ASP.NET MVC Custom Validation in View Model Best Practice

Question

I am trying to use a combination of Domain Driven Design with Test Driven Development for this application I am building in ASP.NET MVC 3. My archictecture is set up with Repositories, Domain Models, View Models, Controllers and Views. All validation will be handled in the view model. I set up my view model to inherit from "IValidatableObject" so that my validation attributes and my custom validation that i set up in the "Validate" method are both executed when my controller method calls "ModelState.IsValid". The problem I am running into is accessing my repository in the Validate method of my view model. I need to access the repository to check for duplicate records in the database. It seems like the best idea would be to create a property of IRepository type and set that property by passing injecting my repository into the constructor of the view model. For example:

public class UserViewModel : IValidatableObject
{
       public UserViewModel(User user, IUserRepository userRepository)
       {
              FirstName = user.FirstName;
              LastName = user.LastName;
              UserRepository = userRepository;
              UserName = user.UserName;
       }
       public string UserName { get; set; }
       public string FirstName { get; set; }
       public string LastName { get; set; }
       public IUserRepository UserRepository { get; set; }
       public IEnumerable<ValidationResult> Validate()
       {
           UserCriteria criteria = new UserCriteria { UserName = this.UserName };
           IList<User> users = UserRepository.SearchUsers(criteria);

           if (users != null && users.count() > 0)
           {
               yield return new ValidationResult("User with username " + this.UserName + " already exists."
           }
       }
}

Do you guys think this is a good idea?

Answer 1

It is good enough but if I were you, I would use

...
private readonly Func<IUserRepository> userRepositoryFactory;
...
public IEnumerable<ValidationResult> Validate()  
   {  
       UserCriteria criteria = new UserCriteria { UserName = this.UserName };  
       using(var UserRepository = userRepositoryFactory())
       {
           IList<User> users = UserRepository.SearchUsers(criteria);  

           if (users != null && users.count() > 0)  
           {  
               yield return new ValidationResult("User with username " + this.UserName + " already exists."  
           }  
       }
   }