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; }
    ...
}  
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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?

You can make your model inherit from IValidatableObject and then put your custom logic into the Validate method. You'll have to remove the RequredAttribute from the property as well. You will have to write some custom javascript to validate this rule on the client as the Validate method doesn't translate into the unobtrusive validation framework. Note I changed your properties to strings to avoid casting.

Also, if you have other validation errors from attributes, those will fire first and prevent the Validate method from being run so you only detect these errors if the attribute-based validation is ok.

public class Party : IValidatableObject
{
    [DisplayName("Your surname")]
    public string surname { get; set; }

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

    public IEnumerable<ValidationResult> Validate( ValidationContext context )
    {
         if (party_type == "P" && string.IsNullOrWhitespace(surname))
         {
              yield return new ValidationResult("Surname is required unless the party is for an organization" );
         }
    }
}

On the client you can do something like:

 <script type="text/javascript">
 $(function() {
      var validator = $('form').validate();
      validator.rules('add', {
          'surname': {
              required: {
                 depends: function(element) {
                      return $('[name=party_type]').val() == 'P';
                 }
              },
              messages: {
                  required: 'Surname is required unless the party is for an organization.'
              }
           }
      });
 });
 </script>

I know this topic has some time, but if you'd like to use only declarative validation for that, you could just use such a simple construction (see this reference for further possibilities):

[RequiredIf(DependentProperty = "party_type", TargetValue = "P")]
public string surname { get; set; }

public string party_type { get; set; }

Update:

Since the ExpressiveAnnotations 2.0, there is a breaking change. Now the same thing can be done in a simpler manner:

[RequiredIf("party_type == 'P'")]
public string surname { get; set; }

In Controller you can check like this: Before if (ModelState.IsValid)

if (model.party_type == 'p')
{
   this.ModelState.Remove("surname");
}