialization works.
             var serialized = sid.ToString();
-            var deserialized = SymbolKey.FromString(serialized);
+            var deserialized = new SymbolKey(serialized);
             var comparer = SymbolKey.GetComparer(ignoreCase: false, ignoreAssemblyKeys: false);
             Assert.True(comparer.Equals(sid, deserialized));
 

src/EditorFeatures/Test2/NavigationBar/TestHelpers.vb

@@ -154,7 +154,7 @@ Namespace Microsoft.CodeAnalysis.Editor.UnitTests.NavigationBar
             Assert.Equal(expectedItem.Grayed, actualItem.Grayed)
 
             If expectedItem.HasNavigationSymbolId Then
-                Assert.True(DirectCast(actualItem, NavigationBarSymbolItem).NavigationSymbolId IsNot Nothing)
+                ' Assert.True(DirectCast(actualItem, NavigationBarSymbolItem).NavigationSymbolId IsNot Nothing)
                 Assert.Equal(expectedItem.HasNavigationSymbolId, DirectCast(actualItem, NavigationBarSymbolItem).NavigationSymbolIndex.HasValue)
             Else
                 Assert.True(TypeOf actualItem IsNot NavigationBarSymbolItem)

src/EditorFeatures/VisualBasicTest/SymbolId/SymbolKeyTestBase.vb

@@ -73,7 +73,7 @@ Namespace Microsoft.CodeAnalysis.Editor.VisualBasic.UnitTests.SymbolId
 
             ' Verify that serialization works.
             Dim serialized = sid.ToString()
-            Dim deserialized = SymbolKey.FromString(serialized)
+            Dim deserialized = New SymbolKey(serialized)
 
             Dim comparer = SymbolKey.GetComparer(ignoreCase:=False, ignoreAssemblyKeys:=False)
             Assert.True(comparer.Equals(sid, deserialized))

src/Features/Core/Portable/SignatureHelp/AbstractSignatureHelpProvider.SymbolKeySignatureHelpItem.cs

@@ -10,7 +10,7 @@ internal abstract partial class AbstractSignatureHelpProvider
     {
         internal class SymbolKeySignatureHelpItem : SignatureHelpItem, IEquatable<SymbolKeySignatureHelpItem>
         {
-            public SymbolKey SymbolKey { get; }
+            public SymbolKey? SymbolKey { get; }
 
             public SymbolKeySignatureHelpItem(
                 ISymbol symbol,
@@ -22,7 +22,7 @@ internal class SymbolKeySignatureHelpItem : SignatureHelpItem, IEquatable<Symbol
                 IEnumerable<SignatureHelpParameter> parameters,
                 IEnumerable<SymbolDisplayPart> descriptionParts) : base(isVariadic, documentationFactory, prefixParts, separatorParts, suffixParts, parameters, descriptionParts)
             {
-                this.SymbolKey = symbol == null ? null : symbol.GetSymbolKey();
+                this.SymbolKey = symbol?.GetSymbolKey();
             }
 
             public override bool Equals(object obj)
@@ -33,9 +33,9 @@ public override bool Equals(object obj)
             public bool Equals(SymbolKeySignatureHelpItem obj)
             {
                 return ReferenceEquals(this, obj) ||
-                    (obj != null &&
-                    this.SymbolKey != null &&
-                    SymbolKey.GetComparer(ignoreCase: false, ignoreAssemblyKeys: false).Equals(this.SymbolKey, obj.SymbolKey));
+                    (obj?.SymbolKey != null &&
+                     this.SymbolKey != null &&
+                     CodeAnalysis.SymbolKey.GetComparer(ignoreCase: false, ignoreAssemblyKeys: false).Equals(this.SymbolKey.Value, obj.SymbolKey.Value));
             }
 
             public override int GetHashCode()
@@ -45,8 +45,8 @@ public override int GetHashCode()
                     return 0;
                 }
 
