Reduce allocations in ProjectState's ctor

src/Workspaces/Core/Portable/Workspace/Solution/ProjectState.cs

@@ -3,7 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -12,7 +11,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.Collections;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Diagnostics.Analyzers.NamingStyles;
 using Microsoft.CodeAnalysis.Host;
@@ -49,10 +47,11 @@ internal sealed partial class ProjectState
     private readonly AsyncLazy<VersionStamp> _lazyLatestDocumentVersion;
     private readonly AsyncLazy<VersionStamp> _lazyLatestDocumentTopLevelChangeVersion;
 
-    // Checksums for this solution state
-    private readonly AsyncLazy<ProjectStateChecksums> _lazyChecksums;
+    // Checksums for this solution state (access via LazyChecksums)
+    private AsyncLazy<ProjectStateChecksums>? _lazyChecksums;
 
-    private readonly AsyncLazy<Dictionary<ImmutableArray<byte>, DocumentId>> _lazyContentHashToDocumentId;
+    // Mapping from content has to document id (access via LazyContentHashToDocumentId)
+    private AsyncLazy<Dictionary<ImmutableArray<byte>, DocumentId>>? _lazyContentHashToDocumentId;
 
     /// <summary>
     /// Analyzer config options to be used for specific trees.
@@ -87,9 +86,6 @@ private ProjectState(
         // holding on. otherwise, these information will be held onto unnecessarily by projectInfo even after
         // the info has changed by DocumentState.
         ProjectInfo = ClearAllDocumentsFromProjectInfo(projectInfo);
-
-        _lazyChecksums = AsyncLazy.Create(static (self, cancellationToken) => self.ComputeChecksumsAsync(cancellationToken), arg: this);
-        _lazyContentHashToDocumentId = AsyncLazy.Create(static (self, cancellationToken) => self.ComputeContentHashToDocumentIdAsync(cancellationToken), arg: this);
     }
 
     public ProjectState(LanguageServices languageServices, ProjectInfo projectInfo, StructuredAnalyzerConfigOptions fallbackAnalyzerOptions)
@@ -128,9 +124,6 @@ public ProjectState(LanguageServices languageServices, ProjectInfo projectInfo,
         // the info has changed by DocumentState.
         // we hold onto the info so that we don't need to duplicate all information info already has in the state
         ProjectInfo = ClearAllDocumentsFromProjectInfo(projectInfoFixed);
-
-        _lazyChecksums = AsyncLazy.Create(static (self, cancellationToken) => self.ComputeChecksumsAsync(cancellationToken), arg: this);
-        _lazyContentHashToDocumentId = AsyncLazy.Create(static (self, cancellationToken) => self.ComputeContentHashToDocumentIdAsync(cancellationToken), arg: this);
     }
 
     public TextDocumentStates<TDocumentState> GetDocumentStates<TDocumentState>()
@@ -158,6 +151,38 @@ private async Task<Dictionary<ImmutableArray<byte>, DocumentId>> ComputeContentH
         return result;
     }
 
+    private AsyncLazy<ProjectStateChecksums> LazyChecksums
+    {
+        get
+        {
+            if (_lazyChecksums is null)
+            {
+                Interlocked.CompareExchange(
+                    ref _lazyChecksums,
+                    AsyncLazy.Create(static (self, cancellationToken) => self.ComputeChecksumsAsync(cancellationToken), arg: this),
+                    null);
+            }
+
+            return _lazyChecksums;
+        }
+    }
+
+    private AsyncLazy<Dictionary<ImmutableArray<byte>, DocumentId>> LazyContentHashToDocumentId
+    {
+        get
+        {
+            if (_lazyContentHashToDocumentId is null)
+            {
+                Interlocked.CompareExchange(
+                    ref _lazyContentHashToDocumentId,
+                    AsyncLazy.Create(static (self, cancellationToken) => self.ComputeContentHashToDocumentIdAsync(cancellationToken), arg: this),
+                    null);
+            }
+
+            return _lazyContentHashToDocumentId;
+        }
+    }
+
     private static ProjectInfo ClearAllDocumentsFromProjectInfo(ProjectInfo projectInfo)
     {
         return projectInfo
@@ -168,7 +193,7 @@ private static ProjectInfo ClearAllDocumentsFromProjectInfo(ProjectInfo projectI
 
     public async ValueTask<DocumentId?> GetDocumentIdAsync(ImmutableArray<byte> contentHash, CancellationToken cancellationToken)
     {
-        var map = await _lazyContentHashToDocumentId.GetValueAsync(cancellationToken).ConfigureAwait(false);
+        var map = await LazyContentHashToDocumentId.GetValueAsync(cancellationToken).ConfigureAwait(false);
         return map.TryGetValue(contentHash, out var documentId) ? documentId : null;
     }
 

src/Workspaces/Core/Portable/Workspace/Solution/ProjectState_Checksum.cs

@@ -16,17 +16,17 @@ namespace Microsoft.CodeAnalysis;
 internal sealed partial class ProjectState
 {
     public bool TryGetStateChecksums([NotNullWhen(true)] out ProjectStateChecksums? stateChecksums)
-        => _lazyChecksums.TryGetValue(out stateChecksums);
+        => LazyChecksums.TryGetValue(out stateChecksums);
 
     public Task<ProjectStateChecksums> GetStateChecksumsAsync(CancellationToken cancellationToken)
-        => _lazyChecksums.GetValueAsync(cancellationToken);
+        => LazyChecksums.GetValueAsync(cancellationToken);
 
     public Task<Checksum> GetChecksumAsync(CancellationToken cancellationToken)
     {
         return SpecializedTasks.TransformWithoutIntermediateCancellationExceptionAsync(
             static (lazyChecksums, cancellationToken) => new ValueTask<ProjectStateChecksums>(lazyChecksums.GetValueAsync(cancellationToken)),
             static (projectStateChecksums, _) => projectStateChecksums.Checksum,
-            _lazyChecksums,
+            LazyChecksums,
             cancellationToken).AsTask();
     }