diff --git a/src/Workspaces/Core/Portable/FindSymbols/IRemoteSymbolFinder.cs b/src/Workspaces/Core/Portable/FindSymbols/IRemoteSymbolFinder.cs
index 8e45ed79e59fb..24169be4667a2 100644
--- a/src/Workspaces/Core/Portable/FindSymbols/IRemoteSymbolFinder.cs
+++ b/src/Workspaces/Core/Portable/FindSymbols/IRemoteSymbolFinder.cs
@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Remote;
@@ -17,19 +17,19 @@ Task FindReferencesAsync(PinnedSolutionInfo solutionInfo, SerializableSymbolAndP
 
         Task FindLiteralReferencesAsync(PinnedSolutionInfo solutionInfo, object value, TypeCode typeCode, CancellationToken cancellationToken);
 
-        Task<IList<SerializableSymbolAndProjectId>> FindAllDeclarationsWithNormalQueryAsync(
+        Task<ImmutableArray<SerializableSymbolAndProjectId>> FindAllDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo, ProjectId projectId, string name, SearchKind searchKind, SymbolFilter criteria, CancellationToken cancellationToken);
 
-        Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithNormalQueryAsync(
+        Task<ImmutableArray<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo, string name, bool ignoreCase, SymbolFilter criteria, CancellationToken cancellationToken);
 
-        Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithNormalQueryAsync(
+        Task<ImmutableArray<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo, ProjectId projectId, string name, bool ignoreCase, SymbolFilter criteria, CancellationToken cancellationToken);
 
-        Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithPatternAsync(
+        Task<ImmutableArray<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithPatternAsync(
             PinnedSolutionInfo solutionInfo, string pattern, SymbolFilter criteria, CancellationToken cancellationToken);
 
-        Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithPatternAsync(
+        Task<ImmutableArray<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithPatternAsync(
             PinnedSolutionInfo solutionInfo, ProjectId projectId, string pattern, SymbolFilter criteria, CancellationToken cancellationToken);
     }
 }
diff --git a/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder.FindReferencesServerCallback.cs b/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder.FindReferencesServerCallback.cs
index 6fe716fd5772d..b7ae067e29832 100644
--- a/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder.FindReferencesServerCallback.cs
+++ b/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder.FindReferencesServerCallback.cs
@@ -15,15 +15,20 @@ public static partial class SymbolFinder
         /// Callback object we pass to the OOP server to hear about the result 
         /// of the FindReferencesEngine as it executes there.
         /// </summary>
-        internal sealed class FindReferencesServerCallback
+        internal sealed class FindReferencesServerCallback : IEqualityComparer<SerializableSymbolAndProjectId>
         {
             private readonly Solution _solution;
             private readonly IStreamingFindReferencesProgress _progress;
             private readonly CancellationToken _cancellationToken;
 
             private readonly object _gate = new object();
-            private readonly Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId> _definitionMap =
-                new Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId>();
+
+            /// <summary>
+            /// Note: for purposes of Equality/Hashing, all that we use is the underlying SymbolKey.  That's because FAR
+            /// only cares if it is looking at the same symbol, it don't care if the symbol came from a different
+            /// project or not.
+            /// </summary>
+            private readonly Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId> _definitionMap;
 
             public FindReferencesServerCallback(
                 Solution solution,
@@ -33,6 +38,7 @@ public FindReferencesServerCallback(
                 _solution = solution;
                 _progress = progress;
                 _cancellationToken = cancellationToken;
+                _definitionMap = new Dictionary<SerializableSymbolAndProjectId, SymbolAndProjectId>(this);
             }
 
             public Task AddItemsAsync(int count) => _progress.ProgressTracker.AddItemsAsync(count);
@@ -85,6 +91,12 @@ public async Task OnReferenceFoundAsync(
 
