vaderpi's random rants

I have been using NMock in my daily test driven development for over a year now. In that time, not much development has taken place. I mentioned in a previous entry that NMock lacks support for events. It also has a few other bugs that have been stacking up over the years. Yet the mailing list has had very little traffic.

Since I found NMock, several other .Net or C#mock object libraries have popped up. I have taken a good look at all of the free ones, and even though NMock has some problems, none of them really impressed me enough to switch away from NMock.

Earlier this week, I ran into a bug with NMock that without a fix, would have caused me to alter one of my designs in order to use it. NMock was unable to mock items in an interface’s parent interface. Traffic on the mailing list suggested that the bug had been fixed or worked around, yet no one had posted any details.

This discussion brought up the mention of a replacement for NMock that the developers had developed based on experiences with jMock. This replacement was checked into the NMock source tree at the end of June under the name NMock2.

When I hit this bug, I was experiencing crunch time here at work. I made request to the list for an NMock fix for the problem, but I really did not have time to wait arround for an answer. So I checked out NMock2 from the SourceForge CVS repository, and started reading the acceptance tests to see how to use it.

What I read really, really impressed me. The mock code that is produced when you use NMock2 reads very well.

I wrote a quick addition to the acceptance tests to see if my bug was there, and I was presented with the Glorious Green Bar of Success. I then added the binary to my project and used for the spots that that NMock did not work with.

Turns out that NMock2 had a different bug related to what I was doing, but this only took me a few minutes to fix. First, I wrote up a quick test that demonstrated the problem, and then I got to work. After a few minutes of tracing and debugging, I found the problem. NMock2 uses Type.GetMethod to get the methods that belong to a type. Even though the documentation suggests otherwise, Type.GetMethod does not get methods from an interface’s parent interfaces. Hmm… this sounds like the bug in NMock as well. My first test must have just sucked. I added a call to Type.GetInterfaces and then recursively called them to look for methods. This worked really well.

I never really dove into the NMock code all that much, but the code for NMock2 is really easy to follow. Furthermore, because of the test driven way that it was developed, I have a pretty high confidence that I did not break any existing functionality.

To make this entry even longer, I am going to share some snippets for code from both implementations that do the same thing. For both of these the following interface will be used to test the ComputeTotal method on a ShoppingCart object. (+10 points if you can figure out what that method does :))

public interface Item {
  double Price { get; }
}

First is the old NMock way of doing things.

[Test]
public void ComputeTotal() {
  // Create the mock object for item1 and have it return a price of 100
  DynamicMock item1Mock = new DynamicMock(typeof(Item));
  item1Mock.Strict = true;
  item1Mock.ExpectAndReturn("Price", 100);
  Item item1 = (Item) item1Mock.MockInstance;

  // Create the mock object for item2 and have it return a price of 50
  DynamicMock item2Mock = new DynamicMock(typeof(Item));
  item2Mock.Strict = true;
  item2Mock.ExpectAndReturn("Price", 50);
  Item item2 = (Item) item2Mock.MockInstance;

  // Add the items to the cart
  ShoppingCart cart = new ShoppingCart();
  cart.Add(item1);
  cart.Add(item2);

  // Get the total and make sure it is 150 (100 + 50)
  int total = cart.ComputeTotal();
  Assert.AreEqual(150, total);

  // Verify that all the expects have been met
  item1Mock.Verify();
  item2Mock.Verify();

}

And now here is the NMock2 way.

[Test]
public void ComputeTotal() {
  Mockery mock = new Mockery();
  // Create the mock object for item1 and have it return a price of 100
  Item item1 = (Item) mock.NewMock(typeof(Item));
  Expect.Once.On(item1).GetProperty("Price").Will(Return.Value(100));

  // Create the mock object for item2 and have it return a price of 50
  Item item2 = (Item) mock.NewMock(typeof(Item));
  Expect.Once.On(item2).GetProperty("Price").Will(Return.Value(50));

  // Add the items to the cart
  ShoppingCart cart = new ShoppingCart();
  cart.Add(item1);
  cart.Add(item2);

  // Get the total and make sure it is 150 (100 + 50)
  int total = cart.ComputeTotal();
  Assert.AreEqual(150, total);

  // Verify that all the expects have been met
  mock.VerifyAllExpectationsHaveBeenMet();

}

Not only is the code shorter, but because of the way that it reads, the comments that I have added for clarity have almost become irrelevant.

I encourage you to go grab NMock2 and give it a shot. I think that it works really well, and I am in the process of replacing NMock with NMock2. It just reads too well not to. There are other pluses about NMock2 that I have not mentioned in this post, like support for events, but I will save that for another time.

I hope that you enjoyed this post, and that you learned something. For you non developers that read this, Wow. Kudos to you for reading this far! If you want, I will give you a special prize. :)

This entry was posted on Thursday, June 9th, 2005 at 11:16 am and is filed under . You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.