C#

private const int WM_NCHITTEST = 0x0084;
private const int HTTRANSPARENT = (-1);

/// <summary>
/// Overrides the standard Window Procedure to ensure the
/// window is transparent to all mouse events.
/// </summary>
/// <param name="m">Windows message to process.</param>
protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_NCHITTEST)
    {
        m.Result = (IntPtr)HTTRANSPARENT;
    }
    else
    {
        base.WndProc(ref m);
    }
}