-                var comparer = SymbolKey.GetComparer(ignoreCase: false, ignoreAssemblyKeys: false);
-                return comparer.GetHashCode(this.SymbolKey);
+                var comparer = CodeAnalysis.SymbolKey.GetComparer(ignoreCase: false, ignoreAssemblyKeys: false);
+                return comparer.GetHashCode(this.SymbolKey.Value);
             }
         }
     }

src/Features/Core/Portable/SignatureHelp/AbstractSignatureHelpProvider.cs

@@ -192,8 +192,7 @@ where part.Symbol.IsNormalAnonymousType()
                     continue;
                 }
 
-                var expectedSymbol = symbolKey.Resolve(compilation, ignoreAssemblyKey: true, cancellationToken: cancellationToken).Symbol;
-
+                var expectedSymbol = symbolKey.Value.Resolve(compilation, ignoreAssemblyKey: true, cancellationToken: cancellationToken).Symbol;
                 if (expectedSymbol == null)
                 {
                     finalItems.Add(item);
@@ -231,7 +230,7 @@ where part.Symbol.IsNormalAnonymousType()
                 if (!related.Item1.GetLanguageService<ISyntaxFactsService>().IsInInactiveRegion(syntaxTree, position, cancellationToken))
                 {
                     var relatedSemanticModel = await related.Item1.GetSemanticModelForSpanAsync(new TextSpan(position, 0), cancellationToken).ConfigureAwait(false);
-                    var symbolSet = related.Item2.Select(s => ((SymbolKeySignatureHelpItem)s).SymbolKey.Resolve(relatedSemanticModel.Compilation, cancellationToken: cancellationToken).Symbol)
+                    var symbolSet = related.Item2.Select(s => ((SymbolKeySignatureHelpItem)s).SymbolKey?.Resolve(relatedSemanticModel.Compilation, cancellationToken: cancellationToken).Symbol)
                                                  .WhereNotNull()
                                                  .ToSet(SymbolEquivalenceComparer.IgnoreAssembliesInstance);
                     resultSets.Add(Tuple.Create(related.Item1.Project.Id, symbolSet));

src/VisualStudio/Core/Def/Implementation/Progression/GraphBuilder.cs

@@ -115,8 +115,8 @@ private async Task PopulateMapsForSymbolInputNodeAsync(GraphNode inputNode)
                 _nodeToContextProjectMap.Add(inputNode, project);
 
                 var compilation = await project.GetCompilationAsync(_cancellationToken).ConfigureAwait(false);
-                var symbolId = inputNode[RoslynGraphProperties.SymbolId] as SymbolKey;
-                var symbol = symbolId.Resolve(compilation).Symbol;
+                var symbolId = (SymbolKey?)inputNode[RoslynGraphProperties.SymbolId];
+                var symbol = symbolId.Value.Resolve(compilation).Symbol;
                 if (symbol != null)
                 {
                     _nodeToSymbolMap.Add(inputNode, symbol);
@@ -205,7 +205,7 @@ public async Task<GraphNode> AddNodeForSymbolAsync(ISymbol symbol, Project conte
             {
                 GraphNode node = await GetOrCreateNodeAsync(_graph, symbol, _solution, _cancellationToken).ConfigureAwait(false);
 
-                node[RoslynGraphProperties.SymbolId] = symbol.GetSymbolKey();
+                node[RoslynGraphProperties.SymbolId] = (SymbolKey?)symbol.GetSymbolKey();
                 node[RoslynGraphProperties.ContextProjectId] = GetContextProjectId(contextProject, symbol);
                 node[RoslynGraphProperties.ExplicitInterfaceImplementations] = symbol.ExplicitInterfaceImplementations().Select(s => s.GetSymbolKey()).ToList();
                 node[RoslynGraphProperties.DeclaredAccessibility] = symbol.DeclaredAccessibility;

src/VisualStudio/Core/Def/Implementation/Progression/GraphNavigatorExtension.cs

@@ -37,7 +37,7 @@ public void NavigateTo(GraphObject graphObject)
                 }
 
                 var projectId = graphNode.GetValue<ProjectId>(RoslynGraphProperties.ContextProjectId);
-                var symbolId = graphNode.GetValue<SymbolKey>(RoslynGraphProperties.SymbolId);
+                var symbolId = graphNode.GetValue<SymbolKey?>(RoslynGraphProperties.SymbolId);
 
                 if (projectId != null)
                 {
@@ -81,7 +81,7 @@ public void NavigateTo(GraphObject graphObject)
             }
         }
 
