how to mock ITable with a concrete type

Question

I am writing unit tests for a repository that use System.Data.Linq DataConext object to get Table of entities from database. Here's the code :

public class ForumRepository : IForumRepository
{
    protected Table<Forum> DataTable;
    IDataContextWrapper DataContext;

    public ForumRepository(IDataContextWrapper DataContext)
    {
        DataTable = DataContext.GetTable<Forum>();
    }

    public Forum GetForumById(int id)
    {
        try
        {
            return DataTable.Single(f => f.tblForumID.Equals(id));
        }
        catch(Exception e)
        {
            return null;
        }
    }

And here's the wrapper implemention :

public class DataContextWrapper<T> : IDataContextWrapper where T : EpixForumDataContext, new()
{
    private readonly T db;

    public DataContextWrapper()
    {
        var t = typeof(T);
        db = (T)Activator.CreateInstance(t);
    }
    public DataContextWrapper(string connectionString)
    {
        var t = typeof(T);
        db = (T)Activator.CreateInstance(t, connectionString);
    }

    public Table<TableName> GetTable<TableName>() where TableName : class
    {
        return (Table<TableName>)db.GetTable(typeof(TableName));
    }

I want to test the repository method.

public class UnitTest1
    {
        [TestMethod]
        public void Can_Get_Forum_ById()
        {
            //arrange
            Forum dummyForum = new Forum() { tblForumID = 1};
            Mock<ITable<Forum>> tableMock = new Mock<ITable<Forum>>();
            tableMock.Object.Attach(dummyForum);
            Mock<IDataContextWrapper> mock = new Mock<IDataContextWrapper>();
            mock.Setup(m => m.GetTable<Forum>()).Returns(tableMock.Object) ;

            //act
            ForumRepository repos = new ForumRepository(mock.Object);
            Forum resultForum = repos.GetForumById(1);

            //assert
            Assert.AreEqual(resultForum.tblForumID, 1);

Where Forum is an auto generated class. I want to setup the forum for Table so that when I do a GetTable on the ContextWrapper I get the table of forums. I do not know if Table.Attach would attach the Forum to the Table or not. Also when I run the test it says

'Type to mock must be an interface or an abstract class or non-sealed class'.

Have I got it all wrong?

Answer 1

I see several issues with what you're trying to do.

1. The posted code does not compile

   IDataContextWrapper.GetTable returns Table<T>, so you can't set it up to return a mocked ITable<T>. An ITable is not a Table, it's the other way around. This leads me to my next point:

2. IDataContextWrapper.GetTable should return an ITable<T>, not Table<T>.

   This will let you mock the return result, since Table<T> is sealed (Moq can't mock sealed classes, which is probably why you got the error you mentioned). It's also good design to program to interfaces rather than concretes.

3. You shouldn't expect the call to Attach to do anything

   You're trying to call a method on a mocked interface like you're expecting it to behave as if something has implemented it. The mock's methods will only do what you tell them to, so in this case (with Loose behavior) it will just do nothing with that call. Instead you should set up what you expect the table to do, but that leads me to:

4. You can't set up the call to Single because it's an extension method

   Moq doesn't support setting up extension methods, since they're static methods. However, you can setup the call to GetEnumerator, which is what Single calls anyways. You'll need to mock the IQueryable<T> members though, since that's what Single is really going to hit.

So, after resolving point 1 above, your test in the end should look like this:

[Test]
public void Can_Get_Forum_ById()
{
    // arrange
    Forum dummyForum = new Forum { tblForumID = 1 };
    IQueryable<Forum> forums = new List<Forum> { dummyForum }.AsQueryable();

    Mock<ITable<Forum>> tableMock = new Mock<ITable<Forum>>();
    tableMock.Setup(p => p.GetEnumerator()).Returns(forums.GetEnumerator());
    tableMock.Setup(r => r.Provider).Returns(forums.Provider);
    tableMock.Setup(r => r.ElementType).Returns(forums.ElementType);
    tableMock.Setup(r => r.Expression).Returns(forums.Expression);

    Mock<IDataContextWrapper> mock = new Mock<IDataContextWrapper>();
    mock.Setup(m => m.GetTable<Forum>()).Returns(tableMock.Object);

    // act
    ForumRepository repos = new ForumRepository(mock.Object);
    Forum resultForum = repos.GetForumById(1);

    // assert
    Assert.AreEqual(resultForum.tblForumID, 1);
}

Note that this is a good, not great test. You could replace the call to Single with a call to First and it would still pass, but obviously be wrong in general. You should at least add a negative of this test, i.e. no object is returned if the Id does not match.

Answer 2

Try something like this

Forum dummyForum = new Forum() { tblForumID = 1};
            Mock<ITable> tableMock = new Mock<ITable>();
            tableMock.Object.Attach(dummyForum);
            Mock<IDataContextWrapper> contextMock = new Mock<IDataContextWrapper>();
            contextMock .Setup(m => m.GetTable<Forum>()).Returns((ITable<Forum>)tableMock.Object) ;

            //act
            ForumRepository repos = new ForumRepository(mock.Object);
            Forum resultForum = repos.GetForumById(1);

            //assert
            Assert.AreEqual(resultForum.tblForumID, 1);