public Form1()        
  {             
         InitializeComponent();
         textBox.Text = "Auto Select Text Demo";             
         textBox.Tag = false;             
         textBox.GotFocus += new EventHandler(textBox_GotFocus);             
         textBox.MouseUp += new MouseEventHandler(textBox_MouseUp);        

 }

  void textBox_MouseUp(object sender, MouseEventArgs e)        
 {            
         //如果鼠標左鍵操作并且標記存在，則執行全選            
       if (e.Button == MouseButtons.Left && (bool)textBox.Tag == true)           
         {                 
                 textBox.SelectAll();            
         }

        //取消全選標記             
      textBox.Tag = false;        
 }

 
 void textBox_GotFocus(object sender, EventArgs e)       
  {             
            textBox.Tag = true;    //設置標記             
         textBox.SelectAll();   //注意1        
 }