Question

I have the following property in my Model :

[StringLength(100, ErrorMessage = "Must be less than 100 Chars", MinimumLength = 3)]
public List<KeyValuePair<int, string>> Authors { get; set; }

How can I Validate each string into the above list with DataAnnotation Validation attribute in MVC3 ?

Is it possible at all ?

Answer 1

Custom validation to the rescue! You need to do the following:

1. Implement the IValidatableObject interface
2. Implement the IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
3. Implement your logic to determine that each string has less than 100 characters

Here’s the code

public class YourModel : IValidatableObject
{
    public List<KeyValuePair<int, string>> Authors { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        foreach(KeyValuePair<int, string> myKvp in Authors)
        {
            if(myKvp.Value.Length >= 100)
            {
                yield return new ValidationResult("Must be less than 100 characters");
            }
        }
    }
}

That way you can do a call to if(Model.IsValid) in your controller action, and return any errors that are reported. If your KeyValuePair entries are referring to a specific entity you can even do something like:

yield return new ValidationResult("Must be less than 100 characters", new string[] { myKvp.Key.ToString() });

You’d need to tailor it to fit the ID of the attribute on your page. This way, the error message could be specific to an input on your page.

I override IValidatableObject in many places as there’s many cases where I do validation that’s dependent on the state of my object. Your case is a bit different, but it’s certainly do-able as can be seen from the above example. (All that’s off the top of my head, however, so may not be perfect!)