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 te

Answer 1

I accomplished this by wrapping Dispatcher in my own IDispatcher interface, and then using Moq to verify the call to it was made.

IDispatcher interface:

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

Real dispatcher implementation:

class RealDispatcher : IDispatcher
{
    private readonly Dispatcher _dispatcher;

    public RealDispatcher(Dispatcher dispatcher)
    {
        _dispatcher = dispatcher;
    }

    public void BeginInvoke(Delegate method, params object[] args)
    {
        _dispatcher.BeginInvoke(method, args);
    }
}

Initializing dispatcher in your class under test:

public ClassUnderTest(IDispatcher dispatcher = null)
{
    _dispatcher = dispatcher ?? new UiDispatcher(Application.Current?.Dispatcher);
}

Mocking the dispatcher inside unit tests (in this case my event handler is OnMyEventHandler and accepts a single bool parameter called myBoolParameter)

[Test]
public void When_DoSomething_Then_InvokeMyEventHandler()
{
    var dispatcher = new Mock();

    ClassUnderTest classUnderTest = new ClassUnderTest(dispatcher.Object);

    Action OnMyEventHanlder = delegate (bool myBoolParameter) { };
    classUnderTest.OnMyEvent += OnMyEventHanlder;

    classUnderTest.DoSomething();

    //verify that OnMyEventHandler is invoked with 'false' argument passed in
    dispatcher.Verify(p => p.BeginInvoke(OnMyEventHanlder, false), Times.Once);
}