alex.cs

    // For users without passwords
    public class BaseRegisterVm
    {
        public Constants.AuthenticationType UserType { get; set; }

        public BaseRegisterVm()
        {
            UserType = Constants.AuthenticationType.Windows;
        }

        [Required]
        [EmailAddress]
        [Display(Name = "Email")]
        public string Email { get; set; }

        public virtual string Password { get; set; }
        public virtual string ConfirmPassword { get; set; }
    }

    public class RegisterVm : BaseRegisterVm
    {
        [Required]
        [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
        [DataType(DataType.Password)]
        [Display(Name = "Password")]
        public override string Password { get; set; }

        [DataType(DataType.Password)]
        [Display(Name = "Confirm password")]
        [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
        public override string ConfirmPassword { get; set; }
    }