                 await _progress.OnReferenceFoundAsync(symbolAndProjectId, referenceLocation).ConfigureAwait(false);
             }
+
+            bool IEqualityComparer<SerializableSymbolAndProjectId>.Equals(SerializableSymbolAndProjectId x, SerializableSymbolAndProjectId y)
+                => y.SymbolKeyData.Equals(x.SymbolKeyData);
+
+            int IEqualityComparer<SerializableSymbolAndProjectId>.GetHashCode(SerializableSymbolAndProjectId obj)
+                => obj.SymbolKeyData.GetHashCode();
         }
     }
 }
diff --git a/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_FindReferences_Current.cs b/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_FindReferences_Current.cs
index fbbae24ec72e0..74f071a6c16ea 100644
--- a/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_FindReferences_Current.cs
+++ b/src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_FindReferences_Current.cs
@@ -46,7 +46,7 @@ internal static async Task FindReferencesAsync(
                             solution,
                             new object[]
                             {
-                                SerializableSymbolAndProjectId.Dehydrate(symbolAndProjectId),
+                                SerializableSymbolAndProjectId.Dehydrate(solution, symbolAndProjectId.Symbol, cancellationToken),
                                 documents?.Select(d => d.Id).ToArray(),
                                 SerializableFindReferencesSearchOptions.Dehydrate(options),
                             },
diff --git a/src/Workspaces/Core/Portable/Remote/RemoteArguments.cs b/src/Workspaces/Core/Portable/Remote/RemoteArguments.cs
index cc9e5a7d1cb7a..9a1b616882dfe 100644
--- a/src/Workspaces/Core/Portable/Remote/RemoteArguments.cs
+++ b/src/Workspaces/Core/Portable/Remote/RemoteArguments.cs
@@ -37,35 +37,30 @@ public FindReferencesSearchOptions Rehydrate()
         }
     }
 
-    internal class SerializableSymbolAndProjectId : IEquatable<SerializableSymbolAndProjectId>
+    internal class SerializableSymbolAndProjectId
     {
         public string SymbolKeyData;
         public ProjectId ProjectId;
 
-        public override int GetHashCode()
-            => Hash.Combine(SymbolKeyData, ProjectId.GetHashCode());
-
-        public override bool Equals(object obj)
-            => Equals(obj as SerializableSymbolAndProjectId);
-
-        public bool Equals(SerializableSymbolAndProjectId other)
-            => other != null && SymbolKeyData.Equals(other.SymbolKeyData) && ProjectId.Equals(other.ProjectId);
-
         public static SerializableSymbolAndProjectId Dehydrate(
-            IAliasSymbol alias, Document document)
+            IAliasSymbol alias, Document document, CancellationToken cancellationToken)
         {
             return alias == null
                 ? null
-                : Dehydrate(new SymbolAndProjectId(alias, document.Project.Id));
+                : Dehydrate(document.Project.Solution, alias, cancellationToken);
         }
 
         public static SerializableSymbolAndProjectId Dehydrate(
-            SymbolAndProjectId symbolAndProjectId)
+            Solution solution, ISymbol symbol, CancellationToken cancellationToken)
         {
+            var symbolKey = symbol.GetSymbolKey(cancellationToken);
+            var projectId = solution.GetExactProjectId(symbol);
+            Contract.ThrowIfNull(projectId, WorkspacesResources.Symbols_project_could_not_be_found_in_the_provided_solution);
+
             return new SerializableSymbolAndProjectId
             {
-                SymbolKeyData = symbolAndProjectId.Symbol.GetSymbolKey().ToString(),
-                ProjectId = symbolAndProjectId.ProjectId
+                SymbolKeyData = symbolKey.ToString(),
+                ProjectId = projectId,
             };
         }
 
@@ -163,12 +158,12 @@ internal class SerializableReferenceLocation
         public CandidateReason CandidateReason { get; set; }
 
         public static SerializableReferenceLocation Dehydrate(
-            ReferenceLocation referenceLocation)
+            ReferenceLocation referenceLocation, CancellationToken cancellationToken)
         {
             return new SerializableReferenceLocation
             {
                 Document = referenceLocation.Document.Id,
-                Alias = SerializableSymbolAndProjectId.Dehydrate(referenceLocation.Alias, referenceLocation.Document),
+                Alias = SerializableSymbolAndProjectId.Dehydrate(referenceLocation.Alias, referenceLocation.Document, cancellationToken),
                 Location = referenceLocation.Location.SourceSpan,
                 IsImplicit = referenceLocation.IsImplicit,
                 SymbolUsageInfo = SerializableSymbolUsageInfo.Dehydrate(referenceLocation.SymbolUsageInfo),
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs b/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
index ee75b5ba7beb5..219a199485557 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
@@ -126,6 +126,29 @@ private static Project CreateProject(ProjectId projectId, Solution solution)
             return projectState == null ? null : GetProject(projectState.Id);
         }
 
