I want to test that setting a certain property (or more generally, executing some code) raises a certain event on my object. In that respect my problem is similar to Unit testing that an event is raised in C#, but I need a lot of these tests and I hate boilerplate. So I'm looking for a more general solution, using reflection.
Ideally, I would like to do something like this:
[TestMethod]
public void TestWidth() {
MyClass myObject = new MyClass();
AssertRaisesEvent(() => { myObject.Width = 42; }, myObject, "WidthChanged");
}
For the implementation of the AssertRaisesEvent
, I've come this far:
private void AssertRaisesEvent(Action action, object obj, string eventName)
{
EventInfo eventInfo = obj.GetType().GetEvent(eventName);
int raisedCount = 0;
Action incrementer = () => { ++raisedCount; };
Delegate handler = /* what goes here? */;
eventInfo.AddEventHandler(obj, handler);
action.Invoke();
eventInfo.RemoveEventHandler(obj, handler);
Assert.AreEqual(1, raisedCount);
}
As you can see, my problem lies in creating a Delegate
of the appropriate type for this event. The delegate should do nothing except invoke incrementer
.
Because of all the syntactic syrup in C#, my notion of how delegates and events really work is a bit hazy. This is also the first time I dabble in reflection. What's the missing part?
This is an old thread but I think it is still a relevant problem to which I have not found a simple solution. At work, I needed something to do event testing and since there was no satisfying existing solution I decided to implement a small library. Since I am quite happy with my solution I have decided to put it on GitHub. Maybe you find it helpful too:
https://github.com/f-tischler/EventTesting
Here is what it looks like in action:
How about this:
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:
Or for a type that implements INotifyPropertyChanged:
And for the case in the original question:
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
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:
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
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.