Async validation with CustomValidator

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Silverlight4

Hi guys,
If I set CustomValidator for some class (the class is defined in DomainService.metadata) then validation is not async. Otherwise, if I set CustomValidator for some properties then validation is async. How can I make async validation with CustomValidator for my class?

First example:

[CustomValidation(typeof(MyValidator), "IsNameAvailable")]
    public partial class MyClass {
       ...
    }

In above example there isn't async validation.

Second example (async validation):

public partial class MyClass {
    [Required]
    [CustomValidation(typeof(MyValidator), "IsNameAvailable")]
    public string name { get; set; }
}

Thanks in advance!