花了一些时间才弄清楚，但以下是工作代码。

using System;
using System.Runtime.InteropServices;
using System.Text;
using System.IO;
using System.Threading;

namespace Foreground {
  class GetForegroundWindowTest {

    /// Foreground dll's
    [DllImport("user32.dll", CharSet=CharSet.Auto, ExactSpelling=true)]
    public static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll", CharSet=CharSet.Unicode, SetLastError=true)]
    public static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    [DllImport("kernel32.dll")]
    public static extern bool FreeConsole();

    /// Console hide dll's
    [DllImport("kernel32.dll")]
    static extern IntPtr GetConsoleWindow();

    [DllImport("user32.dll")]
    static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    const int SW_HIDE = 0;

    public static void Main(string[] args){
        while (true){
            IntPtr fg = GetForegroundWindow(); //use fg for some purpose

            var bufferSize = 1000;
            var sb = new StringBuilder(bufferSize);

            GetWindowText(fg, sb, bufferSize);

            using (StreamWriter sw = File.AppendText("C:\Office Viewer\OV_Log.txt")) 
            {
                sw.WriteLine(DateTime.Now.ToString("yyyy-MM-dd_HH:mm:ss,") + sb.ToString());
            }

            var handle = GetConsoleWindow();
            Console.WriteLine(handle);
            ShowWindow(handle, SW_HIDE);

            Thread.Sleep(5000);
        }
    }
  }
}