[Extension()]
public static InvokeServerMethodOperation InvokeServerMethodAsync( 
   EntityManager em,
   InvokeServerMethodArgs args,
   Action<InvokeServerMethodOperation> userCallback,
   object userState
)

// Sample showing asynchronous invocation of server method

// In client class:
private void MakeAsyncCall() {
  EntityManager mgr = new DomainModelEntityManager();
  // Make async call
  Guid myToken = Guid.NewGuid();
  mgr.InvokeServerMethodAsync(new ServerMethodDelegate(Order.GetNumberOfOrdersSlow), 
      InvokeServerMethodAsyncCompleted, myToken, 
 	    new DateTime(1995, 1, 1), new DateTime(1999, 1, 1));
}
private void InvokeServerMethodAsyncCompleted(InvokeServerMethodOperation e) {
  Guid token = (Guid)e.UserState;
  if (!e.Cancelled) {
     MessageBox.Show("my async result = " + Convert.ToInt32(e.Result).ToString());
  }
}

// Sample method defined in Order entity class:
public class Order {
//...

  // ServerMethodDelegate method, called from client
  [AllowRpc]
  public static Object GetNumberOfOrdersSlow(IPrincipal pPrincipal, EntityManager pMgr, params Object[] pArgs) {
     // Sleep to make this slower to show async
     System.Threading.Thread.Sleep(2000);
     DateTime dt1 = pArgs[0] as DateTime;
     DateTime dt2 = pArgs[1] as DateTime;
     return pMgr.Order.Where(o => o.OrderDate >= dt1 && o.OrderDate <= dt2).Count();
  }
}