public async Task CreateUserAsync_ValidateDuplicatedEmail_ValidationException() { try { var user = new User() { Email = "*****@*****.**" }; Expression <Func <IUserRepository, Task <User> > > getUserByEmailAsync = x => x.GetUserByEmailAsync(It.IsAny <string>()); this._mockOfUserRepository.Setup(getUserByEmailAsync).ReturnsAsync(user).Verifiable(); await _service.CreateUserAsync(user); _mockOfUserRepository.Verify(getUserByEmailAsync, Times.Once); } catch (ValidationException ex) { Assert.IsInstanceOfType(ex, typeof(ValidationException)); } }