Unit Testing: hard dependency MessageBox.Show()

Question

What ways can the SampleConfirmationDialog be unit tested? The SampleConfirmationDialog would be exercised via acceptance tests, however how could we unit test it, seeing as MessageBox is not abstract and no matching interface?

public interface IConfirmationDialog
{
    /// <summary>
    /// Confirms the dialog with the user
    /// </summary>
    /// <returns>True if confirmed, false if not, null if cancelled</returns>
    bool? Confirm();
}


/// <summary>
/// Implementation of a confirmation dialog
/// </summary>
public class SampleConfirmationDialog : IConfirmationDialog
{
    /// <summary>
    /// Confirms the dialog with the user
    /// </summary>
    /// <returns>True if confirmed, false if not, null if cancelled</returns>
    public bool? Confirm()
    {
        return MessageBox.Show("do operation x?", "title", MessageBoxButton.YesNo, MessageBoxImage.Question) == MessageBoxResult.Yes;
    }
}

Answer 1

You can't, it's untestable in it's current state. For this particular class, there is also no value in unit testing it ... it is but a light wrapper around a built-in framework feature, so all you'd be doing is testing the framework.

If you absolutely must test it, the IConfirmationDialog interface should have another dependency that you can mock up in the unit test.

Answer 2

You should look into Typemock, a commercial mocking framework that lets you unit test these kinds of situations by using the .NET performance profiling libraries. See their website for more information.

Answer 3

I think it's OK to stop testing it at that level. Your interaction with IConfirmationDialog is more important than verifying that MessageBox.Show is actually getting called. Since that is an interface and is easily mockable, I think you are pretty well covered.