public void TestAddNumbersViaInterface(int x, int y, bool isSuccessful, int expectedResult) { var mockExternalService = new Mock <IExternalService>(); mockExternalService.Setup(m => m.DoGreatThings()).Returns(isSuccessful); var objExternalService = mockExternalService.Object; var calcService = new CalcService(); var result = calcService.AddNumbersIfSuccessful(x, y, objExternalService); Assert.Equal(expectedResult, result); }