单元测试在事件触发时在类上执行void方法

Question

Suppose you had 2 classes, Listener and Talker. Talker has an event Talking, and when this event is fired, Listener should execute a void method HeardTalk, along these lines:

public class Talker
{
   public event EventHandler Talking;

   public void Talk()
   {
      if (Talking != null)
      {
         Talking(this, null);
      }
   }
}

public class Listener
{
   public void StartListening(Talker talker)
   {
      talker.Talking += HeardTalk;
   }

   public void HeardTalk(object sender, EventArgs e)
   {
      // do something private here
   }
}

How would you go about unit testing that once StartListening has been called, HeardTalk gets called, if there was no public state reflecting that the method was called? I could just add such a state for the purpose of validation, but it seems clumsy. Ideally, I would like to assert the call was made, in a manner similar to what Mocking frameworks do, but I can't mock the class under test.

Is there an elegant way to assert that a method was called on the SUT, without modifying it just for the purpose of testability?

Answer 1

There must be at least some measurable side effect from the event being triggered. Perhaps you are logging the event, saving something to a data-store, or changing the internal state of your class.

If you are calling to any external dependencies those should be passed in via constructor/property injection and mocked. In the case of changing internal state then even doing that must have some side effect that can be observed externally. Otherwise what would be the point?

In the end if all you want to check is that the method got called, then what are you actually testing? Again, there must be a reason that method gets called, and that reason has to have some impact that can be observed somewhere...

Answer 2

You can do such things easily with the commercial Typemock Isolator , which allows you to selectively 'mock' individual methods on 'real' objects.
The test would look like this:

[Test, Isolated]
public void HeardTalk_GetsCalled()
{
    // --- Arrange ---
    var talker = new Talker();
    var listener = new Listener();
    bool heardTalkWasCalled = false;
    Isolate.WhenCalled(() => listener.HeardTalk(null, null))                     // Selectively 'mock' the call to 'listener.HeardTalk()'
                                     .DoInstead(x => heardTalkWasCalled = true); // on the live object (arguments are ignored by default)

    // --- Act ---
    listener.StartListening(talker);
    talker.Talk();

    // --- Assert ---
    Assert.IsTrue(heardTalkWasCalled);
}

Admittedly, Typemock Isolator comes with some license costs. But if you're serious about testing and need to test a lot of stuff like the above, it's worth every penny because of its strength and flexibility.

Notes:

• You can do the same with the free MS Moles framework (for Visual Studio 2010), which is a suitable alternative to Typemock, if you have to do such tests only occasionally. It generally requires somewhat more work/code, adds more complexity to the tests, and is more performance-demanding especially during compilation, but is sufficient for smaller test suites.
• There is also a commercial alternative from Telerik called JustMock . Because it is quite new, I can't say nothing about it except that it exists...

HTH!
Thomas