Make sure the server is shut down before we try to start it up

Author: davidwengier

src/EditorFeatures/Core/LanguageServer/AbstractInProcLanguageClient.cs

@@ -110,10 +110,16 @@ internal abstract partial class AbstractInProcLanguageClient(
 
     public event AsyncEventHandler<EventArgs>? StartAsync;
 
+    public event AsyncEventHandler<EventArgs>? StopAsync;
+
     /// <summary>
-    /// Unused, implementing <see cref="ILanguageClient"/>
+    /// Stops the server if it has been started.
     /// </summary>
-    public event AsyncEventHandler<EventArgs>? StopAsync { add { } remove { } }
+    /// <remarks>
+    /// Per the documentation on <see cref="ILanguageClient.StopAsync"/>, the event is ignored if the server has not been started.
+    /// </remarks>
+    public Task StopServerAsync()
+        => StopAsync?.InvokeAsync(this, EventArgs.Empty) ?? Task.CompletedTask;
 
     public async Task<Connection?> ActivateAsync(CancellationToken cancellationToken)
     {

src/EditorFeatures/Core/LanguageServer/AlwaysActiveLanguageClientEventListener.cs

@@ -66,12 +66,15 @@ private void Load()
 
         async Task LoadAsync()
         {
-
             // Explicitly switch to the bg so that if this causes any expensive work (like mef loads) it 
             // doesn't block the UI thread. Note, we always yield because sometimes our caller starts
             // on the threadpool thread but is indirectly blocked on by the UI thread.
             await TaskScheduler.Default.SwitchTo(alwaysYield: true);
 
+            // Sometimes the editor can be slow to stop the old server instance when the old solution is closed, so we force it here.
+            // This will no-op if the server hasn't been started yet.
+            await _languageClient.StopServerAsync().ConfigureAwait(false);
+
             await _languageClientBroker.Value.LoadAsync(new LanguageClientMetadata(
             [
                 ContentTypeNames.CSharpContentType,