How to add an item to a Mock DbSet (using Moq)

I'm trying to set up a mock DbSet for testing purposes. I used the tutorial here, http://www.loganfranken.com/blog/517/mocking-dbset-queries-in-ef6/ and slightly modified it so calling GetEnumerator returns a new enumerator each time (another problem i was having). However, I am having difficulty adding items to the DbSet.

The output is preCount = 3 postCount = 3. However, I expect it to be precount = 3 postCount = 4. Any help is greatly appreciated.

static void Main(string[] args)
    {
        Debug.WriteLine("hello debug");

        List<string> stringList = new List<string>
        {
            "a", "b", "c"
        };

        DbSet<string> myDbSet = GetQueryableMockDbSet(stringList);
        int preCount = myDbSet.Count();
        myDbSet.Add("d");
        int postCount = myDbSet.Count();
        Debug.WriteLine("preCount = " + preCount + " postCount = " + postCount);
    }

    private static DbSet<T> GetQueryableMockDbSet<T>(List<T> sourceList) where T : class
    {
        var queryable = sourceList.AsQueryable();

        var dbSet = new Mock<DbSet<T>>();
        dbSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(queryable.Provider);
        dbSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(queryable.Expression);
        dbSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(queryable.ElementType);
        dbSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(() => queryable.GetEnumerator());

        return dbSet.Object;
    }

myDbSet is not real implementation of DbSet but a mock which means it's fake and it needs to be setup for all methods you need. The Add is not exception so it needs to be set up to do what you need otherwise it does nothing.

Add something like the following and when the myDbSet.Add("d"); is called then the 'd' is added to the list and can be returned later.

dbSet.Setup(d => d.Add(It.IsAny<T>())).Callback<T>((s) => sourceList.Add(s));

Complete code

private static DbSet<T> GetQueryableMockDbSet<T>(List<T> sourceList) where T : class
{
    var queryable = sourceList.AsQueryable();

    var dbSet = new Mock<DbSet<T>>();
    dbSet.As<IQueryable<T>>().Setup(m => m.Provider).Returns(queryable.Provider);
    dbSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(queryable.Expression);
    dbSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(queryable.ElementType);
    dbSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(() => queryable.GetEnumerator());
    dbSet.Setup(d => d.Add(It.IsAny<T>())).Callback<T>((s) => sourceList.Add(s));

    return dbSet.Object;
}