public int EndUpdateContact(System.IAsyncResult result) {
                object[] _args = new object[0];
                int _result = ((int)(base.EndInvoke("UpdateContact", _args, result))); //вот в этой строке
                return _result;
            }

private void update_Click(object sender, RoutedEventArgs e)
        {
            if (Validate())
            {
                ServiceContacts1Client contacts = new ServiceContacts1Client();
                contacts.UpdateContactCompleted += new EventHandler<UpdateContactCompletedEventArgs>(contacts_UpdateContactCompleted);
                ServiceContacts contact = new ServiceContacts();
                ServiceContacts tempcont = new ServiceContacts();
                tempcont = (ServiceContacts)dataGrid1.SelectedItem;
                contact.idCont = tempcont.idCont;
                contact.first_name = firstName.Text;
                contact.last_name = lastName.Text;
                contact.mobile_phone = mobile.Text;
                contact.home_phone = homePhone.Text;

                if (image1.Source != null)
                {
                    contact.photo = GetBytesFromImage((BitmapImage)image1.Source);
                }
                contacts.UpdateContactAsync(contact);
            }
        }