[DllImport("user32.dll")]
static extern Int32 FlashWindowEx(ref FLASHWINFO pwfi);

[StructLayout(LayoutKind.Sequential)]
public struct FLASHWINFO
{
    public UInt32 cbSize;
    public IntPtr hwnd;
    public Int32 dwFlags;
    public UInt32 uCount;
    public Int32 dwTimeout;
}

public static void Flash()
{
    FLASHWINFO fw = new FLASHWINFO();

    fw.cbSize = Convert.ToUInt32(Marshal.SizeOf(typeof(FLASHWINFO)));
    fw.hwnd = Process.GetCurrentProcess().MainWindowHandle;
    fw.dwFlags = 2;
    fw.uCount = 5;

    FlashWindowEx(ref fw);
}