Why is the DBSet Add failing, and also destroying the existing Customer entries in the DBSet?

Question

Our programming involves some Mock testing using In Memory Data.

      // Let us create some in-memory data
        // Create a list of Customer
        List<Customer> listOfCustomers =  new List<BlahProjectBlahExample.Domain.Objects.Customer>()
                                   { new Customer { CustomerID = "1    ", CompanyName = "Chicago Bulls", ContactName = "Michael Jordan", ContactTitle = "top basket ball player", Address = "332 testing lane", City = "Chicago", Region = "Illinois", PostalCode = "484894", Country = "USA", Phone = "3293993", Fax = "39393" },
                                     new Customer { CustomerID = "2    ", CompanyName = "Miami Heat", ContactName = "Lebron James", ContactTitle = "second best basket ball player", Address = "90 test street", City = "Miami", Region = "Florida", PostalCode = "4869394", Country = "USA", Phone = "3293213", Fax = "33393" },
                                     new Customer { CustomerID = "3    ", CompanyName = "Oklahoma City Thunder", ContactName = "Kevin Durant", ContactTitle = "current top basket ball player", Address = "35 test row", City = "Oklahoma City", Region = "Oklahoma", PostalCode = "480290", Country = "USA", Phone = "304923", Fax = "33325" }
                                   };
        // Convert the list to an IQueryable list
        IQueryable<Customer> queryableListOfCustomerInMemoryData = listOfCustomers.AsQueryable();


        // Let us create a Mocked DbSet object.
        Mock<DbSet<BlahProjectBlahExample.Domain.Objects.Customer>> mockDbSet = new Mock<DbSet<BlahProjectBlahExample.Domain.Objects.Customer>>();

     // Force DbSet to return the IQueryable members
        // of our converted list object as its 
        // data source
        mockDbSet.As<IQueryable<BlahProjectBlahExample.Domain.Objects.Customer>>().Setup(m => m.Provider).Returns(queryableListOfCustomerInMemoryData.Provider);
        mockDbSet.As<IQueryable<BlahProjectBlahExample.Domain.Objects.Customer>>().Setup(m => m.Expression).Returns(queryableListOfCustomerInMemoryData.Expression);
        mockDbSet.As<IQueryable<BlahProjectBlahExample.Domain.Objects.Customer>>().Setup(m => m.ElementType).Returns(queryableListOfCustomerInMemoryData.ElementType);
        mockDbSet.As<IQueryable<BlahProjectBlahExample.Domain.Objects.Customer>>().Setup(m => m.GetEnumerator()).Returns(queryableListOfCustomerInMemoryData.GetEnumerator());



         Mock<BlahProjectBlahDataContext> mockedReptryCtxt = new Mock<BlahProjectBlahDataContext>();


         mockedReptryCtxt.Setup(q => q.Customers).Returns(mockDbSet.Object);
         mockedReptryCtxt.Setup(q => q.Set<Customer>()).Returns(mockDbSet.Object);
        mockedReptryCtxt.CallBase = true;




        DbSet<Customer> inMemoryDbSetCustomer = mockedReptryCtxt.Object.Set<Customer>();

In the following code, I used a loop to see the contents of the inMemoryDbSetCustomer, and it contained the expected data.

      Customer something;
      foreach (var entry in inMemoryDbSetCustomer)
      {
          something = entry as Customer;
      }

Sadly, when I try to add a new customer, 1) the inMemoryDbSetCustomer fails to add the customer 2) the inMemoryDbSetCustomer.Add(someCust) returns NULL 3) the inMemoryDbSetCustomer seems to have lost all it's other Customer entries.

        Customer someCust = new Customer { CustomerID = "4    ", CompanyName = "Kolkota Knights", ContactName = "Sachin Tendulkar", ContactTitle = "current top cricket player", Address = "35 test row", City = "Kolkota", Region = "West Bengal", PostalCode = "3454534", Country = "India", Phone = "304923", Fax = "33325" };
        try
        {
            Customer returnCust = (Customer)(inMemoryDbSetCustomer.Add(someCust));
        }
        catch(Exception ex ){

        }

Why is the DBSet Add failing, and also destroying the existing Customer entries in the DBSet?

Update With Answer

Thanks to @Werlang suggestion. The following code addition helped:

 mockDbSet.Setup(m => m.Add(It.IsAny<Customer>()))
                   .Callback<Customer>((Customer c) => { listOfCustomers.Add(c); })
                   .Returns((Customer c) => c);

Also, if you mocking DBSets with Moq Framework like I am then the following stackoverflow posting will be helpful because you might face a similar problem:

What steps to get rid of Collection was modified; enumeration operation may not execute. Error?

Answer 1

You've mocked the queryable to return a fixed set of data, when accessed via IQueryable. But Add's are maintained in a separated list on DbContext internal in the form of DbEntry's.

I suggest you to mock the Add() method too, addind the new element to mocked array (listOfCustomers).