Unit testing that an event is raised in C#, using reflection

A solution in the style you propose that covers ALL cases will be extremely difficult to implement. But if you're willing to accept that delegate types with ref and out parameters or return values won't be covered, you should be able to use a DynamicMethod.

At design time, create a class to hold the count, lets call it CallCounter.

In AssertRaisesEvent:

• create an instance of your CallCounterclass, keeping it in a strongly typed variable

• initialize the count to zero

• construct a DynamicMethod in your counter class

  new DynamicMethod(string.Empty, typeof(void), parameter types extracted from the eventInfo, typeof(CallCounter))

• get the DynamicMethod's MethodBuilder and use reflection.Emit to add the opcodes for incrementing the field

  • ldarg.0 (the this pointer)
  • ldc_I4_1 (a constant one)
  • ldarg.0 (the this pointer)
  • ldfld (read the current value of the count)
  • add
  • stfld (put the updated count back into the member variable)

• call the two-parameter overload of CreateDelegate, first parameter is the event type taken from eventInfo, second parameter is your instance of CallCounter

• pass the resulting delegate to eventInfo.AddEventHandler (you've got this) Now you're ready to execute the test case (you've got this).

• finally read the count in the usual way.

The only step I'm not 100% sure how you'd do is getting the parameter types from the EventInfo. You use the EventHandlerType property and then? Well, there's an example on that page showing that you just grab the MethodInfo for the Invoke method of the delegate (I guess the name "Invoke" is guaranteed somewhere in the standard) and then GetParameters and then pull out all the ParameterType values, checking that there are no ref/out parameters along the way.

I recently wrote a series of blog posts on unit testing event sequences for objects that publish both synchronous and asynchronous events. The posts describe a unit testing approach and framework, and provides the full source code with tests.

I describe the implementation of an "event monitor" which allows writing event sequencing unit tests to be written more cleanly i.e. getting rid of all the messy boilerplate code.

Using the event monitor described in my article, tests can be written like so:

var publisher = new AsyncEventPublisher();

Action test = () =>
{
    publisher.RaiseA();
    publisher.RaiseB();
    publisher.RaiseC();
};

var expectedSequence = new[] { "EventA", "EventB", "EventC" };

EventMonitor.Assert(publisher, test, expectedSequence);

Or for a type that implements INotifyPropertyChanged:

var publisher = new PropertyChangedEventPublisher();

Action test = () =>
{
    publisher.X = 1;
    publisher.Y = 2;
};

var expectedSequence = new[] { "X", "Y" };

EventMonitor.Assert(publisher, test, expectedSequence);

And for the case in the original question:

MyClass myObject = new MyClass();
EventMonitor.Assert(myObject, () => { myObject.Width = 42; }, "Width");

The EventMonitor does all the heavy lifting and will run the test (action) and assert that events are raised in the expected sequence (expectedSequence). It also prints out nice diagnostic messages on test failure. Reflection and IL are used under the hood to get the dynamic event subscription working, but this is all nicely encapsulated, so only code like the above is required to write event tests.

There's a lot of detail in the posts describing the issues and approaches, and source code too:

http://gojisoft.com/blog/2010/04/22/event-sequence-unit-testing-part-1/

With lambdas you can do this with very little code. Just assign a lambda to the event, and set a value in the handler. No need for reflection and you gain strongly typed refactoring

[TestFixture]
public class TestClass
{
    [Test]
    public void TestEventRaised()
    {
        // arrange
        var called = false;

        var test = new ObjectUnderTest();
        test.WidthChanged += (sender, args) => called = true;

        // act
        test.Width = 42;

        // assert
        Assert.IsTrue(called);
    }

    private class ObjectUnderTest
    {
        private int _width;
        public event EventHandler WidthChanged;

        public int Width
        {
            get { return _width; }
            set
            {
                _width = value; OnWidthChanged();
            }
        }

        private void OnWidthChanged()
        {
            var handler = WidthChanged;
            if (handler != null)
                handler(this, EventArgs.Empty);
        }
    }
}