Moq: Invalid setup on a non-overridable member: x => x.GetByTitle(“asdf”)

In order to control the behavior of a mock object (in Moq, at least), you either need to mock an interface, or make sure that the behavior you’re trying to control is marked virtual. In your comment, I understand it so that the instantiating of _mockArticleDao is done something like this:

_mockArticleDao = new Mock<ArticleDAO>();

If you want to keep it as so, you need to mark the GetArticle method virtual:

public class ArticleDAO :  GenericNHibernateDAO(IArticle, int>, IArticleDAO
{
    public virtual IArticle GetByTitle(string title)
    {
        // ...
    }
}

Otherwise (and this is what I recommend), mock the interface instead.

_mockArticleDao = new Mock<IArticleDAO>();