using System;

public class Program

{

    public static void Main()

    {

        var texts = new[] { "", "hello", "привет", "hello_with_underscore", "русское_с_подчркиванием", "digits_123", "цифры_1234"};

        foreach(var text in texts)

        {

        Console.WriteLine(text);

        Test(text);

            Console.WriteLine(new String('-', 50));

        }

    }

    private static void Test(string plainText)

    {       

        var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);

        var encoded =  System.Convert.ToBase64String(plainTextBytes);

        Console.WriteLine(encoded); 

        var base64EncodedBytes = System.Convert.FromBase64String(encoded);

        var decoded =  System.Text.Encoding.UTF8.GetString(base64EncodedBytes);

        Console.WriteLine(decoded); 

    }

}