How to test a virtual method defined in an abstract class?

Question

I need to unit-test a virtual method defined in an abstract class. But the base class is abstract, so I can't create an instance of it. What do you recommend me to do?

This is a follow up to the following question: I am thinking about if it is possible to test via an instance of a subclass of the abstract class. Is it a good way? How can I do it?

Answer 1

I'm not sure what your abstract class looks like, but if you have something like:

public abstract class SomeClass
{
    public abstract bool SomeMethod();

    public abstract int SomeOtherMethod();

    public virtual int MethodYouWantToTest()
    {
        // Method body
    }
}

And then, as @David suggested in the comments:

public class Test : SomeClass
{
    // You don't care about this method - this is just there to make it compile
    public override bool SomeMethod()
    {
        throw new NotImplementedException();
    }

    // You don't care about this method either
    public override int SomeOtherMethod()
    {
        throw new NotImplementedException();
    }

    // Do nothing to MethodYouWantToTest
}

Then you just instantiate Test for your unit test:

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        SomeClass test = new Test();
        // Insert whatever value you expect here
        Assert.AreEqual(10, test.MethodYouWantToTest());
    }
}

Answer 2

There's no rule that says a unit test can't define its own classes. This is a fairly common practice (at least for me anyway).

Consider the structure of a standard unit test:

public void TestMethod()
{
    // arrange
    // act
    // assert
}

That "arrange" step can include any reasonable actions (without side-effects outside of the test) which set up what you're trying to test. This can just as easily include creating an instance of a class whose sole purpose is to run the test. For example, something like this:

private class TestSubClass : YourAbstractBase { }

public void TestMethod()
{
    // arrange
    var testObj = new TestSubClass();

    // act
    var result = testObj.YourVirtualMethod();

    // assert
    Assert.AreEqual(123, result);
}