有关写入app.config文件的问题

我从这篇文章中获得了一个示例.

这是App.config文件:

I have an example from the article.

here is the App.config file:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
 <appSettings>
    <add key="Setting1" value="Very" />
    <add key="Setting2" value="Easy" />
 </appSettings>
</configuration>

这是控制台程序write.cs:

here is the console program write.cs:

using System;
using System.Collections.Generic;
using System.Text;
using System.Configuration;

namespace AppSettings
{
   class Program
   {
     static void ShowConfig()
     {

        // For read access you do not need to call OpenExeConfiguraton
        foreach(string key in ConfigurationManager.AppSettings)
        {
           string value = ConfigurationManager.AppSettings[key];
           Console.WriteLine("Key: {0}, Value: {1}", key, value);
        }
     }

     static void Main(string[] args)
     {

        ShowConfig();

        // Open App.Config of executable
        System.Configuration.Configuration config =
         ConfigurationManager.OpenExeConfiguration
                    (ConfigurationUserLevel.None);

        // Add an Application Setting.
        config.AppSettings.Settings.Add("ModificationDate",
                       DateTime.Now.ToLongTimeString() + " ");

        // Save the changes in App.config file.
        config.Save(ConfigurationSaveMode.Modified);

        // Force a reload of a changed section.
        ConfigurationManager.RefreshSection("appSettings");
        ShowConfig();
      }
   }
}

我尝试过的事情:

我期望此ModificationDate将出现在App.config中，但事实证明是在write.exe.config文件中.为什么会这样?

What I have tried:

I am expecting this ModificationDate shall come up in App.config, but it turns out in write.exe.config file. why did this happen? the new variable can not be added into App.Config directly?