public partial class Form1 : Form
{
  /*
   * 理解了下面的几个概念,就能完全明白相对坐标的变化.
   * MouseEventArgs e 为事件鼠标参数,因此,e.Location 指示了位于事件源上的光标坐标
   * Cursor.Position 获取的是相对于用户屏幕的光标坐标
   * PointToClient() 方法可将屏幕坐标 Cursor.Position 换算成工作区的坐标
   * 因此,换算后的 Cursor.Position 减去 e.Location 得到的始终是事件源的 Location
  */

  /// 

  /// 鼠标按下为true
  /// 
  private bool Mousedown;

  /// 

  /// 鼠标在事件源的位置
  /// 
  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)
    {
      // 获取当前屏幕的光标坐标
       Point pTemp = new Point(Cursor.Position.X, Cursor.Position.Y);
      // 转换成工作区坐标
       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;
    }
  }
}