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.