>>511
むしろ
フォームをドラッグで検索かけると>>512か

[DllImport( "user32.dll" )]
private static extern int SendMessage( IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam );
[DllImport( "user32.dll" )]
private static extern bool ReleaseCapture();

private void Form1_MouseDown( object sender, MouseEventArgs e )
{
　if( e.Button == MouseButtons.Left )
　{
　　const int WM_SYSCOMMAND = 0x0112;
　　const int SC_MOVE = 0xf010;

　　ReleaseCapture();
　　SendMessage( this.Handle, WM_SYSCOMMAND, (IntPtr)(SC_MOVE | 2), IntPtr.Zero );
　}
}

の方が一般的だと思うが