public async Task AddCity_ShouldNotAdd_WhenCountryIsInvalid() { // Arrange var responseMessage = "Invalid country selected."; var cityForCreationDto = new CityForCreationDto() { CountryId = 1, Name = "Belgrade" }; _unitOfWorkMock.Setup(x => x.Countries.GetById(It.IsAny <int>())) .ReturnsAsync(() => null); // Act var result = await _sut.AddCity(cityForCreationDto); // Assert Assert.Equal(responseMessage, result.Value); }