Mocking static methods – Code musings in C#

I’m working on a pretty old application with no unit tests whatsoever, and I’m trying to write some as I add features and fix bugs.

After successively finding out about MS Fakes, then that it’s only included in VS2012+ Premium/Ultimate and very hard to include in a continuous integration chain, I was pretty stumped until I found this, which works incredibly well.

Another option to transform the static method into a static Func or Action. For instance.

Original code:

C#

class Math { public static int Add(int x, int y) { return x + y; } }

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class Math
{
    public static int Add(int x, int y)
    {
        return x + y;
    }
}

You want to “mock” the Add method, but you can’t. Change the above code to this:

C#

public static Func<int, int, int> Add = (x, y) => { return x + y; };

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public static Func<int, int, int> Add = (x, y) =>
{
    return x + y;
};

Existing client code doesn’t have to change (maybe recompile), but source stays the same.

Now, from the unit-test, to change the behavior of the method, just reassign an in-line function to it:

C#

[TestMethod] public static void MyTest() { Math.Add = (x, y) => { return 11; }; }

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[TestMethod]
public static void MyTest()
{
    Math.Add = (x, y) =>
    {
        return 11;
    };
}

Put whatever logic you want in the method, or just return some hard-coded value, depending on what you’re trying to do.

This may not necessarily be something you can do each time, but in practice, I found this technique works just fine.