One of the coolest (and scariest) things I’ve seen lately is mocking static classes and methods with JustMock. I was working on some legacy code, and was making some changes, so I wanted to make sure that I got everything tested first. When I went to write the tests, I noticed that I needed to wrap and hide away a static method that was being called.

My eventual intent, however, is to get rid of the static class and method calls if possible. Writing the wrapper around it temporarily, so I can get tests in place before refactoring has always seemed like a pain. I went to check and see if JustMock would be able to mock a static class. That seemed like one of those crazy things that commercial mocking frameworks can usually do. I was right. It could mock the static class. (SCARY!!)

I wrote some code that looked like this: (Sorry I can’t show you my clients’ code in my blog without their permission, so you get sample code.)

public static class MyStaticClass

{

    public static int DoSomeDependentStuffThatIsHardToFix()

    {

        // Pretend this does more than just returning 1

        return 1;

    }

}

In my example, I need to test some code calling this static method. Normally, I would put a wrapper around this class, write my tests, refactor, and eventually change this to not be a static dependency anymore.

I decided to try this. Here is a test class that uses mocking to demonstrate how I can decide any result for this method and have control of this dependency. Notice that my method always returns 1. Pretend that a different result were possible.

Now when I go to write my test, I can just Mock out the static and define that I want that method to be called and for it to return 2 instead of 1. I will then assert that it worked as I expected in my test. Once I confirm that the mock works as expected using this type of test, I can put it in place in my actual test. (Yes, I sometimes write temporary tests to confirm what I expect before I write the whole test. Little steps.)

[TestFixture]

public class MyTestClass

{

    [Test]

    public void WhatIsTestedDoesNotMatter()

    {

        Mock.SetupStatic(typeof(MyStaticClass));

        Mock.Arrange(() => MyStaticClass.DoSomeDependentStuffThatIsHardToFix()).Returns(2);

        Assert.AreEqual(2, MyStaticClass.DoSomeDependentStuffThatIsHardToFix());

    }

}

Then when I go and run my tests, I get this nice result showing me that everything is working as I expect it to. Yippee!

How it achieves this? I am not sure. I intend to find out though. When I do, I will write a post explaining it.

Have a good day!

One of the most powerful tool available to developers testing a legacy code base is the ability to mock out classes that their code depends on. This is of great importance since our unit tests need to limit the scope, and we do this by trying to limit our dependencies. Through mocking we can exchange one dependency on the infrastructure of our application for an in-memory mock.

Sometimes mocking is overused, and I am not just talking about cases where every objected gets mocked to the point where we’re testing nothing. I am talking about a different issue in mocking. I am talking about where developers put on their mocking blinders when unit testing. It’s an easy thing to do, and I’ve done it plenty of times myself.

Setting Up Our Example

We will start of by defining our example. We will have a method to do some sort of calculation (an ideal and easy testing scenario). It will be a legacy code base, which means that it is untested code and probably hard to test. We’ll start off by making it a private static method and put in a nice dependency that one might try to mock to avoid.

private static decimal CalculateFooOnXyz(Xyz xyzItem, 
    decimal calculationParameter1)
{
    var numbersInCalculation = Repository.GetNumbers()
        .Where(n => n.IsActive);

    foreach (Number number in numbersInCalculation)
    {
        // Some code that executes for each one.
    }
}

private static decimal CalculateFooOnXyz(Xyz xyzItem, 
    decimal calculationParameter1, IRepository repository)
{
    var numbersInCalculation = repository.GetNumbers()
        .Where(n => n.IsActive);

    foreach (Number number in numbersInCalculation)
    {
        // Some code that executes for each one.
    }
}

Why This is Wrong

My first question is, what is the name of the method? CalculateFooOnXyz. Notice that I didn’t say, “GetDataFromTheDatabase”. That’s because we shouldn’t be doing that. It isn’t part of the calculation. The numbers returned from the database are required for calculating, so that means that we should have that object as our dependency instead.

How We Change It

So instead of making the repository our parameter, we should make the collection of numbers our parameter. This way we’re depending on the in-memory collection of CLR objects. This is much less of a dependency, and in is not one that needs to be mocked. By doing this alternative we’re better following the Single Responsibility and Dependency Inversion principles.

Our best code for testing will look like this.

private static decimal CalculateFooOnXyz(Xyz xyzItem, 
    decimal calculationParameter1, List<CalcNumbers> numbersInCalculation)
{
  foreach (Number number in numbersInCalculation)
  {
    // Some code that executes for each one.
  }
}

One pain point which comes along often when working with others' libraries are the classes that are not open and implementing interfaces. A lot of the classes we developers use every day implement no interfaces. Since the class is out of my control, I obviously cannot give it an interface, so I need some other way to work with it. This creates a problem when we need to mock out the class. There are ways in which we can get around this though.

Wrapping Classes

In my opinion, the most dependable workaround to be able to mock out and test a class is creating an interface-implementing wrapper around the class we want to mock and using that instead. This one works very well, but it forces you to create an interface and a class even though there is already a class in existence.

Partial Class Interface

As the name of this post says, partial classes can be very important when presented with this problem. If a class is not implementing an interface, but it is a partial class, you can give it an interface it is already implement.

If a class is partial you can give it an interface which defines the methods it already implements.

So now I am going to present an example. For this example, we will have a class called "BrendanMailer". This class is not implementing an interface, which means that if we want to remove this dependency we need to come up with some solution that lets us program against an interface.

public partial class BrendanMailer
{
    public void SendEmail()
    {
    }

    public void SomeNonImportantMethod()
    {
    }
}

For this we could obviously use the wrapper method, but we can also do something a lot easier. Thanks to the smart creator of that class, there is a partial keyword in there. This allows us to leverage a very powerful trick, because we can now create our own interface. The interface can include only the methods we want it to, which is in some ways better than if the interface had been predefined.

public interface IBrendanMailer
{
    void SendEmail();
}

So now I have a custom interface to work with and program against. I now just add that into another part of this partial class and tell that one I am implementing the interface. I don't have to implement the methods, because they already are implemented in the original part of this partial.

public partial class BrendanMailer : IBrendanMailer
{
}

In my production code I will inject the BrendanMailer class, and in the tests I will mock it out or use a fake or something. This is very powerful and doesn't clutter much at all, because I can hide this partial away in a folder in my project and ignore it for a long time.