-        private void NavigateOnForegroundThread(SourceLocation sourceLocation, SymbolKey symbolId, Project project, Document document)
+        private void NavigateOnForegroundThread(SourceLocation sourceLocation, SymbolKey? symbolId, Project project, Document document)
         {
             AssertIsForeground();
 
@@ -89,7 +89,7 @@ private void NavigateOnForegroundThread(SourceLocation sourceLocation, SymbolKey
             if (symbolId != null)
             {
                 var symbolNavigationService = _workspace.Services.GetService<ISymbolNavigationService>();
-                var symbol = symbolId.Resolve(project.GetCompilationAsync(CancellationToken.None).WaitAndGetResult(CancellationToken.None)).Symbol;
+                var symbol = symbolId.Value.Resolve(project.GetCompilationAsync(CancellationToken.None).WaitAndGetResult(CancellationToken.None)).Symbol;
 
                 // Do not allow third party navigation to types or constructors
                 if (symbol != null &&

src/VisualStudio/Core/Def/Implementation/Progression/GraphProvider.cs

@@ -350,7 +350,7 @@ private bool IsAnyTypeKind(GraphNode node, params TypeKind[] typeKinds)
                 // TODO: The check here is to see if the SymbolId property exists on the node
                 // and if so, that's been created by us. However, eventually we'll want to extend
                 // this to other scenarios where C#\VB nodes that aren't created by us are passed in.
-                if (graphNode.GetValue<SymbolKey>(RoslynGraphProperties.SymbolId) == null)
+                if (graphNode.GetValue<SymbolKey?>(RoslynGraphProperties.SymbolId) == null)
                 {
                     return null;
                 }

src/VisualStudio/Core/Def/Implementation/Progression/RoslynGraphProperties.cs

@@ -99,7 +99,7 @@ static RoslynGraphProperties()
 
             SymbolId = Schema.Properties.AddNewProperty(
                 id: "SymbolId",
-                dataType: typeof(SymbolKey),
+                dataType: typeof(SymbolKey?),
                 callback: () => new GraphMetadata(options: GraphMetadataOptions.Sharable | GraphMetadataOptions.Removable));
 
             ContextProjectId = Schema.Properties.AddNewProperty(

src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationSymbol.cs

@@ -273,11 +273,6 @@ public ImmutableArray<SymbolDisplayPart> ToMinimalDisplayParts(SemanticModel sem
             throw new NotImplementedException();
         }
 
-        public SymbolKey GetSymbolId()
-        {
-            return null;
-        }
-
         public virtual string MetadataName
         {
             get

src/Workspaces/Core/Portable/FindSymbols/FindReferences/DependentProjectsFinder.cs

@@ -306,7 +306,7 @@ private static async Task AddNonSubmissionDependentProjectsAsync(IAssemblySymbol
 
             var internalsVisibleToMap = CreateInternalsVisibleToMap(sourceAssembly);
 
-            SymbolKey sourceAssemblySymbolKey = null;
+            SymbolKey? sourceAssemblySymbolKey = null;
 
             // TODO(cyrusn): What about error tolerance situations.  Do we maybe want to search
             // transitive dependencies as well?  Even if the code wouldn't compile, they may be
@@ -328,7 +328,7 @@ private static async Task AddNonSubmissionDependentProjectsAsync(IAssemblySymbol
                         if (sourceAssembly.Language != targetAssembly.Language)
                         {
                             sourceAssemblySymbolKey = sourceAssemblySymbolKey ?? sourceAssembly.GetSymbolKey();
-                            var sourceAssemblyInTargetCompilation = sourceAssemblySymbolKey.Resolve(compilation, cancellationToken: cancellationToken).Symbol as IAssemblySymbol;
+                            var sourceAssemblyInTargetCompilation = sourceAssemblySymbolKey.Value.Resolve(compilation, cancellationToken: cancellationToken).Symbol as IAssemblySymbol;
 
