MVC ValidationAttribute to ensure property is empty. Sounds strange but I create for an anti-bot field to which is hidden in css but bots still try filling in.

MustBeEmptyAttribute.cs

public class MustBeEmptyAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value == null)
        {
            return true;
        }
        if (value is string)
        {
            if (((string) value).Length == 0)
                return true;
        }
        return false;
    }
}