How to Unit Test an Abstract Class

How to unit test an abstract class? Or a whole class hierarchy that depends on that abstract class? Let’s see.

Unit testing an abstract class

Imagine you work on a people software in a university and have the following code:

public class Student
{
    public string Name { get; set; }
    public void EnrollInCourse(Course course)
    {
        /* ... */
    }
    public string GetSignature()
    {
        return $"Best regards,\r\n{Name},\r\nStudent at MIT";
    }
}
public class Professor
{
    public string Name { get; set; }
    public void ApplyForFacultyJob(Faculty faculty)
    {
        /* ... */
    }
    public string GetSignature()
    {
        return $"Best regards,\r\n{Name},\r\nProfessor at MIT";
    }
}

In this example, Student and Professor share common functionality, and so you decide to extract it into a Person abstract base class:

public class Student : Person
{
    public void EnrollInCourse(Course course)
    {
        /* ... */
    }
    protected override string GetTitle() => "Student";
}
public class Professor : Person
{
    public void ApplyForFacultyJob(Faculty faculty)
    {
        /* ... */
    }
    protected override string GetTitle() => "Professor";
}
public abstract class Person
{
    public string Name { get; set; }
    protected abstract string GetTitle();
    public string GetSignature()
    {
        return $"Best regards,\r\n{Name},\r\n{GetTitle()} at MIT";
    }
}

This version looks much better because it follows the DRY principle.

Now, the question is — how to test it?

There are two options:

• Unit test all classes — When you test each class separately (Student, Professor, and Person)
• Unit test only concrete classes — When you test only the non-abstract classes (Student and Professor)

Let’s discuss them separately.

Test class per each production class

Testing each class in the hierarchy provides the benefit of not repeating your tests. Both Student and Professor derive the…