Code for decoding/encoding a modified base64 URL

Question

Also check class HttpServerUtility with UrlTokenEncode and UrlTokenDecode methods that is handling URL safe Base64 encoding and decoding.

Note 1: The result is not a valid Base64 string. Some unsafe characters for URL are replaced.

Note 2: The result differs from the base64url algorithm in RFC4648.

///<summary>
/// Base 64 Encoding with URL and Filename Safe Alphabet using UTF-8 character set.
///</summary>
///<param name="str">The origianl string</param>
///<returns>The Base64 encoded string</returns>
public static string Base64ForUrlEncode(string str)
{
    byte[] encbuff = Encoding.UTF8.GetBytes(str);
    return HttpServerUtility.UrlTokenEncode(encbuff);
}
///<summary>
/// Decode Base64 encoded string with URL and Filename Safe Alphabet using UTF-8.
///</summary>
///<param name="str">Base64 code</param>
///<returns>The decoded string.</returns>
public static string Base64ForUrlDecode(string str)
{
    byte[] decbuff = HttpServerUtility.UrlTokenDecode(str);
    return Encoding.UTF8.GetString(decbuff);
}

This ought to pad it out correctly:-

 base64 = base64.PadRight(base64.Length + (4 - base64.Length % 4) % 4, '=');

Not enough points to comment, but in case it helps, the code snippet that Sushil found in the link provided (JSON Web Signature ietf draft) works for when encoding Base 64 as a parameter in URL.

Copied snippet below for those that are lazy:

    static string Base64UrlEncode(byte[] arg)
    {
        string s = Convert.ToBase64String(arg); // Regular base64 encoder
        s = s.Split('=')[0]; // Remove any trailing '='s
        s = s.Replace('+', '-'); // 62nd char of encoding
        s = s.Replace('/', '_'); // 63rd char of encoding
        return s;
    }

    static byte[] Base64UrlDecode(string arg)
    {
        string s = arg;
        s = s.Replace('-', '+'); // 62nd char of encoding
        s = s.Replace('_', '/'); // 63rd char of encoding
        switch (s.Length % 4) // Pad with trailing '='s
        {
            case 0: break; // No pad chars in this case
            case 2: s += "=="; break; // Two pad chars
            case 3: s += "="; break; // One pad char
            default: throw new System.Exception(
              "Illegal base64url string!");
        }
        return Convert.FromBase64String(s); // Standard base64 decoder
    }

I hit here while looking for code to do encode/decode for base64url encoding which is little different than base64 as explained in the question.

Found c# code snippet in this document. JSON Web Signature ietf draft

In comparison to the accepted answer, here is how you would fundamentally decode a base64 encoded url, using C#:

Decode:

string codedValue = "base64encodedUrlHere";

string decoded;
byte[] buffer =  Convert.FromBase64String(codedValue);
decoded = Encoding.UTF8.GetString(buffer);