using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ConvertToBase64(@"ciao mamma"));

        Console.WriteLine(ConvertToString(@"Y2lhbyBtYW1tYQ=="));

    }

​

    public static string ConvertToBase64(string s)

    {

        byte[] bytesFromString = Encoding.UTF8.GetBytes(s);  // don't use Default encoding, see https://docs.microsoft.com/en-us/dotnet/api/system.text.encoding.default?view=net-5.0

        return Convert.ToBase64String(bytesFromString);

    }

​

    public static string ConvertToString(string s)

    {

        byte[] bytesFromBase64 = Convert.FromBase64String(s);

        return Encoding.UTF8.GetString(bytesFromBase64);

    }

}

​