public void GetBill_calls_BillingApi_correctly() { // arrange var mockBillingApi = new Mock<IBillingApi>(MockBehavior.Strict); mockBillingApi.Setup(x => x.GetBill()).Returns(new Bill()); var billingService = new BillingService(mockBillingApi.Object); // act billingService.GetBill(); // assert mockBillingApi.Verify(x => x.GetBill(), Times.Once); }