Validation in Business Layer: How to call service methods?

Question

I have created a struct on validating models on Business Layer which is based on Steven's answer .

It is working well but something confuses my mind. I inject UserService in CreateUserValidator to able to use GetUser method. This means I call validator in UserService and create a new UserService instance to check whether user exist.

UserService -> [ValidateUser -> new UserService().GetUser()]

It works but seems to be a very bad design. But I have to use that method.

Could you please let me know how I can solve this problem, or Shouldn't I worry about it?

public class CreateUser
{
    public string Name { get; set; }
    public string Email { get; set; }
}

public sealed class CreateUserValidator : Validator<CreateUser>
{
    private IUserService _userService;
    public CreateUserValidator(IUserService userService)
    {
        _userService = userService;
    }
    protected override IEnumerable<ValidationResult> Validate(
        CreateUser entity)
    {

        var user = _userService.GetUserByEmail(entity.Email);
        if (user != null)
        {
            yield return new ValidationResult("Email", "Email address is already exist!");
        }
    }
}

UserService.cs

public partial class UserService : IUserService
{
    IGenericUnitofWork _uow = null;
    private readonly IValidationProvider _validationProvider;
    public UserService(IGenericUnitofWork uow, IValidationProvider validationProvider)
    {
        _uow = uow;
        _validationProvider = validationProvider;
    }

    public User CreateUser(CreateUser createUser)
    {
        this._validationProvider.Validate(createUser);

        var user = new User()
        {
            Email = createUser.Email,
            Name = createUser.Name,
        };
        _uow.Repository<User>().Insert(User);
        _uow.SaveChanges();
        return user;
    }

    public User GetUser(string email)
    {
        var user = _uow.Repository<User>().Where(m => m.Email == email).FirstOrDefault();
        return user;
    }
}

Answer 1

You dependency graph is cyclic. As described in section 6.3 of Dependency Injection in .NET second edition , dependency cycles are often caused by Single Responsibility Principle violations, as is the case in your design.

The problem is that UserService has too many responsibilities: Creating a user is a different responsibility than getting a user. Creating a user can become a very complex use case, as the validation logic hints at, while getting a user is something typically quite simple. It would therefore be beneficial to split UserService into multiple smaller classes. This would allow the validator to depend on the service that allows retrieving the user by its mail address, while the 'create user' service can depend on the validator.

To take it even one step further, you might want to remove validation from the 'create user' service completely. Validation is a cross-cutting concern, and mixing it with the class that contains the business logic, makes such class harder to maintain.

A design that might benefit you is one where you place all state changing business actions behind a common abstraction, as described here .