+        /// <summary>
+        /// Given a <paramref name="symbol"/> returns the <see cref="ProjectId"/> of the <see cref="Project"/> it came
+        /// from.  Returns <see langword="null"/> if <paramref name="symbol"/> does not come from <paramref name="symbol"/>.
+        /// </summary>
+        /// <remarks>
+        /// This function differs from <see cref="GetProject(IAssemblySymbol, CancellationToken)"/> in terms of how it
+        /// treats <see cref="IAssemblySymbol"/>s.  Specifically, say there is the following:
+        ///
+        /// <c>
+        /// Project-A, containing Symbol-A.<para/>
+        /// Project-B, with a reference to Project-A, and usage of Symbol-A.
+        /// </c>
+        ///
+        /// It is possible (with retargeting, and other complex cases) that Symbol-A from Project-B will be a different
+        /// symbol than Symbol-A from Project-A.  However, <see cref="GetProject(IAssemblySymbol, CancellationToken)"/>
+        /// will always try to return Project-A for either of the Symbol-A's, as it prefers to return the original
+        /// Source-Project of the original definition, not the project that actually produced the symbol.  For many
+        /// features this is an acceptable abstraction.  However, for some cases (Find-References in particular) it is
+        /// necessary to resolve symbols back to the actual project/compilation that produced them for correctness.
+        /// </remarks>
+        internal ProjectId? GetExactProjectId(ISymbol symbol)
+            => _state.GetExactProjectId(symbol);
+
         /// <summary>
         /// True if the solution contains the document in one of its projects
         /// </summary>
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.State.cs b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.State.cs
index 03466e2521705..d98698cc435e4 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.State.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.State.cs
@@ -7,6 +7,7 @@
 using System;
 using System.Collections.Immutable;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using Microsoft.CodeAnalysis;
 using Roslyn.Utilities;
 
@@ -30,7 +31,10 @@ private class State
                 /// <summary>
                 /// The base <see cref="State"/> that starts with everything empty.
                 /// </summary>
-                public static readonly State Empty = new State(compilation: null, declarationOnlyCompilation: null, generatorDriver: new TrackedGeneratorDriver(null));
+                public static readonly State Empty = new State(
+                    compilation: null, declarationOnlyCompilation: null,
+                    generatorDriver: new TrackedGeneratorDriver(null),
+                    assemblyAndModuleSet: null);
 
                 /// <summary>
                 /// A strong reference to the declaration-only compilation. This compilation isn't used to produce symbols,
@@ -46,6 +50,21 @@ private class State
 
                 public TrackedGeneratorDriver GeneratorDriver { get; }
 
+                /// <summary>
+                /// Weak table of the assembly and module symbols that this compilation tracker has created.  This can
+                /// be used to determine which project an assembly symbol came from after the fact.  This is needed as
+                /// the compilation an assembly came from can be GC'ed and further requests to get that compilation (or
+                /// any of it's assemblies) may produce new assembly symbols.
+                /// </summary>
+                /// <remarks>
+                /// Ideally this would just be <c>ConditionalWeakSet&lt;ISymbol&gt;</c>.  Effectively we just want to
+                /// hold onto the symbols as long as someone else is keeping them alive.  And we don't actually need
+                /// them to map to anything.  We just use their existence to know if our project was the project it came
+                /// from.  However, ConditionalWeakTable is the best tool we have, so we simulate a set by just using a
+                /// table and mapping the keys to the <see langword="null"/> value.
+                /// </remarks>
+                public readonly ConditionalWeakTable<ISymbol, object?>? AssemblyAndModuleSet;
+
                 /// <summary>
                 /// Specifies whether <see cref="FinalCompilation"/> and all compilations it depends on contain full information or not. This can return
                 /// <see langword="null"/> if the state isn't at the point where it would know, and it's necessary to transition to <see cref="FinalState"/> to figure that out.
@@ -57,7 +76,11 @@ private class State
                 /// </summary>
                 public virtual ValueSource<Optional<Compilation>>? FinalCompilation => null;
 
