LogicClass.cs

public class UserController : Controller
{
    [HttpPost]
    public ActionResult Create(string email, string password, string firstName, string lastName)
    {
      new UserLogic().Create(email, password, firstName, lastName);
      
      return View("UserCreated");      
    }
}

public class UserLogic
{
  public void Create(string email, string password, string firstName, string lastName)
  {    
      if(string.IsNullOrWhiteSpace(email))
        throw new InvalidEmailException();
      
      if(!new Regex("^[A-Za-z]\w{6,}[A-Za-z]$").IsMatch(password))
        throw new InvalidPasswordException();
    
      if(string.IsNullOrWhiteSpace(firstName) || string.IsNullOrWhiteSpace(lastame))
        throw new InvalidNameException();
      
      var repo = new UserRepository();
      repo.Create(new UserEntity{
        Email = email;
        Password = password;
        FirstName = firstName;
        LastName = lastName;
      });
  }
}