Mocking DbContext Entity Framework 4 Code First CTP5 with NSubstitute

Take an Entity Framework 4 Code First model, something like this:

   1: public class Customer

   2: {

   3:    public int Id { get; set; }

   4:    public string Name { get; set; }

   5: }

   6:  

   7: public class CustomerContext : DbContext, ICustomerContext

   8: {

   9:    public DbSet<Customer> Customers { get; set; }

  11: }

I now want to write some tests and I'm going to mock the DbContext using NSubstitute.  I first try something like this:

   1: [Test]

   2: public void can_mock_customer_context()

   3: {

   4:    var context = Substitute.For<CustomerContext>();

   5:    context.Customers.Returns(

   6:       new DbSet<Customer>(

   7:          new[]

   8:          {

   9:             new Customer {Name = "Sean"}

  10:          })

  11:       );

  12:    Assert.AreEqual(1, context.Customers.Count());

  13: }

The problem is that the DbSet constructor is internal (as of EF4 Code First CTP5).  So, let's abstract our DB access to a simple interface and replace DbSet<T> with the IQueryable<T> interface, ending up with the below:

   1: public interface ICustomerContext

   2: {

   3:    IQueryable<Customer> Customers { get; set; }

   4: }

This interface can be implemented like so:

   1: public class CustomerContext : DbContext, ICustomerContext

   2:   {

   3:      public DbSet<Customer> Customers { get; set; }

   4:      IQueryable<Customer> ICustomerContext.Customers { get { return Customers; } }

   5:   }

Now all we need to do is to use an implementation of IQueryable<T> in our mock.  I'm going to use EnumerableQuery<T> which gives me the following test that now passes:

   1: [Test]

   2: public void can_mock_customer_context()

   3: {

   4:    var context = Substitute.For<ICustomerContext>();

   5:    context.Customers.Returns(

   6:       new EnumerableQuery<Customer>(

   7:          new[]

   8:          {

   9:             new Customer {Name = "Sean"}

  10:          })

  11:       );

  12:    Assert.AreEqual(1, context.Customers.Count());

  13: }

  14:    }

I'm new to NSubstitute but it seem to be the lowest friction mocking library out there.  Just perfect for use with Entity Framework 4 Code First - certainly the lowest friction ORM there is today!

Note that we could have used the repository pattern to wrap the DbContext instead of a simple interface, the approach is almost identical.

Update: Ro Miller has an alternative approach using fakes that does a better job of surfacing IDbSet.  Check it out here: http://romiller.com/2010/09/07/ef-ctp4-tips-tricks-testing-with-fake-dbcontext/.