I was sharing what I’d learned about unit testing private methods with some co-workers, and the idea of testing a private method of an abstract class was presented.

PrivateObject deals with this handily by allowing you to specify the type being tested in one of its constructors.

Here’s a simple example. Consider we have an abstract class, Mammal, and a derived class, Human.

public abstract class Mammal
{
    private void SecretMammalStuff()
    {
    }
}

public class Human : Mammal
{
}

We can test Mammal’s private method by doing the following:

var human = new Human();
var po = new PrivateObject(human, new PrivateType(typeof(Mammal)));
po.Invoke("SecretMammalStuff");

If we don’t specify the PrivateType argument in the PrivateObject constructor, we’re not able to access the base class’s private methods. This is the key to testing private methods in an abstract class.