internal static class NativeMethods
{
    [DllImport("user32")]
    internal static extern bool SetForegroundWindow(IntPtr hwnd);

    [DllImport("SecBridge.dll")]
    internal static extern Int32 CheckSecurity();

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern bool SetDllDirectory(string lpPathName);
}

private IntPtr ExistingAppInstanceHandle()
{
    var currentProcessId = Process.GetCurrentProcess().Id;
    var currentProcessName = Process.GetCurrentProcess().ProcessName;

    var instances = Process.GetProcessesByName(currentProcessName);

    if (!instances.Any() || !instances.Where( x => x.Id != currentProcessId).Any())
    {
        return IntPtr.Zero;
    }

    return instances.Where( x => x.Id != currentProcessId).First().MainWindowHandle;
}