asp mvc custom validation how to get other values - asp.net-mvc

Question

I am comparing a username using a custom validation. I am checking if it's the same as the old value or if it passes the regex it is acceptable but if it is not then throw an error. How do i get the UserID from the viewmodel if possible?
[EmailValidation]
public string Username{ get; set; }
public int UserID { get; set; }
public class EmailValidationAttribute : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
User user= User.getUserByID(UserID); //How to get UserID?
string username= value.ToString();
if (user.Username== username || IsValid(username))
{
return ValidationResult.Success;
}
else
{
return new ValidationResult("Error");
}
}

Answer 1

You can get UserID from ObjectInstance of ValidationContext. In your case it will be instance of your User class.
public class EmailValidationAttribute : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
UserDto dto = (UserDto)validationContext.ObjectInstance;
User user= User.getUserByID(dto.UserID);
string username = value.ToString();
if (user.Username == username || IsValid(username))
{
return ValidationResult.Success;
}
else
{
return new ValidationResult("Error");
}
}
}
If you want something more universal you can add property that specify property name and use reflection to get the value.

Answer 2

The easiest way is to move your validation to the class instead of being a field validator.
For reference: http://jeffhandley.com/archive/2009/10/16/validator.aspx
class MyClass : IValidatableObject {
public string EmailAddress { get; set; }
public int UserID { get; set; }
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext) {
User user= User.getUserByID(UserID); // UserID is available because this is a method on the object
string username= value.ToString();
if (user.Username== username || IsValid(username)) {
// it's fine?
} else {
yield return new ValidationResult("Error");
}
}
}

Answer 3

I would recommend a 2 step process here so that you may use this attribute in the future with any new classes that come up:
1) Create an abstract class that contains the UserID in it, and have any classes that have this EmailValidationAttribute extend that class.
2) In your validation you can cast value to the abstract class type, and just grab the UserID from there.