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

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