webservice and event SelectionChanging

[tuan]

Hi All,

I use web service for my application silverlight and I have problem as follows:
    In my application, i use RadGridView and event SelectionChanging 
  And In event SelectionChanging, I call web service for check business logic and I want assign

e.Cancel by result of web service, but can not.

Ex:

 

void grdView_SelectionChanging(object sender, Telerik.Windows.Controls.SelectionChangingEventArgs e)
 {
            TestServiceClient client = new TestServiceClient();
            client.DoWorkCompleted += ((s, e1) =>
                        {
                            e.Cancel = e1.Result;
                        });
            client.DoWorkAsync();
}

who is have solution for this problem?
thanks