设置自己的区域性或更改日期时间的当前区域性

我正在尝试在我的项目中设置自定义区域性。但是我有一些问题，我已经搜索了谷歌并找到了以下代码。但是我有一些问题，请在评论中观察。

using System;
using System.IO;
using System.Globalization;
public class Example 
{
    public static void Main() 
    {
        // Persist the date and time data.
        StreamWriter sw = new StreamWriter(@".'DateData.dat");
        // Create a DateTime value.      
        DateTime dtIn = DateTime.Now;
        // Retrieve a CultureInfo object.
        CultureInfo invC = CultureInfo.InvariantCulture;
        // Convert the date to a string and write it to a file.
        sw.WriteLine(dtIn.ToString("r", invC));//what r mean?. if r is the custem culture      variabel then how we determin it.
        sw.Close();
        // Restore the date and time data.
        StreamReader sr = new StreamReader(@".'DateData.dat");
        String input;
        while ((input = sr.ReadLine()) != null) 
        {
            Console.WriteLine("Stored data: {0}'n" , input);    
            // Parse the stored string.
            DateTime dtOut = DateTime.Parse(input, invC, DateTimeStyles.RoundtripKind);
            // Create a French (France) CultureInfo object.
            CultureInfo frFr = new CultureInfo("fr-FR");
            // Displays the date formatted for the "fr-FR" culture.
            Console.WriteLine("Date formatted for the {0} culture: {1}" , 
                       frFr.Name, dtOut.ToString("f", frFr));// f?
            // Creates a German (Germany) CultureInfo object.
            CultureInfo deDe= new CultureInfo("de-De");
            // Displays the date formatted for the "de-DE" culture.
            Console.WriteLine("Date formatted for {0} culture: {1}" , 
                       deDe.Name, dtOut.ToString("f", deDe));
        }
        sr.Close();
    }
}