Bug 5021 - BackgroundWorker RunWorkerCompleted does not run within UI thread
Summary: BackgroundWorker RunWorkerCompleted does not run within UI thread
Alias: None
Product: Android
Classification: Xamarin
Component: Mono runtime / AOT Compiler ()
Version: 4.1.x
Hardware: PC Windows
: Low normal
Target Milestone: ---
Assignee: Jonathan Pryor
Depends on:
Reported: 2012-05-11 00:05 UTC by Rob
Modified: 2012-06-01 14:33 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Rob 2012-05-11 00:05:39 UTC
attempting to access any UI objects from Main Activity from a BackgroundWorker RunWorkerCompleted event causes an unknown exception. Without accessing UI objects , no exception is thrown. This leads me to believe RunWorkerCompleted is not executed within the UI thread.

public class MainActivity : Activity
   private ProgressDialog progressDialog;
   protected override void OnCreate(Bundle bundle)
      Button searchButton = FindViewById<Button>(Resource.Id.SubmitButton);
      searchButton.Click += new EventHandler(searchButton_Click);
   void searchButton_Click(object sender, EventArgs e)
    this.progressDialog = ProgressDialog.Show(this, "Search", "Please Wait...");
    // find information 
    BackgroundWorker worker = new BackgroundWorker();
    worker.DoWork += new DoWorkEventHandler(worker_DoWork);
    worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);

    void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
           //******** CRASH BELOW ***********//

    void worker_DoWork(object sender, DoWorkEventArgs e)
      // do stuff
Comment 1 Jonathan Pryor 2012-05-21 22:39:23 UTC
The problem is that we haven't done any integration between .NET's SynchronizationContext and Android's main thread.

We need to make System.Threading.SynchronizationContext.Current, in effect, return the Android.App.Application.SynchronizationContext instance:


That will allow (in this case) AsyncOperationManager.SynchronizationContext calls to be invoked on the UI thread, as expected.
Comment 2 Jonathan Pryor 2012-06-01 14:33:18 UTC
Fixed in commit 45ea51bc.