ValidationAttribute's value always null

Question

Here's a simple validation attribute I wrote to enforce number-only strings (I use it to validate phone numbers, postal codes and alike):

public class NumericAttribute : ValidationAttribute {

    public NumericAttribute() {
        ErrorMessage = SharedModelResources.ValidationStrings.Numeric; // Message coming from a .resx file
    }

    public override bool IsValid(object value) {

        string stringValue = value as string;

        if (stringValue.IsNotNull())
            foreach (var c in stringValue)
                if (!char.IsDigit(c))
                    return false;

        return true;
    }

}

The problem is that when used to validate other than strings, let's say a decimal, short, int or anything else, object value is ALWAYS null. Why validate numeric only decimals? Because if I don't, I get the default validation message 'The value 'x' is not valid for 'propertyName'. I'm aware of the solutions for the specific problem of the default validation message.

I just want to understand WHY it's always null...

BTW, I'm using VS2008, ASP.NET MVC 2 (final), .NET 3.5 SP1.

Answer 1

Ok, thanks @mcl for suggesting debugging all the way down (View > Controller > Model) to the validation attribute!

Here's what happened: when inserting invalid input in a textbox binded to anything that's not a string in the model, the ´DefaultModelBinder´ sets it's value to ´null´ before it reaches even the controller! (Anyone have more info on that? I would be glad!)