Don't get IVsAsyncFileChangeEx on background thread

`VisualStudioFileChangeTrackerFactory` retrieves an `IVsAsyncFileChangeEx` service in its constructor. Because this is an `[ImportingConstructor]` it will likely be called on the thread pool and implicitly marshal to the UI thread to perform a QI as part of retrieving the service. This can cause a hang if the UI thread is tied up with other work, such as when a solution is loaded.

This change avoids retrieving `IVsAsyncFileChangeEx` until its needed. This does block on retrieving the service. However, avoiding that will require a more substantial change that I'll follow up with on `main`.

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/Documents/VisualStudioFileChangeTrackerFactory.cs

@@ -13,10 +13,10 @@ namespace Microsoft.VisualStudio.Editor.Razor.Documents;
 [Export(typeof(IFileChangeTrackerFactory))]
 internal class VisualStudioFileChangeTrackerFactory : IFileChangeTrackerFactory
 {
-    private readonly IErrorReporter _errorReporter;
-    private readonly IVsAsyncFileChangeEx _fileChangeService;
-    private readonly ProjectSnapshotManagerDispatcher _projectSnapshotManagerDispatcher;
+    private readonly ProjectSnapshotManagerDispatcher _dispatcher;
     private readonly JoinableTaskContext _joinableTaskContext;
+    private readonly IErrorReporter _errorReporter;
+    private readonly JoinableTask<IVsAsyncFileChangeEx> _getFileChangeServiceTask;
 
     [ImportingConstructor]
     public VisualStudioFileChangeTrackerFactory(
@@ -25,10 +25,17 @@ internal class VisualStudioFileChangeTrackerFactory : IFileChangeTrackerFactory
         ProjectSnapshotManagerDispatcher dispatcher,
         IErrorReporter errorReporter)
     {
-        _errorReporter = errorReporter;
-        _fileChangeService = (IVsAsyncFileChangeEx)serviceProvider.GetService(typeof(SVsFileChangeEx));
-        _projectSnapshotManagerDispatcher = dispatcher;
+        _dispatcher = dispatcher;
         _joinableTaskContext = joinableTaskContext;
+        _errorReporter = errorReporter;
+
+        var jtf = _joinableTaskContext.Factory;
+        _getFileChangeServiceTask = jtf.RunAsync(async () =>
+        {
+            await jtf.SwitchToMainThreadAsync();
+
+            return (IVsAsyncFileChangeEx)serviceProvider.GetService(typeof(SVsFileChangeEx));
+        });
     }
 
     public IFileChangeTracker Create(string filePath)
@@ -38,6 +45,9 @@ internal class VisualStudioFileChangeTrackerFactory : IFileChangeTrackerFactory
             throw new ArgumentException(SR.ArgumentCannotBeNullOrEmpty, nameof(filePath));
         }
 
-        return new VisualStudioFileChangeTracker(filePath, _errorReporter, _fileChangeService, _projectSnapshotManagerDispatcher, _joinableTaskContext);
+        // TODO: Make IFileChangeTrackerFactory.Create(...) asynchronous to avoid blocking here.
+        var fileChangeService = _getFileChangeServiceTask.Join();
+
+        return new VisualStudioFileChangeTracker(filePath, _errorReporter, fileChangeService, _dispatcher, _joinableTaskContext);
     }
 }