Task ContinueWith() Not Updating Cursor on UI Thread

Question

I'm working on a WPF app and I simply want to alter the cursor before and after the task runs. I have this code:

this.Cursor = Cursors.Wait;

Task.Factory.StartNew(() => PerformMigration(legacyTrackerIds)).ContinueWith(_ => this.Cursor = Cursors.Arrow);

The cursor indeed changes to the wait cursor, but it doesn't change back to the arrow when the task is done. If I put a breakpoint in the ContinueWith() method, it gets hit. But the cursor doesn't change back to an arrow. Why?

This was the old way I was trying it. The cursor changed back to an arrow, but I don't want to Wait() for the task.

this.Cursor = Cursors.Wait;

Task.Factory.StartNew(() => PerformMigration(legacyTrackerIds)).Wait();

this.Cursor = Cursors.Arrow;

Answer 1

The cursor change needs to be done on the UI thread. You can either use the the overload of ContinueWith that takes a task scheduler:

var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext(); 

Task.Factory
  .StartNew(() => PerformMigration(legacyTrackerIds))
  .ContinueWith(_ => this.Cursor = Cursors.Arrow, uiScheduler);

Or use the Dispatcher.Invoke method:

Task.Factory
  .StartNew(() => PerformMigration(legacyTrackerIds))
  .ContinueWith(_ => { Dispatcher.Invoke(() => { this.Cursor = Cursors.Arrow; }); });

Answer 2

I think you need to use the correct synchronization context:

this.Cursor = Cursors.Wait; 

var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext()); 

Task.Factory.StartNew(() => PerformMigration(legacyTrackerIds))
            .ContinueWith(_ => this.Cursor = Cursors.Arrow, uiScheduler);

Answer 3

The problem is that the continuation needs to run in the UI thread. Currently it's being done in a background thread.

Add TaskScheduler.FromCurrentSynchronizationContext() to the second parameter of ContinueWith to have it run in the UI thread.