MVVMLight command CanExecute not firing after async call

I'm using MVVMLight in a WPF4 project, and have set up an ApplyCommand and an UndoCommand, which are bound to buttons on the view. All standard stuff, done many times. When the user clicks a button, a WCF service method is called, and a local variable is updated. The UndoCommandCanExecute method uses this local variable to determine if the UndoCommand can be run.

This works fine if I make the WCF service call synchronously. However, if I use an async call, the CanExecute methods don't get fired, and so the undo button is not enabled/disabled correctly.

Here is a simplified version of a private method on the view model that is called by both commands. In the real code, parameters are passed in, which are used in the service call.

  private void CallServiceMethod() {
    IsBusy = true;
    Task t = new Task(() => {
      Thread.Sleep(2000); // would really call a WCF service
      SetUndoMessage();
      IsBusy = false;
    });
    t.Start();
  }

As noted, the WCF service call has been replaced by a call to Thread.Sleep for simplicity. The SetUndoMessage() method sets the local variable which the UndoCommandCanExecute method uses. IsBusy is a bool bound to a Telerik busy indicator on the view, so the user can see that something is happening.

When this is run, the undo button is not enabled or disabled correctly until you click somewhere on the window. I added Debug.WriteLine statements all over the place, and I could see that the UndoCommandCanExecute method was not being called when the task callback ended.

I tried adding the following two lines at the end of the task callback, but it didn't help...

    CommandManager.InvalidateRequerySuggested();
    UndoCommand.RaiseCanExecuteChanged();

If I remove the Thread.Sleep, then it works fine, which sounds like it's the delay that's the problem. This method is ending before the task has ended, and so the UndoCommandCanExecute method isn't firing. I don't understand why it doesn't fire when I call its RaiseCanExecuteChanged method or the CommandManager.

I tried adding t.Wait() right after I started the thread, but then the busy indicator didn't show up until after the service call had ended, negating the whole purpose.

Anyone any ideas how to fix this? I want to keep the async call, as that allows me to use the busy indicator, which is standard throughout this large application, and also gives the user some feedback that something is happening.

I'm stuck using C# 4 by the way, so the new async features of C# 5 aren't available to me, even if they would solve the problem.

Thanks for any help.

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