Question

I’m performing some unit tests on my entities and I’ve had a bit of a mental block mocking a property. Take the following entities:

public class Teacher
{
    public int MaxBobs { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}

public class Student
{
    public string Name { get; set; }
    public virtual Teacher Teacher { get; set; }
}

I have a method on Teacher called AddStudent which first checks whether a teacher has too many students called Bob assigned. If so, then I raise a custom exception saying too many bobs. The method looks like this:

public void AddStudent(Student student)
{
    if (student.Name.Equals("Bob"))
    {
        if (this.Students.Count(s => s.Name.Equals("Bob")) >= this.MaxBobs)
        {
            throw new TooManyBobsException("Too many Bobs!!");
        }
    }

    this.Students.Add(student);
}

I’d like to unit test this using Moq mocks – specifically I want to mock the .Count method of Teacher.Students where I can pass it any expression and it’ll return a number suggesting there are currently 10 Bobs assigned to that teacher. I’m setting it up like this:

[TestMethod]
[ExpectedException(typeof(TooManyBobsException))]
public void Can_not_add_too_many_bobs()
{
    Mock<ICollection<Student>> students = new Mock<ICollection<Student>>();
    students.Setup(s => s.Count(It.IsAny<Func<Student, bool>>()).Returns(10);

    Teacher teacher = new Teacher();
    teacher.MaxBobs = 1;

    // set the collection to the Mock - I think this is where I'm going wrong
    teacher.Students = students.Object; 

    // the next line should raise an exception because there can be only one
    // Bob, yet my mocked collection says there are 10
    teacher.AddStudent(new Student() { Name = "Bob" });
}

I’m expecting my custom exception, but what I’m actually getting is System.NotSupportedException which infers that the .Count method of ICollection isn’t virtual and therefore cannot be mocked. How do I mock this particular function?

Any help always appreciated!

Answer 1

You can’t mock the Count method you are using, because it is an extension method. It isn’t a method defined on ICollection<T>.
The simplest solution would be to simply assign a list with 10 bobs to the Students property:

teacher.Students = Enumerable.Repeat(new Student { Name = "Bob" }, 10)
                             .ToList();