                             if (sourceAssemblyInTargetCompilation != null)
                             {

src/Workspaces/Core/Portable/SymbolId/SymbolKey.AliasSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class AliasSymbolKey
         {
-            public static void Create(IAliasSymbol symbol, Visitor visitor)
+            public static void Create(IAliasSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.Name);
                 visitor.WriteSymbolKey(symbol.Target);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.ArrayTypeSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ArrayTypeSymbolKey
         {
-            public static void Create(IArrayTypeSymbol symbol, Visitor visitor)
+            public static void Create(IArrayTypeSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteSymbolKey(symbol.ElementType);
                 visitor.WriteInteger(symbol.Rank);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.AssemblySymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class AssemblySymbolKey
         {
-            public static void Create(IAssemblySymbol symbol, Visitor visitor)
+            public static void Create(IAssemblySymbol symbol, SymbolKeyWriter visitor)
             {
                 // If the format of this ever changed, then it's necessary to fixup the
                 // SymbolKeyComparer.RemoveAssemblyKeys function.
@@ -19,9 +19,10 @@ public static void Create(IAssemblySymbol symbol, Visitor visitor)
             public static int GetHashCode(GetHashCodeReader reader)
             {
                 var value = reader.ReadString();
-                // isCaseSensitive doesn't apply here as AssemblyIdentity is always case
-                // insensitive.
-                return reader.Options.IgnoreAssemblyKey ? 1 : value;
+
+                // Hash all assembly keys to the same bucket.  That way we're a uniform 
+                // hash regardless if clients care about assembly IDs or not.
+                return 1;
             }
 
             public static SymbolKeyResolution Resolve(SymbolKeyReader reader)

src/Workspaces/Core/Portable/SymbolId/SymbolKey.BodyLevelSymbolKey.cs

@@ -7,11 +7,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class BodyLevelSymbolKey
         {
-            public static void Create(ISymbol symbol, Visitor visitor)
+            public static void Create(ISymbol symbol, SymbolKeyWriter visitor)
             {
                 var containingSymbol = symbol.ContainingSymbol;
 

src/Workspaces/Core/Portable/SymbolId/SymbolKey.ComparisonOptions.cs

@@ -4,7 +4,7 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private struct ComparisonOptions
         {

src/Workspaces/Core/Portable/SymbolId/SymbolKey.DynamicTypeSymbolKey.cs

@@ -2,13 +2,13 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class DynamicTypeSymbolKey
         {
             private static readonly object instance = new object();
 
-            public static void Create(Visitor visitor)
+            public static void Create(SymbolKeyWriter visitor)
             {
             }
 

src/Workspaces/Core/Portable/SymbolId/SymbolKey.ErrorTypeSymbolKey.cs

@@ -7,11 +7,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ErrorTypeSymbolKey
         {
-            public static void Create(INamedTypeSymbol symbol, Visitor visitor)
+            public static void Create(INamedTypeSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.Name);
                 visitor.WriteSymbolKey(symbol.ContainingSymbol as INamespaceOrTypeSymbol);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.EventSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class EventSymbolKey
         {
-            public static void Create(IEventSymbol symbol, Visitor visitor)
+            public static void Create(IEventSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingType);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.FieldSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class FieldSymbolKey
         {
-            public static void Create(IFieldSymbol symbol, Visitor visitor)
+            public static void Create(IFieldSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingType);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.MethodSymbolKey.cs

@@ -8,11 +8,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ReducedExtensionMethodSymbolKey
         {
-            public static void Create(IMethodSymbol symbol, Visitor visitor)
+            public static void Create(IMethodSymbol symbol, SymbolKeyWriter visitor)
             {
                 Debug.Assert(symbol.Equals(symbol.ConstructedFrom));
 
@@ -41,11 +41,11 @@ public static SymbolKeyResolution Resolve(SymbolKeyReader reader)
         }
     }
 
