What is Object Mocking and when do I need it?

Many people use Mock Objects when they are writing unit tests. What is a Mock Object? Why would I ever need one? Do I need a Mock Object Framework?

Object Mocking is used to keep dependencies out of your unit test. Sometimes you'll have a test like "SelectPerson" which will select a person from the database and return a Person object.

To do this, you would normally need a dependency on the database, however with object mocking you can simulate the interaction with the database with a mock framework, so it might return a dataset which looks like one returned from the database and you can then test your code to ensure that it handles translating a dataset to a person object, rather than using it to test that a connection to the database exists.

Several people have already answered the 'what', but here are a couple of quick 'whys' that I can think of:

1. Performance

   Because unit tests should be fast, testing a component that interacts with a network, a database, or other time-intensive resource does not need to pay the penalty if it's done using mock objects. The savings add up quickly.

2. Collaboration

   If you are writing a nicely encapsulated piece of code that needs to interact with someone else's code (that hasn't been written yet, or is in being developed in parallel - a common scenario), you can exercise your code with mock objects once an interface has been agreed upon. Otherwise your code may not begin to be tested until the other component is finished.