public void Login_Login_Failed()
        {
            // arrange
            var loggedInUser = new UserModel();

            loggedInUser.AddError("invalid username or password");
            this._contactService.Setup(x => x.Login(It.IsAny <string>(), It.IsAny <string>())).Returns(Task.FromResult <UserModel>(loggedInUser));

            ILoginViewModel loginViewModel = this.CreateSubject();

            // act
            loginViewModel.ShowDialog();
            loginViewModel.UserName = "******";
            loginViewModel.Password = "******";
            loginViewModel.LoginCommand.Execute(null);

            // assert
            Assert.IsNull(loginViewModel.DialogResult);
            this._contactService.Verify(x => x.Login(It.IsAny <string>(), It.IsAny <string>()), Times.Once);
        }