Question

I wrote an implementation for a priority queue I needed, and now I would like to test it. I decided on using moq, as I’ve already used rhino mocks at work and want to try something new/possibly easier.

The interface for my PriorityQueue is pretty straight-forward:

public interface IPriorityQueue<TKey, TValue>  where TKey : IComparable
{
    void Enqueue(TKey priority, TValue value);
    bool IsEmpty();
    TValue Peek();
    TValue Dequeue();
}

I went to write my first test, which tests the Enqueue method. Here’s the implementation for it:

public void Enqueue(TKey priority, TValue value)
{
    if (priority == null) { throw new ArgumentNullException("priority"); }

    if (_queue.ContainsKey(priority))
    {
        // queue contains key, therefore just add value to existing key's list
        _queue[priority].Add(value);
    }

    // otherwise just add pair
    else
    {
        _queue.Add(priority, new List<TValue>() { value });
    }
}

The first unit test I wrote was to test if the key was null, it should throw an argument null exception.

[TestMethod]
public void EnqueueNullKey_ThrowsArgumentNullException()
{
    /* Arrange */
    var mock = new Mock<IPriorityQueue<string, IMessage>>();

    // string implements the IComparable interface, and is nullable.
    mock
        .Setup(x => x.Enqueue(null, It.IsAny<IMessage>()))
        .Throws<ArgumentNullException>();

    /* Assert */
    mock.VerifyAll();   
}

So I realize now, that my method Enqueue will never be called, because I am instantiating an instance of my interface, not the implementation. Then the question begs to be asked, if I’m supposed to test with using my interfaces (at-least that was the impression I got after watching Roy Osherove’s TDD – Understanding Mock Objects video) how am I to test my implementation?

Am I misunderstanding the advice to test with interfaces?

In the video he created a class within the test he was writing, and used that to test. I don’t see how that would help me test my implementation of PriorityQueue (specifically the Enqueue method).

Thanks stack overflow!

edit: Here’s the following (ugly) working test that I’ve came up with. I’m extremely unhappy with it, it feels so primitive. Can anyone suggest a better way of doing this? From the responses below, it seems like the framework is completely unnecessary for this unit test.

However, here it is:

[TestMethod]
public void EnqueueNullKey_ThrowsArgumentNullException()
{
    /* Arrange */
    var pq = new PriorityQueue<string, IMessage>();

    try
    {
        pq.Enqueue(null, null);
    }
    catch(ArgumentNullException)
    {
        Assert.IsTrue(true);
        return;
    }

    // failure condition if we don't catch the exception
    Assert.IsTrue(false);   
}

Answer 1

You are getting it wrong. You don’t need a mock of IPriorityQueue when you are testing methods on one of its implementation. What you need is a mock of queue

The responsibilities of Enqueue method are

1. throwing exception if priority is null
2. doing something on if condition
3. doing something on else condition

For #1 testing is quite easy and a stub implementation of queue will suffice, for #2 and #3 you will need a mock of queue. For #2 and #3 you should test if mock queue’s Add methods is called with correct parameters