public void MembershipAdded() { var moqCustomer = new Mock <ICustomer>(); var moqCR = new Mock <ICustomerRetriever>(); moqCR.Setup(x => x.RetrieveCustomer(It.IsAny <int>())).Returns(moqCustomer.Object); var moqPO = new PurchaseOrder("Customer: 123456", moqCR.Object, moqRefererRetriever.Object); moqPO.AddMembership(MembershipType.Books); moqCustomer.Verify(x => x.AddMembership(MembershipType.Books)); }