How can I mock a method of a returned mock object?

Question

I am novice to Moq. I am unit testing an Entity Framework 6 project, following guidance provided here . So I have a templated method to create the fake tables:

    protected Mock<DbSet<TheType>> MockDBSet<TheType>(List<TheType> data) where TheType : class
    {
        var mockSet = new Mock<DbSet<TheType>>();
        var dataSet = data.AsQueryable();
        mockSet.As<IQueryable<TheType>>().Setup(m => m.Provider).Returns(dataSet.Provider);
        mockSet.As<IQueryable<TheType>>().Setup(m => m.Expression).Returns(dataSet.Expression);
        mockSet.As<IQueryable<TheType>>().Setup(m => m.ElementType).Returns(dataSet.ElementType);
        mockSet.As<IQueryable<TheType>>().Setup(m => m.GetEnumerator()).Returns(dataSet.GetEnumerator());
        mockSet.Setup(x => x.Add(It.IsAny<TheType>()))
            .Returns(new Func<TheType, TheType>(x =>
            {
                data.Add(x);
                return data.Last();
            }));

        return mockSet;
    }

Using the above works fine for adding and querying the fake database:

var db = new Mock<BloggingContext>();
db.Setup(m => m.Blog s).Returns(MockDBSet<Blog >(
    new List<Blog>() 
).Object);

BloggingContext context = db.Object;

Blog blog= new Blog();
context.Blogs.Add(blog); //fine
Assert.IsTrue(context.Blogs.Count() == 1); //fine

EF also provides the "Local" property on the entity collections which give access to un-saved entities. So, when not mocking, BloggingContext.Blogs.Local passes back a ObservableCollection collection of unsaved entities. Objects are moved from BloggingContext.Blogs.Local to BloggingContext.Blogs when the BloggingContext.SaveChanges() is called.

I wanted to mock this behavior, so I created a new class:

    public class FakeBlogs : List<Blog>
    {
        ObservableCollection<Blog> _local = new ObservableCollection<Blog>();
        ObservableCollection<Blog> Local { get { return _local; } }

        public void Add (Blog item)
        {
            _local.Add(item);
        }
    }

When unit testing, the following code works:

var db = new Mock<BloggingContext>();
db.Setup(m => m.Blog s).Returns(MockDBSet<Blog>(
    new FakeBlogs() //<===== Changed to use FakeBlogs
).Object);

BloggingContext context = db.Object;

Blog blog= new Blog();    
context.Blogs.Add(blog); //fine
Assert.IsTrue(context.Blogs.Count() == 1); //fine

However, a using the Local property throws a NPE as the Local property is null.

var blog = (from i in context.Blogs.Local select i).FirstOrDefault();//throws NPE

How can I mock the Local property successfully?

Answer 1

Fixed. I added the following line to the MockDBSet template function:

        mockSet.Setup(m => m.Local).Returns(data.Local);

And declared the "Local" property of FakeTable as public. Thanks to all who took an interest.