-                protected State(ValueSource<Optional<Compilation>>? compilation, Compilation? declarationOnlyCompilation, TrackedGeneratorDriver generatorDriver)
+                protected State(
+                    ValueSource<Optional<Compilation>>? compilation,
+                    Compilation? declarationOnlyCompilation,
+                    TrackedGeneratorDriver generatorDriver,
+                    ConditionalWeakTable<ISymbol, object?>? assemblyAndModuleSet)
                 {
                     // Declaration-only compilations should never have any references
                     Contract.ThrowIfTrue(declarationOnlyCompilation != null && declarationOnlyCompilation.ExternalReferences.Any());
@@ -65,6 +88,7 @@ protected State(ValueSource<Optional<Compilation>>? compilation, Compilation? de
                     Compilation = compilation;
                     DeclarationOnlyCompilation = declarationOnlyCompilation;
                     GeneratorDriver = generatorDriver;
+                    AssemblyAndModuleSet = assemblyAndModuleSet;
                 }
 
                 public static State Create(
@@ -90,6 +114,25 @@ public static ValueSource<Optional<Compilation>> CreateValueSource(
                         ? new WeakValueSource<Compilation>(compilation)
                         : (ValueSource<Optional<Compilation>>)new ConstantValueSource<Optional<Compilation>>(compilation);
                 }
+
+                public static ConditionalWeakTable<ISymbol, object?> GetAssemblyAndModuleSet(Compilation compilation)
+                {
+                    var result = new ConditionalWeakTable<ISymbol, object?>();
+
+                    var compAssembly = compilation.Assembly;
+                    result.Add(compAssembly, null);
+
+                    foreach (var reference in compilation.References)
+                    {
+                        var symbol = compilation.GetAssemblyOrModuleSymbol(reference);
+                        if (symbol == null)
+                            continue;
+
+                        result.Add(symbol, null);
+                    }
+
+                    return result;
+                }
             }
 
             /// <summary>
@@ -106,7 +149,8 @@ public InProgressState(
                     ImmutableArray<(ProjectState state, CompilationAndGeneratorDriverTranslationAction action)> intermediateProjects)
                     : base(compilation: new ConstantValueSource<Optional<Compilation>>(inProgressCompilation),
                            declarationOnlyCompilation: null,
-                           generatorDriver: inProgressGeneratorDriver)
+                           generatorDriver: inProgressGeneratorDriver,
+                           GetAssemblyAndModuleSet(inProgressCompilation))
                 {
                     Contract.ThrowIfTrue(intermediateProjects.IsDefault);
                     Contract.ThrowIfFalse(intermediateProjects.Length > 0);
@@ -121,7 +165,10 @@ public InProgressState(
             private sealed class LightDeclarationState : State
             {
                 public LightDeclarationState(Compilation declarationOnlyCompilation)
-                    : base(compilation: null, declarationOnlyCompilation: declarationOnlyCompilation, generatorDriver: new TrackedGeneratorDriver(null))
+                    : base(compilation: null,
+                           declarationOnlyCompilation: declarationOnlyCompilation,
+                           generatorDriver: new TrackedGeneratorDriver(null),
+                           assemblyAndModuleSet: null)
                 {
                 }
             }
@@ -133,7 +180,10 @@ public LightDeclarationState(Compilation declarationOnlyCompilation)
             private sealed class FullDeclarationState : State
             {
                 public FullDeclarationState(Compilation declarationCompilation, TrackedGeneratorDriver generatorDriver)
-                    : base(new WeakValueSource<Compilation>(declarationCompilation), declarationCompilation.Clone().RemoveAllReferences(), generatorDriver)
+                    : base(new WeakValueSource<Compilation>(declarationCompilation),
+                           declarationCompilation.Clone().RemoveAllReferences(),
+                           generatorDriver,
+                           GetAssemblyAndModuleSet(declarationCompilation))
                 {
                 }
             }
@@ -160,8 +210,12 @@ public FinalState(
                     ValueSource<Optional<Compilation>> compilationWithoutGeneratedFilesSource,
                     Compilation compilationWithoutGeneratedFiles,
                     TrackedGeneratorDriver generatorDriver,
-                    bool hasSuccessfullyLoaded)
-                    : base(compilationWithoutGeneratedFilesSource, compilationWithoutGeneratedFiles.Clone().RemoveAllReferences(), generatorDriver)
+                    bool hasSuccessfullyLoaded,
+                    ConditionalWeakTable<ISymbol, object?>? compilationAssemblies)
+                    : base(compilationWithoutGeneratedFilesSource,
+                           compilationWithoutGeneratedFiles.Clone().RemoveAllReferences(),
+                           generatorDriver,
+                           compilationAssemblies)
                 {
                     HasSuccessfullyLoaded = hasSuccessfullyLoaded;
                     FinalCompilation = finalCompilationSource;
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs
index 69d01dff8860e..a937aa92e4f3f 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs
@@ -100,6 +100,14 @@ public bool HasCompilation
                 }
             }
 
