diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/Project.cs b/src/Workspaces/Core/Portable/Workspace/Solution/Project.cs
index f8f4981ead216..d20005fd748bb 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/Project.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/Project.cs
@@ -26,10 +26,11 @@ namespace Microsoft.CodeAnalysis;
 [DebuggerDisplay("{GetDebuggerDisplay(),nq}")]
 public partial class Project
 {
-    private ImmutableDictionary<DocumentId, Document?> _idToDocumentMap = ImmutableDictionary<DocumentId, Document?>.Empty;
-    private ImmutableDictionary<DocumentId, SourceGeneratedDocument> _idToSourceGeneratedDocumentMap = ImmutableDictionary<DocumentId, SourceGeneratedDocument>.Empty;
-    private ImmutableDictionary<DocumentId, AdditionalDocument?> _idToAdditionalDocumentMap = ImmutableDictionary<DocumentId, AdditionalDocument?>.Empty;
-    private ImmutableDictionary<DocumentId, AnalyzerConfigDocument?> _idToAnalyzerConfigDocumentMap = ImmutableDictionary<DocumentId, AnalyzerConfigDocument?>.Empty;
+    // Only access these dictionaries when holding them as a lock
+    private Dictionary<DocumentId, Document?>? _idToDocumentMap;
+    private Dictionary<DocumentId, SourceGeneratedDocument>? _idToSourceGeneratedDocumentMap;
+    private Dictionary<DocumentId, AdditionalDocument?>? _idToAdditionalDocumentMap;
+    private Dictionary<DocumentId, AnalyzerConfigDocument?>? _idToAnalyzerConfigDocumentMap;
 
     internal Project(Solution solution, ProjectState projectState)
     {
@@ -239,19 +240,48 @@ public bool ContainsAnalyzerConfigDocument(DocumentId documentId)
     /// Get the document in this project with the specified document Id.
     /// </summary>
     public Document? GetDocument(DocumentId documentId)
-        => ImmutableInterlocked.GetOrAdd(ref _idToDocumentMap, documentId, s_tryCreateDocumentFunction, this);
+        => GetOrAddDocumentUnderLock(documentId, ref _idToDocumentMap, s_tryCreateDocumentFunction, this);
 
     /// <summary>
     /// Get the additional document in this project with the specified document Id.
     /// </summary>
     public TextDocument? GetAdditionalDocument(DocumentId documentId)
-        => ImmutableInterlocked.GetOrAdd(ref _idToAdditionalDocumentMap, documentId, s_tryCreateAdditionalDocumentFunction, this);
+        => GetOrAddDocumentUnderLock(documentId, ref _idToAdditionalDocumentMap, s_tryCreateAdditionalDocumentFunction, this);
 
     /// <summary>
     /// Get the analyzer config document in this project with the specified document Id.
     /// </summary>
     public AnalyzerConfigDocument? GetAnalyzerConfigDocument(DocumentId documentId)
-        => ImmutableInterlocked.GetOrAdd(ref _idToAnalyzerConfigDocumentMap, documentId, s_tryCreateAnalyzerConfigDocumentFunction, this);
+        => GetOrAddDocumentUnderLock(documentId, ref _idToAnalyzerConfigDocumentMap, s_tryCreateAnalyzerConfigDocumentFunction, this);
+
+    private static TDocument GetOrAddDocumentUnderLock<TDocument, TArg>(DocumentId documentId, ref Dictionary<DocumentId, TDocument>? idMap, Func<DocumentId, TArg, TDocument> tryCreate, TArg arg)
+    {
+        if (idMap == null)
+        {
+            // First call assigns a new dictionary. Any other simultaneous requests will not assign the
+            // dictionary they created to idMap. At that point, normal locking rules apply.
+            Interlocked.CompareExchange(ref idMap, new Dictionary<DocumentId, TDocument>(), null);
+        }
+
+        lock (idMap)
+        {
+            return idMap.GetOrAdd(documentId, tryCreate, arg);
+        }
+    }
+
+    private static bool TryGetDocumentValueUnderLock<TDocument>(DocumentId documentId, ref Dictionary<DocumentId, TDocument>? idMap, out TDocument? document)
+    {
+        if (idMap == null)
+        {
+            document = default;
+            return false;
+        }
+
+        lock (idMap)
+        {
+            return idMap.TryGetValue(documentId, out document);
+        }
+    }
 
     /// <summary>
     /// Gets a document or a source generated document in this solution with the specified document ID.
@@ -288,7 +318,7 @@ public async ValueTask<IEnumerable<SourceGeneratedDocument>> GetSourceGeneratedD
 
