A question about writing into app.config file

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

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();
      }
   }
}