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

Question

Not sure how I can fix this, trying to do a unit test on the method "GetByTitle"

Here are my definitions:

public class ArticleDAO :  GenericNHibernateDAO(IArticle, int>, IArticleDAO
{
    public IArticle GetByTitle(string title)
    {
        IQuery query = Session.CreateQuery("...")
        return query.UniqueResult<IArticle>();
    }
}

public interface IArticleDAO
{
    IArticle GetByTitle(string title);
}

unit test:

[Test]
public void can_load_by_title()
{
    _mockDaoFactory.Setup(x => x.GetArticleDao())
                                .Returns(_mockArticleDao.Object);
    _mockArticleDao.Setup(x => x.GetByTitle("some title"))
                                .Returns(article1.Object);

    _articleManager.LoadArticle("some title");

    Assert.IsNotNull(_articleManager.Article);
}

Running the test gives me the error:

System.ArgumentException: Invalid setup on a non-overridable member:
x => x.GetByTitle("some title")

Update

My [Setup] looks like:

[Setup]
public void SetUp()
{
     _mockDaoFactory = new Mock<IDaoFactory>();
     _mockArticleDao = new Mock<ArticleDao>();

     _articleManager = new ArticleManager(_mockDaoFactory.Object);    
}

Answer 1

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>();