Silverlight：纠结的快捷键问题

void MainPage_KeyDown(object sender,KeyEventArgs e)
{

    e.Handled = true;
    ModifierKeys keys = Keyboard.Modifiers;
    bool b1 = false;
    if ((e.Key >= Key.A && e.Key <= Key.Z) || (e.Key>=Key.D0 && e.Key<=Key.D9) || (e.Key>=Key.NumPad0 && (e.Key<=Key.NumPad9))) 
    {
        b1 = true;
    }
    bool b2 = (keys != ModifierKeys.None);
    bool b3 = (b1 && b2);
    string msg = string.Format("按键={0}，控制键={1}，按键是否有效={2}，控制键是否有效={3}，组合键是否有效={4}", e.Key, keys, b1, b2, b3);            
    System.Diagnostics.Debug.WriteLine(msg);
    test.Text = msg;

    if (b3) 
    {
        MessageBox.Show(string.Format("捕获到的组合键为：{0}+{1}", keys.ToString(), e.Key.ToString()));
    }
}