         // return an iterator to avoid eagerly allocating all the document instances
         return generatedDocumentStates.States.Values.Select(state =>
-            ImmutableInterlocked.GetOrAdd(ref _idToSourceGeneratedDocumentMap, state.Id, s_createSourceGeneratedDocumentFunction, (state, this)));
+            GetOrAddDocumentUnderLock(state.Id, ref _idToSourceGeneratedDocumentMap, s_createSourceGeneratedDocumentFunction, (state, this)));
     }
 
     internal async IAsyncEnumerable<Document> GetAllRegularAndSourceGeneratedDocumentsAsync([EnumeratorCancellation] CancellationToken cancellationToken)
@@ -312,7 +342,7 @@ internal async IAsyncEnumerable<Document> GetAllRegularAndSourceGeneratedDocumen
             return null;
 
         // Quick check first: if we already have created a SourceGeneratedDocument wrapper, we're good
-        if (_idToSourceGeneratedDocumentMap.TryGetValue(documentId, out var sourceGeneratedDocument))
+        if (TryGetDocumentValueUnderLock(documentId, ref _idToSourceGeneratedDocumentMap, out var sourceGeneratedDocument))
             return sourceGeneratedDocument;
 
         // We'll have to run generators if we haven't already and now try to find it.
@@ -325,7 +355,7 @@ internal async IAsyncEnumerable<Document> GetAllRegularAndSourceGeneratedDocumen
     }
 
     internal SourceGeneratedDocument GetOrCreateSourceGeneratedDocument(SourceGeneratedDocumentState state)
-        => ImmutableInterlocked.GetOrAdd(ref _idToSourceGeneratedDocumentMap, state.Id, s_createSourceGeneratedDocumentFunction, (state, this));
+        => GetOrAddDocumentUnderLock(state.Id, ref _idToSourceGeneratedDocumentMap, s_createSourceGeneratedDocumentFunction, (state, this));
 
     /// <summary>
     /// Returns the <see cref="SourceGeneratedDocumentState"/> for a source generated document that has already been generated and observed.
@@ -346,7 +376,7 @@ internal SourceGeneratedDocument GetOrCreateSourceGeneratedDocument(SourceGenera
             return null;
 
         // Easy case: do we already have the SourceGeneratedDocument created?
-        if (_idToSourceGeneratedDocumentMap.TryGetValue(documentId, out var document))
+        if (TryGetDocumentValueUnderLock(documentId, ref _idToSourceGeneratedDocumentMap, out var document))
             return document;
 
         // Trickier case now: it's possible we generated this, but we don't actually have the SourceGeneratedDocument for it, so let's go
@@ -355,7 +385,7 @@ internal SourceGeneratedDocument GetOrCreateSourceGeneratedDocument(SourceGenera
         if (documentState == null)
             return null;
 
-        return ImmutableInterlocked.GetOrAdd(ref _idToSourceGeneratedDocumentMap, documentId, s_createSourceGeneratedDocumentFunction, (documentState, this));
+        return GetOrAddDocumentUnderLock(documentId, ref _idToSourceGeneratedDocumentMap, s_createSourceGeneratedDocumentFunction, (documentState, this));
     }
 
     internal ValueTask<ImmutableArray<Diagnostic>> GetSourceGeneratorDiagnosticsAsync(CancellationToken cancellationToken)
diff --git a/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs b/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
index 0ad1c652735c5..20f5a637864e8 100644
--- a/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
+++ b/src/Workspaces/Core/Portable/Workspace/Solution/Solution.cs
@@ -27,9 +27,8 @@ namespace Microsoft.CodeAnalysis;
 /// </summary>
 public partial class Solution
 {
-
-    // Values for all these are created on demand.
-    private ImmutableDictionary<ProjectId, Project> _projectIdToProjectMap;
+    // Values for all these are created on demand. Only access when holding the dictionary as a lock.
+    private readonly Dictionary<ProjectId, Project> _projectIdToProjectMap = [];
 
     /// <summary>
     /// Result of calling <see cref="WithFrozenPartialCompilationsAsync"/>.
@@ -46,7 +45,6 @@ private Solution(
         SolutionCompilationState compilationState,
         AsyncLazy<Solution>? cachedFrozenSolution = null)
     {
-        _projectIdToProjectMap = ImmutableDictionary<ProjectId, Project>.Empty;
         CompilationState = compilationState;
 
         _cachedFrozenSolution = cachedFrozenSolution ??
@@ -152,7 +150,10 @@ public bool ContainsProject([NotNullWhen(returnValue: true)] ProjectId? projectI
     {
         if (this.ContainsProject(projectId))
         {
-            return ImmutableInterlocked.GetOrAdd(ref _projectIdToProjectMap, projectId, s_createProjectFunction, this);
+            lock (_projectIdToProjectMap)
+            {
+                return _projectIdToProjectMap.GetOrAdd(projectId, s_createProjectFunction, this);
+            }
         }
 
         return null;