1. 添加引用：
   using System.ComponentModel;





2.添加窗体Closing事件
private void Form1_Closing( object sender, CancelEventArgs e ) 
{ 
  string text = "确定退出程序?";
  string caption = "关闭程序";
  if (MessageBox.Show( text, caption, MessageBoxButtons.YesNo) == DialogResult.No ) 
    e.Cancel = true; 
}