Creating and using a mock object is semantically very much like using a stub, except that a mock will do a little more than a stub - it will save the history of communication so that it can later be verified. Let's add a new requirement to our LogAnalyzer class. It will have to interact with an external web service that will receive an error message whenever the LogAnalyzer encounters a filename whose length is not too long.

[img_assist|nid=5444|title=|desc=|link=url|url=http://www.manning.com/osherove/|align=left|width=142|height=178]Unfortunately, the web service we'd like to test against is still not fully functional, and even if it were, it would have taken too long to use it as part of our tests. Because of that, we'll refactor our design and create a new interface that we can use to later create a mock object of. The interface will have the methods we will need to call on our web service, and nothing else.

Figure 1 shows how the test will work with our MockWebService.

[img_assist|nid=5443|title=|desc=|link=none|align=none|width=485|height=350]

First off, let's extract a simple interface that we can use in our code under test, instead of talking directly to the web service.

public interface IWebService
{
void LogError(string message);
}

This interface will serve us when we want to create stubs as well as mocks, without needing to have an external dependency on a project we have no control of. Next, we'll create the mock object itself. It may look like a stub, but it contains one little bit of extra code that makes it a mock object.

public class MockService:IWebService
{
	public string LastError;
	public void LogError(string message)
	{
	LastError = message;
	}
}

Our mock implements an interface just like a stub, but saves some state for later, so that our test can then assert and verify that our mock was called correctly. Listing 1 shows what the test might look like:

Listing 1: The log analyzer under test actually calls our mock object, which holds on to the string that is passed into the implemented method, and later asserted in the test.

[Test]
public void Analyze_TooShortFileName_CallsWebService()
{
MockService mockService = new MockService();
	LogAnalyzer log = new LogAnalyzer(mockService);
	string tooShortFileName="abc.ext";
	log.Analyze(tooShortFileName);
		Assert.AreEqual("Filename too short:abc.ext", |#1
							  mockService.LastError); |#1
}
public class LogAnalyzer
{
	private IWebService service;
	public LogAnalyzer(IWebService service)
	{
	this.service = service;
	}
	public void Analyze(string fileName)
	{
		if(fileName.Length<8)
		{
		service.LogError("Filename too short:" |#2
								  + fileName); |#2
		}
	}
}

Annotation #1:
The assert is done against the mock service, and not against the class under test
Annotation #2:
The method part we're going to test

Notice how the assert is performed against the mock object, and not against the LogAnalyzer class. That's because we are testing the interaction between LogAnalyzer and the web service. We still use the same dependency injection techniques, but this time the stub also makes or breaks the test. That's why it's a mock object. Also notice that we are not writing the tests directly inside the mock object code. There are several reasons for this.

We'd like to be able to re-use the mock object in other test cases, with other asserts on the message. If the assert is found inside the mock object, whoever is reading the test will have no idea what we are asserting. We will be hiding essential information from the test code, which hinders the readability and maintainability aspect of the test.

In many scenarios in your tests, you might find that you need to replace more than one object for the test to be able to work, which involves combining mocks and stubs.

This article is taken from the book The Art of Unit Testing. As part of a chapter on interactive testing using mock objects, this segment shows how to create and use a mock object and helps differentiate mock objects from stubs.