Initialize MEF services before switching to the main thread

Fixes dotnet#54159

src/VisualStudio/Core/Impl/CodeModel/CodeModelState.cs

@@ -38,6 +38,8 @@ public CodeModelState(
  Debug.Assert(languageServices != null);
  Debug.Assert(workspace != null);
 
+ // ⚠ This code runs on the main thread. Language services accessed here should be preloaded in
+ // ProjectCodemodelFactory to avoid blocking MEF operations.
  this.ThreadingContext = threadingContext;
  this.ServiceProvider = serviceProvider;
  this.CodeModelService = languageServices.GetService<ICodeModelService>();

src/VisualStudio/Core/Impl/CodeModel/ProjectCodeModelFactory.cs

@@ -7,15 +7,18 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.ComponentModel.Composition;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.Editor;
+using Microsoft.CodeAnalysis.CodeGeneration;
 using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
-using Microsoft.CodeAnalysis.PooledObjects;
+using Microsoft.CodeAnalysis.LanguageServices;
 using Microsoft.CodeAnalysis.Shared.TestHooks;
 using Microsoft.CodeAnalysis.SolutionCrawler;
 using Microsoft.VisualStudio.Shell;
+using Microsoft.VisualStudio.Threading;
 using Roslyn.Utilities;
 using Task = System.Threading.Tasks.Task;
 
@@ -76,6 +79,27 @@ private async Task ProcessNextDocumentBatchAsync(
  const int MaxTimeSlice = 15;
  var delayBetweenProcessing = TimeSpan.FromMilliseconds(50);
 
+ Debug.Assert(!_threadingContext.JoinableTaskContext.IsOnMainThread, "The following context switch is not expected to cause runtime overhead.");
+ await TaskScheduler.Default;
+
+ // Ensure MEF services used by the code model are initially obtained on a background thread.
+ // This code avoids allocations where possible.
+ // https://github.com/dotnet/roslyn/issues/54159
+ string? previousLanguage = null;
+ foreach (var (_, projectState) in _visualStudioWorkspace.CurrentSolution.State.ProjectStates)
+ {
+ if (projectState.Language == previousLanguage)
+ {
+ // Avoid duplicate calls if the language did not change
+ continue;
+ }
+
+ previousLanguage = projectState.Language;
+ projectState.LanguageServices.GetService<ICodeModelService>();
+ projectState.LanguageServices.GetService<ISyntaxFactsService>();
+ projectState.LanguageServices.GetService<ICodeGenerationService>();
+ }
+
  await _threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
 
  var stopwatch = SharedStopwatch.StartNew();