public class CreateUserCommandHandlerTest
{
    private Mock<UserManager<ApplicationUser>> _userManager;
    private CreateUserCommandHandler _systemUnderTest;

    public CreateUserCommandHandlerTest()
    {
        _userManager = MockUserManager.GetUserManager<ApplicationUser>();
        var user = new ApplicationUser() { UserName = "ancon1", Email = "ancon@mail.com", RoleType = RoleTypes.Anonymous };
        _userManager
            .Setup(u => u.CreateAsync(user, "ancon2")).ReturnsAsync(IdentityResult.Success);
        _systemUnderTest = new CreateUserCommandHandler(_userManager.Object);
    }

    [Fact]
    public async void Handle_GivenValidInput_ReturnsCreatedResponse()
    {
        var command = new CreateUserCommand { Username = "ancon1", Email = "ancon@mail.com", Password = "ancon2", RoleType = RoleTypes.Anonymous };
        var result = await _systemUnderTest.Handle(command, default(CancellationToken));
        Assert.NotNull(result);
        Assert.IsType<Application.Commands.CreatedResponse>(result);
    }
}

public static class MockUserManager
{
    public static Mock<UserManager<TUser>> GetUserManager<TUser>()
        where TUser : class
    {
        var store = new Mock<IUserStore<TUser>>();
        var passwordHasher = new Mock<IPasswordHasher<TUser>>();
        IList<IUserValidator<TUser>> userValidators = new List<IUserValidator<TUser>>
        {
            new UserValidator<TUser>()
        };
        IList<IPasswordValidator<TUser>> passwordValidators = new List<IPasswordValidator<TUser>>
        {
            new PasswordValidator<TUser>()
        };
        userValidators.Add(new UserValidator<TUser>());
        passwordValidators.Add(new PasswordValidator<TUser>());
        var userManager = new Mock<UserManager<TUser>>(store.Object, null, passwordHasher.Object, userValidators, passwordValidators, null, null, null, null);
        return userManager;
    }
}

public class CreateUserCommandHandler : IRequestHandler<CreateUserCommand, BaseCommandResponse>
{
    private readonly UserManager<ApplicationUser> _userManager;

    public CreateUserCommandHandler(UserManager<ApplicationUser> userManager)
    {
        _userManager = userManager;
    }

    public async Task<BaseCommandResponse> Handle(CreateUserCommand createUserCommand, CancellationToken cancellationToken)
    {
        var user = new ApplicationUser { UserName = createUserCommand.Username, Email = createUserCommand.Email, RoleType = createUserCommand.RoleType };
        var result = await _userManager.CreateAsync(user, createUserCommand.Password);
        if (result.Succeeded)
        {
            return new CreatedResponse();
        }

        ErrorResponse errorResponse = new ErrorResponse(result.Errors.Select(e => e.Description).First());

        return errorResponse;
    }
}