Using the WPF Dispatcher in unit tests

Question

I'm having trouble getting the Dispatcher to run a delegate I'm passing to it when unit testing. Everything works fine when I'm running the program, but, during a unit test the following code will not run:

this.Dispatcher.BeginInvoke(new ThreadStart(delegate {     this.Users.Clear();      foreach (User user in e.Results)     {         this.Users.Add(user);     } }), DispatcherPriority.Normal, null); 

I have this code in my viewmodel base class to get a Dispatcher:

if (Application.Current != null) {     this.Dispatcher = Application.Current.Dispatcher; } else {     this.Dispatcher = Dispatcher.CurrentDispatcher; } 

Is there something I need to do to initialise the Dispatcher for unit tests? The Dispatcher never runs the code in the delegate.

Answer 1

By using the Visual Studio Unit Test Framework you don’t need to initialize the Dispatcher yourself. You are absolutely right, that the Dispatcher doesn’t automatically process its queue.

You can write a simple helper method “DispatcherUtil.DoEvents()” which tells the Dispatcher to process its queue.

C# Code:

public static class DispatcherUtil {     [SecurityPermissionAttribute(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]     public static void DoEvents()     {         DispatcherFrame frame = new DispatcherFrame();         Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background,             new DispatcherOperationCallback(ExitFrame), frame);         Dispatcher.PushFrame(frame);     }      private static object ExitFrame(object frame)     {         ((DispatcherFrame)frame).Continue = false;         return null;     } } 

You find this class too in the WPF Application Framework (WAF).

Answer 2

We've solved this issue by simply mocking out the dispatcher behind an interface, and pulling in the interface from our IOC container. Here's the interface:

public interface IDispatcher {     void Dispatch( Delegate method, params object[] args ); } 

Here's the concrete implementation registered in the IOC container for the real app

[Export(typeof(IDispatcher))] public class ApplicationDispatcher : IDispatcher {     public void Dispatch( Delegate method, params object[] args )     { UnderlyingDispatcher.BeginInvoke(method, args); }      // -----      Dispatcher UnderlyingDispatcher     {         get         {             if( App.Current == null )                 throw new InvalidOperationException("You must call this method from within a running WPF application!");              if( App.Current.Dispatcher == null )                 throw new InvalidOperationException("You must call this method from within a running WPF application with an active dispatcher!");              return App.Current.Dispatcher;         }     } } 

And here's a mock one that we supply to the code during unit tests:

public class MockDispatcher : IDispatcher {     public void Dispatch(Delegate method, params object[] args)     { method.DynamicInvoke(args); } } 

We also have a variant of the MockDispatcher which executes delegates in a background thread, but it's not neccessary most of the time