private Point mouseOff;//鼠標移動位置變量
  private bool leftFlag;//鼠標是否為左鍵
  private void Form1_MouseDown(object sender, MouseEventArgs e)
  {
   if(e.Button == MouseButtons.Left)
   {
    mouseOff = new Point(-e.X, -e.Y);//獲得當前鼠標的坐標
    leftFlag = true;
   }
  }

  private void Form1_MouseMove(object sender, MouseEventArgs e)
  {
   if (leftFlag)
   {
    Point mouseSet = Control.MousePosition;//獲得移動后鼠標的坐標
    mouseSet.Offset(mouseOff.X, mouseOff.Y);//設置移動后的位置
    Location = mouseSet;
   }
  }

  private void Form1_MouseUp(object sender, MouseEventArgs e)
  {
   if (leftFlag)
   {
    leftFlag = false;
   }
  }