+            public bool ContainsAssemblyOrModule(ISymbol assemblyOrModule)
+            {
+                Debug.Assert(assemblyOrModule.Kind == SymbolKind.Assembly || assemblyOrModule.Kind == SymbolKind.NetModule);
+                var state = this.ReadState();
+                var assemblyAndModuleSet = state.AssemblyAndModuleSet;
+                return assemblyAndModuleSet != null && assemblyAndModuleSet.TryGetValue(assemblyOrModule, out _);
+            }
+
             /// <summary>
             /// Creates a new instance of the compilation info, retaining any already built
             /// compilation state as the now 'old' state
@@ -188,7 +196,8 @@ public CompilationTracker FreezePartialStateWithTree(SolutionState solution, Doc
                         new ConstantValueSource<Optional<Compilation>>(inProgressCompilation),
                         inProgressCompilation,
                         generatorDriver: new TrackedGeneratorDriver(null),
-                        hasSuccessfullyLoaded: false));
+                        hasSuccessfullyLoaded: false,
+                        State.GetAssemblyAndModuleSet(inProgressCompilation)));
             }
 
             /// <summary>
@@ -710,7 +719,8 @@ private async Task<CompilationInfo> FinalizeCompilationAsync(
                             State.CreateValueSource(compilationWithoutGeneratedFiles, solution.Services),
                             compilationWithoutGeneratedFiles,
                             generatorDriver,
-                            hasSuccessfullyLoaded),
+                            hasSuccessfullyLoaded,
+                            State.GetAssemblyAndModuleSet(compilation)),
                         solution.Services);
 
                     return new CompilationInfo(compilation, hasSuccessfullyLoaded);
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.SymbolToProjectId.cs b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.SymbolToProjectId.cs
new file mode 100644
index 0000000000000..c0f9834ef284f
--- /dev/null
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.SymbolToProjectId.cs
@@ -0,0 +1,81 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable enable
+
+using System.Runtime.CompilerServices;
+using Microsoft.CodeAnalysis.Shared.Extensions;
+using Roslyn.Utilities;
+
+namespace Microsoft.CodeAnalysis
+{
+    internal partial class SolutionState
+    {
+        /// <inheritdoc cref="Solution.GetExactProjectId"/>
+        public ProjectId? GetExactProjectId(ISymbol? symbol)
+        {
+            LazyInitialization.EnsureInitialized(ref _assemblyOrModuleSymbolToProjectId, s_createTable);
+
+            // Walk up the symbol so we can get to the containing namespace/assembly that will be used to map
+            // back to a project.
+
+            while (symbol != null)
+            {
+                var result = GetProjectIdDirectly(symbol, _assemblyOrModuleSymbolToProjectId);
+                if (result != null)
+                    return result;
+
+                symbol = symbol.ContainingSymbol;
+            }
+
+            return null;
+        }
+
+        private ProjectId? GetProjectIdDirectly(
+            ISymbol symbol, ConditionalWeakTable<ISymbol, ProjectId?> assemblyOrModuleSymbolToProjectId)
+        {
+            if (symbol.IsKind(SymbolKind.Namespace, out INamespaceSymbol? ns))
+            {
+                if (ns.ContainingCompilation != null)
+                {
+                    // A namespace that spans a compilation.  These don't belong to an assembly/module directly.
+                    // However, as we're looking for the project this corresponds to, we can look for the
+                    // source-module component (the first in the constituent namespaces) and then search using that.
+                    return GetExactProjectId(ns.ConstituentNamespaces[0]);
+                }
+            }
+            else if (symbol.IsKind(SymbolKind.Assembly) ||
+                     symbol.IsKind(SymbolKind.NetModule))
+            {
+                if (!assemblyOrModuleSymbolToProjectId.TryGetValue(symbol, out var projectId))
+                {
+                    foreach (var (id, tracker) in _projectIdToTrackerMap)
+                    {
+                        if (tracker.ContainsAssemblyOrModule(symbol))
+                        {
+                            projectId = id;
+                            break;
+                        }
+                    }
+
+                    // Have to lock as there's no atomic AddOrUpdate in netstandard2.0 and we could throw if two
+                    // threads tried to add the same item.
+#if NETSTANDARD
+                    lock (assemblyOrModuleSymbolToProjectId)
+                    {
+                        assemblyOrModuleSymbolToProjectId.Remove(symbol);
+                        assemblyOrModuleSymbolToProjectId.Add(symbol, projectId);
+                    }
+#else
+                    assemblyOrModuleSymbolToProjectId.AddOrUpdate(symbol, projectId);
+#endif
+                }
+
+                return projectId;
+            }
+
+            return null;
+        }
+    }
+}
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs
index a9434d23c40c6..46acdd99680f9 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs
@@ -56,6 +56,14 @@ internal partial class SolutionState
         // holds on data calculated based on the AnalyzerReferences list
         private readonly Lazy<HostDiagnosticAnalyzers> _lazyAnalyzers;
 