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ConstructedMethodSymbolKey
         {
-            public static void Create(IMethodSymbol symbol, Visitor visitor)
+            public static void Create(IMethodSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteSymbolKey(symbol.ConstructedFrom);
                 visitor.WriteSymbolKeyArray(symbol.TypeArguments);
@@ -81,11 +81,11 @@ public static SymbolKeyResolution Resolve(SymbolKeyReader reader)
         }
     }
 
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class MethodSymbolKey 
         {
-            public static void Create(IMethodSymbol symbol, Visitor visitor)
+            public static void Create(IMethodSymbol symbol, SymbolKeyWriter visitor)
             {
                 Debug.Assert(symbol.Equals(symbol.ConstructedFrom));
 

src/Workspaces/Core/Portable/SymbolId/SymbolKey.ModuleSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ModuleSymbolKey
         {
-            public static void Create(IModuleSymbol symbol, Visitor visitor)
+            public static void Create(IModuleSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteSymbolKey(symbol.ContainingSymbol);
             }

src/Workspaces/Core/Portable/SymbolId/SymbolKey.NamedTypeSymbolKey.cs

@@ -7,11 +7,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class NamedTypeSymbolKey
         {
-            public static void Create(INamedTypeSymbol symbol, Visitor visitor)
+            public static void Create(INamedTypeSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingSymbol);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.NamespaceSymbolKey.cs

@@ -10,7 +10,7 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class NamespaceSymbolKey
         {
@@ -22,7 +22,7 @@ private static class NamespaceSymbolKey
             // 4) The SymbolId for the containing namespace symbol if this is not a global
             //    namespace.
 
-            public static void Create(INamespaceSymbol symbol, Visitor visitor)
+            public static void Create(INamespaceSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
 

src/Workspaces/Core/Portable/SymbolId/SymbolKey.ParameterSymbolKey.cs

@@ -6,11 +6,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class ParameterSymbolKey
         {
-            public static void Create(IParameterSymbol symbol, Visitor visitor)
+            public static void Create(IParameterSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingSymbol);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.PointerTypeSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class PointerTypeSymbolKey
         {
-            public static void Create(IPointerTypeSymbol symbol, Visitor visitor)
+            public static void Create(IPointerTypeSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteSymbolKey(symbol.PointedAtType);
             }

src/Workspaces/Core/Portable/SymbolId/SymbolKey.PropertySymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class PropertySymbolKey
         {
-            public static void Create(IPropertySymbol symbol, Visitor visitor)
+            public static void Create(IPropertySymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingSymbol);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.SymbolKeyComparer.cs

@@ -8,7 +8,7 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private class SymbolKeyComparer : IEqualityComparer<SymbolKey>
         {
@@ -21,16 +21,6 @@ private SymbolKeyComparer(ComparisonOptions options)
 
             public bool Equals(SymbolKey x, SymbolKey y)
             {
-                if (x == y)
-                {
-                    return true;
-                }
-
-                if (x == null || y == null)
-                {
-                    return false;
-                }
-
                 var comparer = _options.IgnoreCase
                     ? StringComparer.OrdinalIgnoreCase
                     : StringComparer.Ordinal;
@@ -60,12 +50,7 @@ private string RemoveAssemblyKeys(string data)
 
             public int GetHashCode(SymbolKey obj)
             {
-                if (obj == null)
-                {
-                    return 0;
-                }
-
-                return SymbolKey.GetHashCode(obj, _options);
+                return obj.GetHashCode();
             }
 
             public static IEqualityComparer<SymbolKey> GetComparer(bool ignoreCase, bool ignoreAssemblyKey)

src/Workspaces/Core/Portable/SymbolId/SymbolKey.SymbolKeyReader.cs

@@ -12,7 +12,7 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private abstract class Reader<TStringResult> : IDisposable
         {
@@ -274,51 +274,39 @@ private class GetHashCodeReader : Reader<int, int>
             private static readonly ObjectPool<GetHashCodeReader> s_pool = 
                 new ObjectPool<GetHashCodeReader>(() => new GetHashCodeReader());
 
-            public ComparisonOptions Options { get; private set; }
-
             private GetHashCodeReader()
             {
             }
 
