private void MainForm_Load(object sender, EventArgs e) 
{ 
  //绑定事件 
  MouseMove += Form_MouseMove; 
  MouseDown += Form_MouseDown; 
} 
private Point _mousePoint; 
private void Form_MouseMove(object sender, MouseEventArgs e) 
{ 
  if (e.Button == MouseButtons.Left) 
  { 
    Top = MousePosition.Y - _mousePoint.Y; 
    Left = MousePosition.X - _mousePoint.X; 
  } 
} 
private void Form_MouseDown(object sender, MouseEventArgs e) 
{ 
  if (e.Button == MouseButtons.Left) 
  { 
    _mousePoint.X = e.X; 
    _mousePoint.Y = e.Y; 
  } 
} 

Top -= SystemInformation.CaptionHeight;

Top -= SystemInformation.FormBorderSize.Height
Left -= SystemInformation.FormBorderSize.Width