+        /// <summary>
+        /// Cache we use to map between assembly and module symbols and the project they came from.  That way if we
+        /// are asked about many symbols from the same assembly/module we can answer the question quickly after
+        /// computing for the first one.  Created on demand.
+        /// </summary>
+        private ConditionalWeakTable<ISymbol, ProjectId?>? _assemblyOrModuleSymbolToProjectId;
+        private static readonly Func<ConditionalWeakTable<ISymbol, ProjectId?>> s_createTable = () => new ConditionalWeakTable<ISymbol, ProjectId?>();
+
         private SolutionState(
             BranchId branchId,
             int workspaceVersion,
diff --git a/src/Workspaces/Core/Portable/WorkspacesResources.resx b/src/Workspaces/Core/Portable/WorkspacesResources.resx
index 1dde52b6f9d25..39e05a55ac404 100644
--- a/src/Workspaces/Core/Portable/WorkspacesResources.resx
+++ b/src/Workspaces/Core/Portable/WorkspacesResources.resx
@@ -553,4 +553,7 @@
   <data name="Unknown" xml:space="preserve">
     <value>Unknown</value>
   </data>
+  <data name="Symbols_project_could_not_be_found_in_the_provided_solution" xml:space="preserve">
+    <value>Symbol's project could not be found in the provided solution</value>
+  </data>
 </root>
\ No newline at end of file
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.cs.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.cs.xlf
index 927aed0ecac19..cd1c8908e9287 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.cs.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.cs.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Přidávání projektů se nepodporuje.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.de.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.de.xlf
index 8edd7c4742e15..64377f1c43998 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.de.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.de.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Das Hinzufügen von Projekten wird nicht unterstützt.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.es.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.es.xlf
index 10a857869d5d1..90b817c0b25bf 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.es.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.es.xlf
@@ -172,6 +172,11 @@
         <target state="translated">No se admite la adición de proyectos.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.fr.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.fr.xlf
index 1e9a44690981e..9a7247c1d20c3 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.fr.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.fr.xlf
@@ -172,6 +172,11 @@
         <target state="translated">L'ajout de projets n'est pas pris en charge.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.it.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.it.xlf
index e390333bf5a71..96ec6d11bc0df 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.it.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.it.xlf
@@ -172,6 +172,11 @@
         <target state="translated">L'aggiunta di progetti non è supportata.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ja.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ja.xlf
index 3b3ff9d7ec695..20b168f0c7d18 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ja.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ja.xlf
@@ -172,6 +172,11 @@
         <target state="translated">プロジェクトの追加はサポートされていません。</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ko.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ko.xlf
index f82e78f4a7042..791183aadef82 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ko.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ko.xlf
@@ -172,6 +172,11 @@
         <target state="translated">프로젝트 추가가 지원되지 않습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pl.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pl.xlf
index ce51a1359c4fc..c0817d3f8b274 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pl.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pl.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Dodawanie projektów nie jest obsługiwane.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pt-BR.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pt-BR.xlf
index e97d1c1ff8249..593d1ff0828c0 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pt-BR.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.pt-BR.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Não há suporte para adicionar projetos.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ru.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ru.xlf
index e1a5d5082256e..757fa537a55c1 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ru.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.ru.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Добавление проектов не поддерживается.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.tr.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.tr.xlf
index 65b15ef347614..980ccc20171ed 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.tr.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.tr.xlf
@@ -172,6 +172,11 @@
         <target state="translated">Projelerin eklenmesi desteklenmiyor.</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hans.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hans.xlf
index 974afac279fcd..700d7440735dc 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hans.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hans.xlf
@@ -172,6 +172,11 @@
         <target state="translated">不支持添加项目。</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hant.xlf b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hant.xlf
index 97722d4bec82c..b097edeb49440 100644
--- a/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hant.xlf
+++ b/src/Workspaces/Core/Portable/xlf/WorkspacesResources.zh-Hant.xlf
@@ -172,6 +172,11 @@
         <target state="translated">不支援新增專案。</target>
         <note />
       </trans-unit>
+      <trans-unit id="Symbols_project_could_not_be_found_in_the_provided_solution">
+        <source>Symbol's project could not be found in the provided solution</source>
+        <target state="new">Symbol's project could not be found in the provided solution</target>
+        <note />
+      </trans-unit>
       <trans-unit id="The_project_already_contains_the_specified_reference">
         <source>The project already contains the specified reference.</source>
         <target state="new">The project already contains the specified reference.</target>
diff --git a/src/Workspaces/Remote/ServiceHub/Services/CodeAnalysisService_SymbolFinder.cs b/src/Workspaces/Remote/ServiceHub/Services/CodeAnalysisService_SymbolFinder.cs
index e1636d7b252ea..07b3fdb8debf5 100644
--- a/src/Workspaces/Remote/ServiceHub/Services/CodeAnalysisService_SymbolFinder.cs
+++ b/src/Workspaces/Remote/ServiceHub/Services/CodeAnalysisService_SymbolFinder.cs
@@ -3,12 +3,12 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.FindSymbols;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Utilities;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
@@ -34,7 +34,7 @@ public Task FindReferencesAsync(
                     var symbolAndProjectId = await symbolAndProjectIdArg.TryRehydrateAsync(
                         solution, cancellationToken).ConfigureAwait(false);
 
