Base64 Codec in C#

Program to an interface, not an implementation. So, first we prepare a Codec interface.

// Codec.cs - Codec interface 
// (ignore namespace)
interface Codec { 
  string Encode(string s);
  string Decode(string s);
}

Next, we provide a Base64 codec implementation to support Codec interface.

// Base64Codec.cs - Base64 Codec implementation 
// (ignore namespace)
using System;
using System.Text;

public class Base64Codec : Codec {
  public string Encode(string s) {
    return Convert.ToBase64String(Encoding.UTF8.GetBytes(s));
  }

  public string Decode(string s) {
    byte[] decodedBytes = Convert.FromBase64String(s);

    Decoder decoder = new UTF8Encoding().GetDecoder();
    int size = decoder.GetCharCount(decodedBytes, 
                                    0, 
                                    decodedBytes.Length);
    char[] decodedChars = new char[size];
    decoder.GetChars(decodedBytes, 
                     0, 
                     decodedBytes.Length, 
                     decodedChars, 
                     0);
    return new String(decodedChars);
  }
}

Done.