Disable reference conversion during Solution Close operations

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -93,6 +93,11 @@ internal abstract partial class VisualStudioWorkspaceImpl : VisualStudioWorkspac
         /// </summary>
         private ImmutableHashSet<DocumentId> _documentsNotFromFiles = ImmutableHashSet<DocumentId>.Empty;
 
+        /// <summary>
+        /// Indicates whether the current solution is closing.
+        /// </summary>
+        private bool _solutionClosing;
+
         [Obsolete("This is a compatibility shim for TypeScript; please do not use it.")]
         internal VisualStudioProjectTracker? _projectTracker;
 
@@ -185,7 +190,10 @@ internal void SubscribeExternalErrorDiagnosticUpdateSourceToSolutionBuildEvents(
         public async Task InitializeUIAffinitizedServicesAsync(IAsyncServiceProvider asyncServiceProvider)
         {
             // Create services that are bound to the UI thread
-            await _threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync();
+            await _threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(_threadingContext.DisposalToken);
+
+            var solutionClosingContext = UIContext.FromUIContextGuid(VSConstants.UICONTEXT.SolutionClosing_guid);
+            solutionClosingContext.UIContextChanged += (_, e) => _solutionClosing = e.Activated;
 
             var openFileTracker = await OpenFileTracker.CreateAsync(this, asyncServiceProvider).ConfigureAwait(true);
 
@@ -1815,21 +1823,32 @@ public void RemoveProjectOutputPath(ProjectId projectId, string outputPath)
                 projectReferenceInformation.OutputPaths.Remove(outputPath);
                 _projectsByOutputPath.MultiRemove(outputPath, projectId);
 
-                if (_projectsByOutputPath.TryGetValue(outputPath, out var remainingProjectsForOutputPath))
+                // When a project is closed, we may need to convert project references to metadata references (or vice
+                // versa). Failure to convert the references could leave a project in the workspace with a project
+                // reference to a project which is not open.
+                //
+                // For the specific case where the entire solution is closing, we do not need to update the state for
+                // remaining projects as each project closes, because we know those projects will be closed without
+                // further use. Avoiding reference conversion when the solution is closing improves performance for both
+                // IDE close scenarios and solution reload scenarios that occur after complex branch switches.
+                if (!_solutionClosing)
                 {
-                    var distinctRemainingProjects = remainingProjectsForOutputPath.Distinct();
-                    if (distinctRemainingProjects.Count() == 1)
+                    if (_projectsByOutputPath.TryGetValue(outputPath, out var remainingProjectsForOutputPath))
+                    {
+                        var distinctRemainingProjects = remainingProjectsForOutputPath.Distinct();
+                        if (distinctRemainingProjects.Count() == 1)
+                        {
+                            // We had more than one project outputting to the same path. Now we're back down to one
+                            // so we can reference that one again
+                            ConvertMetadataReferencesToProjectReferences_NoLock(distinctRemainingProjects.Single(), outputPath);
+                        }
+                    }
+                    else
                     {
-                        // We had more than one project outputting to the same path. Now we're back down to one
-                        // so we can reference that one again
-                        ConvertMetadataReferencesToProjectReferences_NoLock(distinctRemainingProjects.Single(), outputPath);
+                        // No projects left, we need to convert back to metadata references
+                        ConvertProjectReferencesToMetadataReferences_NoLock(projectId, outputPath);
                     }
                 }
-                else
-                {
-                    // No projects left, we need to convert back to metadata references
-                    ConvertProjectReferencesToMetadataReferences_NoLock(projectId, outputPath);
-                }
             }
         }