private Point mouseOff;//鼠标移动位置变量
  private bool leftFlag;//鼠标是否为左键
  private void Form1_MouseDown(object sender, MouseEventArgs e)
  {
   if(e.Button == MouseButtons.Left)
   {
    mouseOff = new Point(-e.X, -e.Y);//获得当前鼠标的坐标
    leftFlag = true;
   }
  }

  private void Form1_MouseMove(object sender, MouseEventArgs e)
  {
   if (leftFlag)
   {
    Point mouseSet = Control.MousePosition;//获得移动后鼠标的坐标
    mouseSet.Offset(mouseOff.X, mouseOff.Y);//设置移动后的位置
    Location = mouseSet;
   }
  }

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