PostMessage到当前活动进程不适用于所有进程？

Question

我有这个代码：

    const UInt32 WM_KEYDOWN = 0x0100;

    const int VK_DOWN = 0x28;
    const int VK_UP = 0x26;

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

    [DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll")]
    private static extern Int32 GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

    private static Process GetProcessByHandle(IntPtr hwnd)
    {
        try
        {
            uint processID;
            GetWindowThreadProcessId(hwnd, out processID);
            return Process.GetProcessById((int)processID);
        }
        catch { return null; }
    }

    public static Process GetActiveProcess()
    {
        IntPtr hwnd = GetForegroundWindow();
        return hwnd != null ? GetProcessByHandle(hwnd) : null;
    }

    public static void KeyUp()
    {
        PostMessage(GetActiveProcess().MainWindowHandle, WM_KEYDOWN, VK_UP, 0);
    }

    public static void KeyDown()
    {
        PostMessage(GetActiveProcess().MainWindowHandle, WM_KEYDOWN, VK_DOWN, 0);
    }

当我调用KeyUp / KeyDown时，它仅适用于某些进程，如text editos，visual studio。

但不适用于Explorer.exe，iTunes.exe，Chrome.exe，Outlook.exe，FileZilla.exe等。

我在控制台上运行调试：

    while (true)
    {
        Process currentProcess = GetActiveProcess();
        if (currentProcess != null)
            Console.WriteLine(currentProcess.MainWindowTitle);
        Thread.Sleep(1000);
    }

这是我从一些应用程序得到的：

ProjName - Microsoft Visual Studio (Administrator)
?PostMessage to Current Active Proccess doesn't work for all proccess ? - StackOverFlow - Google Chrome?
FileZilla
Inbox - Outlook Data File - Microsoft Outlook

在Explorer.exe上我得到空行..