-                    var progressCallback = new FindReferencesProgressCallback(EndPoint, cancellationToken);
+                    var progressCallback = new FindReferencesProgressCallback(solution, EndPoint, cancellationToken);
 
                     if (!symbolAndProjectId.HasValue)
                     {
@@ -74,7 +74,17 @@ await SymbolFinder.FindLiteralReferencesInCurrentProcessAsync(
             }, cancellationToken);
         }
 
-        public Task<IList<SerializableSymbolAndProjectId>> FindAllDeclarationsWithNormalQueryAsync(
+        private ImmutableArray<SerializableSymbolAndProjectId> Convert(ImmutableArray<SymbolAndProjectId> items, Solution solution, CancellationToken cancellationToken)
+        {
+            using var _ = ArrayBuilder<SerializableSymbolAndProjectId>.GetInstance(out var result);
+
+            foreach (var item in items)
+                result.Add(SerializableSymbolAndProjectId.Dehydrate(solution, item.Symbol, cancellationToken));
+
+            return result.ToImmutable();
+        }
+
+        public Task<ImmutableArray<SerializableSymbolAndProjectId>> FindAllDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo,
             ProjectId projectId,
             string name,
@@ -94,12 +104,12 @@ public Task<IList<SerializableSymbolAndProjectId>> FindAllDeclarationsWithNormal
                     var result = await DeclarationFinder.FindAllDeclarationsWithNormalQueryInCurrentProcessAsync(
                         project, query, criteria, cancellationToken).ConfigureAwait(false);
 
-                    return (IList<SerializableSymbolAndProjectId>)result.SelectAsArray(SerializableSymbolAndProjectId.Dehydrate);
+                    return Convert(result, solution, cancellationToken);
                 }
             }, cancellationToken);
         }
 
-        public Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithNormalQueryAsync(
+        public Task<ImmutableArray<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo,
             string name,
             bool ignoreCase,
@@ -114,12 +124,12 @@ public Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclaration
                     var result = await DeclarationFinder.FindSourceDeclarationsWithNormalQueryInCurrentProcessAsync(
                         solution, name, ignoreCase, criteria, cancellationToken).ConfigureAwait(false);
 
-                    return (IList<SerializableSymbolAndProjectId>)result.SelectAsArray(SerializableSymbolAndProjectId.Dehydrate);
+                    return Convert(result, solution, cancellationToken);
                 }
             }, cancellationToken);
         }
 
-        public Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithNormalQueryAsync(
+        public Task<ImmutableArray<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithNormalQueryAsync(
             PinnedSolutionInfo solutionInfo,
             ProjectId projectId,
             string name,
@@ -137,12 +147,12 @@ public Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarations
                     var result = await DeclarationFinder.FindSourceDeclarationsWithNormalQueryInCurrentProcessAsync(
                         project, name, ignoreCase, criteria, cancellationToken).ConfigureAwait(false);
 
-                    return (IList<SerializableSymbolAndProjectId>)result.SelectAsArray(SerializableSymbolAndProjectId.Dehydrate);
+                    return Convert(result, solution, cancellationToken);
                 }
             }, cancellationToken);
         }
 
