Unit Testing Critical Legacy Code

I once worked on a medical project which required intense Unit Testing in C#. A problem I encountered is that all of the critical sections of the code can't be accessed or required a massive amount of object dependencies. The dilemma I was in drove me to search for a new soluion. Good thing I found TypeMock Isolator.

TypeMock Isolator is a mocking framework that can take control of function calls regardless if it is accessible or not such as privately declared functions and etc. This Framework is center-focused on white box testing legacy code; although, can be used for black box testing.

Why use it?

• Code not Written using SOLID Principles
• Tested code is not following TDD(Test Driven Development)
• Testing legacy code
• Unrefactorable tested code
• Excellent for mission critical code.

Mock Instantiation

Using Typemock Isolator, object instantiations can be ignored.

• Ignore Constructor

ClassToTest obj = Isolate.Fake.Instance<ClassToTest> (Members.CallOriginal, ConstructorWillBe.Ignored);

• Ignore Base Class

ClassToTest obj = Isolate.Fake.Instance<ClassToTest> (Members.CallOriginal, ConstructorWillBe.Ignored, BaseConstructorWillBe.Ignored);

Mock Behaviour

A powerful feature of this framework is its ability to switch out function calls regardless of its access modifier.

• Non-Public Method

Isolate.NonPublic.WasCalled(objectToTest, "PrivateMethod" ).DoInstead(context => { /*Substitute Method */});
    "context" can be use to :
        1.) Get method’s instance
        2.) Get/Set arguments

• Public Method

Isolate.WasCalled(objectToTest, "PublicMethod").WillReturn(VALUE);

• Properties

Isolate.NonPublic.Property.WhenSetCalled(objectToTest,"Property").Doinstead(context => {/* Do Something */});

Invoke!

Another useful feature is being able to Invoke inaccessible functions so it can be tested.

• Invoke NonPublic methods

Isolate.Invoke.Method(objectToTest, "PrivateMethod"), new object[] {Argument});

• Fire events

Isolate.Invoke.Event(() => {eventTested += null;}, new object[]{Argument});

Example 1

This example shows that private functions can be invoked and tested without having to care about dependencies in constructor.

Code to Test

Unit Test

Example 2

This example shows that you don't have to think about dependencies on the function calls inside your unit on test. In this case, Coolness function's behaviour is being swaped by a dummy lambda function.

Code to Test

Unit Test

Example 3

Lastly, let us look into this really interesting test problem. Some critical systems will definitely need to have some variables set to a certain value to operate correctly. In the constructor, in the middle of operation, _hunger needs to be in a certain value before and after SearchFood() function. Normally, this is almost impossible to do. We can test SearchFood() function as a hook to extract the current state of _hunger.

Code to Test

Unit Test

Conclusion

If you are stuck in a similar dilemma as I did wherein  I could not access anything, Here is the solution to the problem! This framework can save you lots and lots of time.

Here is the official website of Typmock.