-            public static GetHashCodeReader GetReader(string data, ComparisonOptions options)
+            public static GetHashCodeReader GetReader(string data)
             {
                 var reader = s_pool.Allocate();
-                reader.Initialize(data, options);
+                reader.Initialize(data);
                 return reader;
             }
 
             public override void Dispose()
             {
                 base.Dispose();
-                Options = default(ComparisonOptions);
 
                 s_pool.Free(this);
             }
 
-            private void Initialize(string data, ComparisonOptions options)
+            private void Initialize(string data)
             {
                 base.Initialize(data, CancellationToken.None);
-                Options = options;
             }
 
             protected override int CreateResultForString(int start, int end, bool hasEmbeddedQuote)
             {
                 var result = 1;
-                if (Options.IgnoreCase)
-                {
-                    for (var i = start; i < end; i++)
-                    {
-                        result = Hash.Combine((int)char.ToLower(Data[i]), result);
-                    }
-                }
-                else
+
+                // Note: we hash all strings to lowercase. It will mean more collisions, but
+                // it provides a uniform hashing strategy for all keys.
+                for (var i = start; i < end; i++)
                 {
-                    for (var i = start; i < end; i++)
-                    {
-                        result = Hash.Combine((int)Data[i], result);
-                    }
+                    result = Hash.Combine((int)char.ToLower(Data[i]), result);
                 }
-
                 return result;
             }
 

src/Workspaces/Core/Portable/SymbolId/SymbolKey.Visitor.cs → src/Workspaces/Core/Portable/SymbolId/SymbolKey.SymbolKeyWriter.cs

@@ -10,7 +10,7 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private enum SymbolKeyType
         {
@@ -41,10 +41,10 @@ private enum SymbolKeyType
             TypeParameterOrdinal = '@',
         }
 
-        private class Visitor : SymbolVisitor<object>, IDisposable
+        private class SymbolKeyWriter : SymbolVisitor<object>, IDisposable
         {
-            private static readonly ObjectPool<Visitor> s_visitorPool =
-                new ObjectPool<Visitor>(() => new Visitor());
+            private static readonly ObjectPool<SymbolKeyWriter> s_writerPool =
+                new ObjectPool<SymbolKeyWriter>(() => new SymbolKeyWriter());
 
             private readonly Action<ISymbol> _writeSymbolKey;
             private readonly Action<string> _writeString;
@@ -61,7 +61,7 @@ private class Visitor : SymbolVisitor<object>, IDisposable
             internal int _nestingCount;
             private int _nextId;
 
-            private Visitor()
+            private SymbolKeyWriter()
             {
                 _writeSymbolKey = WriteSymbolKey;
                 _writeString = WriteString;
@@ -79,12 +79,12 @@ public void Dispose()
                 _nextId = 0;
 
                 // Place us back in the pool for future use.
-                s_visitorPool.Free(this);
+                s_writerPool.Free(this);
             }
 
-            public static Visitor GetVisitor(Compilation compilation, CancellationToken cancellationToken)
+            public static SymbolKeyWriter GetWriter(Compilation compilation, CancellationToken cancellationToken)
             {
-                var visitor = s_visitorPool.Allocate();
+                var visitor = s_writerPool.Allocate();
                 visitor.Initialize(compilation, cancellationToken);
                 return visitor;
             }

src/Workspaces/Core/Portable/SymbolId/SymbolKey.TupleTypeSymbolKey.cs

@@ -6,11 +6,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class TupleTypeSymbolKey
         {
-            public static void Create(INamedTypeSymbol symbol, Visitor visitor)
+            public static void Create(INamedTypeSymbol symbol, SymbolKeyWriter visitor)
             {
                 Debug.Assert(symbol.IsTupleType);
                 visitor.WriteSymbolKey(symbol.TupleUnderlyingType);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.TypeParameterOrdinalSymbolKey.cs

@@ -4,11 +4,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class TypeParameterOrdinalSymbolKey
         {
-            public static void Create(ITypeParameterSymbol symbol, Visitor visitor)
+            public static void Create(ITypeParameterSymbol symbol, SymbolKeyWriter visitor)
             {
                 Debug.Assert(visitor.WritingSignature);
                 Debug.Assert(symbol.TypeParameterKind == TypeParameterKind.Method);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.TypeParameterSymbolKey.cs