-        public Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithPatternAsync(
+        public Task<ImmutableArray<SerializableSymbolAndProjectId>> FindSolutionSourceDeclarationsWithPatternAsync(
             PinnedSolutionInfo solutionInfo, string pattern, SymbolFilter criteria, CancellationToken cancellationToken)
         {
             return RunServiceAsync(async () =>
@@ -154,12 +164,12 @@ public Task<IList<SerializableSymbolAndProjectId>> FindSolutionSourceDeclaration
                     var result = await DeclarationFinder.FindSourceDeclarationsWithPatternInCurrentProcessAsync(
                         solution, pattern, criteria, cancellationToken).ConfigureAwait(false);
 
-                    return (IList<SerializableSymbolAndProjectId>)result.SelectAsArray(SerializableSymbolAndProjectId.Dehydrate);
+                    return Convert(result, solution, cancellationToken);
                 }
             }, cancellationToken);
         }
 
-        public Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithPatternAsync(
+        public Task<ImmutableArray<SerializableSymbolAndProjectId>> FindProjectSourceDeclarationsWithPatternAsync(
             PinnedSolutionInfo solutionInfo, ProjectId projectId, string pattern, SymbolFilter criteria, CancellationToken cancellationToken)
         {
             return RunServiceAsync(async () =>
@@ -172,7 +182,7 @@ public Task<IList<SerializableSymbolAndProjectId>> FindProjectSourceDeclarations
                     var result = await DeclarationFinder.FindSourceDeclarationsWithPatternInCurrentProcessAsync(
                         project, pattern, criteria, cancellationToken).ConfigureAwait(false);
 
-                    return (IList<SerializableSymbolAndProjectId>)result.SelectAsArray(SerializableSymbolAndProjectId.Dehydrate);
+                    return Convert(result, solution, cancellationToken);
                 }
             }, cancellationToken);
         }
@@ -203,13 +213,15 @@ public Task ItemCompletedAsync()
 
         private sealed class FindReferencesProgressCallback : IStreamingFindReferencesProgress, IStreamingProgressTracker
         {
+            private readonly Solution _solution;
             private readonly RemoteEndPoint _endPoint;
             private readonly CancellationToken _cancellationToken;
 
             public IStreamingProgressTracker ProgressTracker { get; }
 
-            public FindReferencesProgressCallback(RemoteEndPoint endPoint, CancellationToken cancellationToken)
+            public FindReferencesProgressCallback(Solution solution, RemoteEndPoint endPoint, CancellationToken cancellationToken)
             {
+                _solution = solution;
                 _endPoint = endPoint;
                 _cancellationToken = cancellationToken;
                 ProgressTracker = this;
@@ -228,15 +240,19 @@ public Task OnFindInDocumentCompletedAsync(Document document)
                 => _endPoint.InvokeAsync(nameof(SymbolFinder.FindReferencesServerCallback.OnFindInDocumentCompletedAsync), new object[] { document.Id }, _cancellationToken);
 
             public Task OnDefinitionFoundAsync(SymbolAndProjectId definition)
-                => _endPoint.InvokeAsync(nameof(SymbolFinder.FindReferencesServerCallback.OnDefinitionFoundAsync), new object[] { SerializableSymbolAndProjectId.Dehydrate(definition) }, _cancellationToken);
+                => _endPoint.InvokeAsync(
+                    nameof(SymbolFinder.FindReferencesServerCallback.OnDefinitionFoundAsync),
+                    new object[] { SerializableSymbolAndProjectId.Dehydrate(_solution, definition.Symbol, _cancellationToken) }, _cancellationToken);
 
             public Task OnReferenceFoundAsync(SymbolAndProjectId definition, ReferenceLocation reference)
-            {
-                return _endPoint.InvokeAsync(
+                => _endPoint.InvokeAsync(
                     nameof(SymbolFinder.FindReferencesServerCallback.OnReferenceFoundAsync),
-                    new object[] { SerializableSymbolAndProjectId.Dehydrate(definition), SerializableReferenceLocation.Dehydrate(reference) },
+                    new object[]
+                    {
+                        SerializableSymbolAndProjectId.Dehydrate(_solution, definition.Symbol, _cancellationToken),
+                        SerializableReferenceLocation.Dehydrate(reference, _cancellationToken),
+                    },
                     _cancellationToken);
-            }
 
             public Task AddItemsAsync(int count)
                 => _endPoint.InvokeAsync(nameof(SymbolFinder.FindReferencesServerCallback.AddItemsAsync), new object[] { count }, _cancellationToken);