public partial class Form1 : Form
{
  /*
   * 理解了下面的幾個概念,就能完全明白相對坐標的變化.
   * MouseEventArgs e 為事件鼠標參數,因此,e.Location 指示了位于事件源上的光標坐標
   * Cursor.Position 獲取的是相對于用戶屏幕的光標坐標
   * PointToClient() 方法可將屏幕坐標 Cursor.Position 換算成工作區的坐標
   * 因此,換算后的 Cursor.Position 減去 e.Location 得到的始終是事件源的 Location
  */

  /// <summary>
  /// 鼠標按下為true
  /// </summary>
  private bool Mousedown;

  /// <summary>
  /// 鼠標在事件源的位置
  /// </summary>
  private int CurX = 0;
  private int CurY = 0;

  public Form1()
  {
    InitializeComponent();
  }

  private void Controls_MouseDown(object sender, MouseEventArgs e)
  {
    CurX = e.X;
    CurY = e.Y;
    Mousedown = true;
    if (sender is TextBox)
    {
      ((TextBox)sender).Cursor = Cursors.Arrow;
    }
  }

  private void Controls_MouseMove(object sender, MouseEventArgs e)
  {
    if (Mousedown)
    {
      // 獲取當前屏幕的光標坐標
       Point pTemp = new Point(Cursor.Position.X, Cursor.Position.Y);
      // 轉換成工作區坐標
       pTemp = this.PointToClient(pTemp);
      // 定位事件源的 Location
      Control control = sender as Control;
      control.Location = new Point(pTemp.X - CurX, pTemp.Y - CurY);        
    }
  }

  private void Controls_MouseUp(object sender, MouseEventArgs e)
  {
    Mousedown = false;
    if (sender is TextBox)
    {
      ((TextBox)sender).Cursor = Cursors.IBeam;
    }
  }
}