Conditional data annotation

Is there a way to make a data annotation conditional? I have a table Party where I store both organisations and persons. If I'm adding an organisation I don't want the field surname to be required, but only if I'm adding a person.

public class Party
{
    [Required(ErrorMessage = "{0} is missing")]
    [DisplayName("Your surname")]
    public object surname { get; set; }

    [DisplayName("Type")]
    public object party_type { get; set; }
    ...
}  

I'd like a condition for the required data annotation of surname, something like:
if (party_type=='P') then surname is required, else the surname can be empty.

EDIT
If I have to move this validation to the controller, how would I do it there? How can I trigger the same error message from there?