Asp.net Core Identity unit test controller actions

Mehod under test should be made async and not use blocking calls ie .Result

[HttpGet("RegisterNewUser")]
public async Task<HttpResponseMessage> RegisterNewUser([FromBody] NewUserRegistration user) {
    if (ModelState.IsValid) {
        var newUser = new ApplicationUser() {
            UserName = user.username,
            Email = user.password
        };
        var result = await _userManager.CreateAsync(newUser, user.password);
        if (result.Errors.Count() > 0) {
            var errors = new IdentityResultErrorResponse().returnResponseErrors(result.Errors);
            return this.WebApiResponse(errors, HttpStatusCode.BadRequest);
        }
    } else {
        var errors = new ViewModelResultErrorResponse().returnResponseErrors(ModelState);
        return this.WebApiResponse(errors, HttpStatusCode.BadRequest);
    }

    return this.WebApiResponse(
                "We have sent a valifation email to you, please click on the verify email account link.",
                HttpStatusCode.OK);
}

Review of Happy path scenario and method under test shows that there is no need to setup the UserStore as test will be overriding the user manager virtual members directly.

Note the test has also been made async as well.

1. Create a valid viewmodel and return a success message

[Fact]
public async Task RegisterNewUser_ReturnsHttpStatusOK_WhenValidModelPosted() {
    //Arrange
    var mockStore = Mock.Of<IUserStore<ApplicationUser>>();
    var mockUserManager = new Mock<UserManager<ApplicationUser>>(mockStore, null, null, null, null, null, null, null, null);

    mockUserManager
        .Setup(x => x.CreateAsync(It.IsAny<ApplicationUser>(), It.IsAny<string>()))
        .ReturnsAsync(IdentityResult.Success);

    var sut = new MembershipController(mockUserManager.Object);
    var input = new NewUserInputBuilder().Build();

    //Act
    var actual = await sut.RegisterNewUser(input);

    //Assert
    actual
        .Should().NotBeNull()
        .And.Match<HttpResponseMessage>(_ => _.IsSuccessStatusCode == true);        
}

2. Create a valid viewmodel but returns a IdentityResponse error (eg. user exists) which gets converted

For this you just need to setup the mock to return a result with errors.

[Fact]
public async Task RegisterNewUser_ReturnsHttpStatusBadRequest_WhenViewModelPosted() {
    //Arrange

    //...code removed for brevity

    mockUserManager
        .Setup(x => x.CreateAsync(It.IsAny<ApplicationUser>(), It.IsAny<string>()))
        .ReturnsAsync(IdentityResult.Failed(new IdentityError { Description = "test"}));

    //...code removed for brevity

    //Assert
    actual
        .Should().NotBeNull()
        .And.Match<HttpResponseMessage>(_ => _.StatusCode == HttpStatusCode.BadRequest);
}

And for

3. Create an invalid viewmodel and returns Modelstate errors

You just need to set the model state of the controller so that it is invalid.

[Fact]
public async Task RegisterNewUser_ReturnsHttpStatusBadRequest_WhenInvalidModelState() {
    //Arrange
    var mockStore = Mock.Of<IUserStore<ApplicationUser>>();
    var mockUserManager = new Mock<UserManager<ApplicationUser>>(mockStore, null, null, null, null, null, null, null, null);

    var sut = new MembershipController(mockUserManager.Object);
    sut.ModelState.AddModelError("", "invalid data");
    var input = new NewUserInputBuilder().Build();

    //Act
    var actual = await sut.RegisterNewUser(input);

    //Assert
    actual
        .Should().NotBeNull()
        .And.Match<HttpResponseMessage>(_ => _.StatusCode == HttpStatusCode.BadRequest);    
}

FluentAssertions were used to do all the assertions. You could just as easily used Assert.* API.

This should be enough to get you on your way with the above question.