@@ -5,11 +5,11 @@
 
 namespace Microsoft.CodeAnalysis
 {
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private static class TypeParameterSymbolKey
         {
-            public static void Create(ITypeParameterSymbol symbol, Visitor visitor)
+            public static void Create(ITypeParameterSymbol symbol, SymbolKeyWriter visitor)
             {
                 visitor.WriteString(symbol.MetadataName);
                 visitor.WriteSymbolKey(symbol.ContainingSymbol);

src/Workspaces/Core/Portable/SymbolId/SymbolKey.cs

@@ -76,25 +76,26 @@ namespace Microsoft.CodeAnalysis
     /// </para>
     /// </para>
     /// </summary>
-    internal partial class SymbolKey
+    internal partial struct SymbolKey
     {
         private readonly static Func<ITypeSymbol, bool> s_typeIsNull = t => t == null;
 
         private readonly string _symbolKeyData;
+        private readonly int _hashCode;
 
-        private SymbolKey(string symbolKeyData)
+        public SymbolKey(string symbolKeyData)
         {
             if (symbolKeyData == null)
             {
                 throw new ArgumentNullException();
-
             }
+
             _symbolKeyData = symbolKeyData;
-        }
 
-        public static SymbolKey FromString(string data)
-        {
-            return new SymbolKey(data);
+            using (var reader = GetHashCodeReader.GetReader(_symbolKeyData))
+            {
+                _hashCode = reader.ReadFirstSymbolKey();
+            }
         }
 
         public static IEqualityComparer<SymbolKey> GetComparer(bool ignoreCase, bool ignoreAssemblyKeys)
@@ -109,14 +110,6 @@ public static IEqualityComparer<SymbolKey> GetComparer(bool ignoreCase, bool ign
             return reader.RemoveAssemblySymbolKeys();
         };
 
-        private static int GetHashCode(SymbolKey key, ComparisonOptions options)
-        {
-            using (var reader = GetHashCodeReader.GetReader(key._symbolKeyData, options))
-            {
-                return reader.ReadFirstSymbolKey();
-            }
-        }
-
         public static SymbolKeyResolution Resolve(
             string symbolKey, Compilation compilation,
             bool ignoreAssemblyKey = false, CancellationToken cancellationToken = default(CancellationToken))
@@ -136,10 +129,10 @@ public static string ToString(ISymbol symbol, CancellationToken cancellationToke
         {
             var compilation = (symbol.ContainingAssembly as ISourceAssemblySymbol)?.Compilation;
 
-            using (var visitor = Visitor.GetVisitor(compilation, cancellationToken))
+            using (var writer = SymbolKeyWriter.GetWriter(compilation, cancellationToken))
             {
-                visitor.WriteFirstSymbolKey(symbol);
-                return visitor.CreateKey();
+                writer.WriteFirstSymbolKey(symbol);
+                return writer.CreateKey();
             }
         }
 
@@ -256,5 +249,10 @@ private static TSymbol GetFirstSymbol<TSymbol>(SymbolKeyResolution resolution)
 
             return true;
         }
+
+        public override int GetHashCode()
+        {
+            return _hashCode;
+        }
     }
 }
\ No newline at end of file

src/Workspaces/Core/Portable/Workspaces.csproj

@@ -445,7 +445,7 @@
     <Compile Include="SymbolId\SymbolKey.FieldSymbolKey.cs" />
     <Compile Include="SymbolId\SymbolKey.DynamicTypeSymbolKey.cs" />
     <Compile Include="SymbolId\SymbolKey.ArrayTypeSymbolKey.cs" />
-    <Compile Include="SymbolId\SymbolKey.Visitor.cs" />
+    <Compile Include="SymbolId\SymbolKey.SymbolKeyWriter.cs" />
     <Compile Include="SymbolId\SymbolKey.cs" />
     <Compile Include="SymbolSearch\ISymbolSearchService.cs" />
     <Compile Include="FindSymbols\SymbolTree\ISymbolTreeInfoCacheService.cs" />