Creating a custom validation through DataAnnotations?

Question

Ok so when I want to have a property be validated, I might write something like this:

    [Required]
    [StringLength(255)]
    [DataType(DataType.EmailAddress)         


        

Answer 1

This SO answer givens an answer for MVC2.

And here is a post for MVC3 and up.

Basically you create an Attribute:

public class MyValidationAttribute: ValidationAttribute
{
   public MyValidationAttribute()
   { }

    protected override ValidationResult IsValid(
           object value, ValidationContext validationContext)
    {

        ...
        if (somethingWrong)
        {
            return new ValidationResult(errorMessage); 
        }
        return null; // everything OK
    }
}

Answer 2

The documentation for ValidationAttribute shows how to create your custom validation attribute.