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