About setWindowLong & getWindowLong

// WINAPI Constants
private const int GWL_EXSTYLE = -20;
private const int WS_EX_LAYERED = 0x80000;
private const int LWA_ALPHA = 2;

// WINAPI Functions
[DllImport("user32.dll", EntryPoint = "GetWindowLongA", SetLastError = true)]
private static extern int GetWindowLong(IntPtr hwnd, int nIndex);

[DllImport("user32.dll", EntryPoint = "SetWindowLongA", SetLastError = true)]
private static extern int SetWindowLong(IntPtr hwnd, int nIndex, int dwNewLong);

[DllImport("user32.dll")]
private static extern bool SetLayeredWindowAttributes(IntPtr hwnd, uint crKey, byte bAlpha, uint dwFlags);

public static void ApplyMenuTransparency(IntPtr Handle, byte Transparency)
{
    SetWindowLong(Handle, GWL_EXSTYLE, GetWindowLong(Handle, GWL_EXSTYLE) | WS_EX_LAYERED);
    SetLayeredWindowAttributes(Handle, 0, Transparency, LWA_ALPHA);
}

protected override void OnRenderMenuItemBackground(ToolStripItemRenderEventArgs e)
{
    if (e.Item.IsOnDropDown)
        Manager.ApplyMenuTransparency(e.ToolStrip.Handle, this.MenuTransparency);
    base.OnRenderMenuItemBackground(e);
}