等待不会在主线程上恢复

Question

Background

I faced this problem a couple of years ago and got this very helpful answer from Stephen Cleary. Problem with VSTO add-ins is that they do not set a SynchronizationContext and therefore async calls do not resume on UI thread, causing all sorts of cross-thread access troubles. The solution, as he mentioned is to manually call SetSynchronizationContext before calling any async function.

I have been using this technique since then and thought that was all there was to it. But today I have seen a situation where even manually setting the context does not force it to resume on calling thread.

Situation

My VSTO add-in contains a WPF pane (inside a CustomTaskPane) which is bound to its ViewModel that contains several AsyncRelayCommand properties (from WCT). One of these commands calls my Data Service which in turn calls a RestSharp methods to fetch data from the API server.

All these calls use async / await and all these call use ConfigureAwait(false) except the one at the top level (ie the command itself). Here is a snapshot of how this call-site looks like:

As you can see, I have manually called SetSynchronizationContext before doing the await call. It also shows that SynchronizationContext.Current is set after it resumes after the await call, but somehow the code is still running on the worker thread. I also verified that the code was running on UI thread when it hit line 259 before drilling down into the await call.

I have already spent a lot of time and effort on this and can't make any sense of it. Can anyone help me figure out if I'm missing something obvious?

Answer 1

You need to use an instance of the WindowsFormsSynchronizationContext class instead.

The WindowsFormsSynchronizationContext class provides a synchronization mechanism for Windows Forms.

The DispatcherSynchronizationContext class is for WPF applications which uses two threads. One thread is background thread for rendering and the other thread is for UI. So, UI elements in one thread are not accessible to other UI threads. Microsoft introduced the dispatcher which is responsible for multiple UI threads interaction.