Pete On Software

RSS Feed

Intro to NMock

If you do Test Driven Development (TDD) for any amount of time and with any amount of serious effort you are going to run across a problem. Sometimes you can’t test one object without other objects being involved. Since you can’t properly test one object without isolating it, you need to fake or mock the others that you aren’t testing. The Wikipedia entry on Mock Objects discusses it in some more detail.

A good candidate for a mock are objects that return current information (and as such are hard to properly test) or objects that will make permanent changes to databases, for instance, and as such are also poor candidates for repeatable testing.

The first (and only) mocking framework that I’ve used is NMock. I chose it because my rule is “when in doubt, choose the product that is named ‘n’ and then what you are doing” (remember my NLog and NUnit posts?).

Examine the code below and my comments about what each section is doing.

This outputs the following to the output window:

However, check out the following code:

This returns the following exception:

One application of how that could be useful would be if you are testing your business tier to make sure that it does not call Save() on an object or into the DataTier more than one time per invocation. There are other options, as well. You can say Expect.AtLeastOnce or Expect.AtLeast(some int) or Expect.AtMost or Expect.Between or Expect.Never. The framework is so flexible. You can define how you want the mock object to behave depending on its inputs.

Examine the following code that makes the SavePost method return true if passed 1 and false if passed -1.

As you can see, NMock is flexible and can be very useful to a programmer who is doing unit tests. Next time, I’d like to take a look at Dependency Injection / Inversion of Control and how it can be used with Mocking to not only make your code flexible, but very testable.

One Comment

Urs Enzler - NMock2 dev team  on December 22nd, 2008

Nice article Pete.

Some comments about your sample code:

IBlogEntry mockBlogEntry = (IBlogEntry) mockery.NewMock(typeof (IBlogEntry));

you can use

IBlogEntry mockBlogEntry = mockery.NewMock();


WithNoArguments is not needed if this is the only overload.

If you want to have a look at the latest and (hopefully) greatest version then have a look at which is based on the version from and offers a bunch of new functionality.

Happy mocking

Leave a Comment