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);
}