How to achieve Base64 URL safe encoding in C#?

Question

It is common to simply swap alphabet for use in urls, so that no %-encoding is necessary; only 3 of the 65 characters are problematic - +, / and =. the most common replacements are - in place of + and _ in place of /. As for the padding: just remove it (the =); you can infer the amount of padding needed. At the other end: just reverse the process:

string returnValue = System.Convert.ToBase64String(toEncodeAsBytes)
        .TrimEnd(padding).Replace('+', '-').Replace('/', '_');

with:

static readonly char[] padding = { '=' };

and to reverse:

string incoming = returnValue
    .Replace('_', '/').Replace('-', '+');
switch(returnValue.Length % 4) {
    case 2: incoming += "=="; break;
    case 3: incoming += "="; break;
}
byte[] bytes = Convert.FromBase64String(incoming);
string originalText = Encoding.ASCII.GetString(bytes);

The interesting question, however, is: is this the same approach that the "common codec library" uses? It would certainly be a reasonable first thing to test - this is a pretty common approach.

You can use class Base64UrlEncoder from namespace Microsoft.IdentityModel.Tokens.

const string StringToEncode = "He=llo+Wo/rld";

var encodedStr = Base64UrlEncoder.Encode(StringToEncode);
var decodedStr = Base64UrlEncoder.Decode(encodedStr);

if (decodedStr == StringToEncode)
    Console.WriteLine("It works!");
else
    Console.WriteLine("Dangit!");

Another option, if you are using ASP.NET Core, would be to use Microsoft.AspNetCore.WebUtilities.WebEncoders.Base64UrlEncode.

If you are not using ASP.NET Core, the WebEncoders source is available under the Apache 2.0 License.

Based off the answers here with some performance improvements, we've published a very easy to use url-safe base64 implementation to NuGet with the source code available on GitHub (MIT licensed).

Usage is as easy as

var bytes = Encoding.UTF8.GetBytes("Foo");
var encoded = UrlBase64.Encode(bytes);
var decoded = UrlBase64.Decode(encoded);

To get an URL-safe base64-like encoding, but not "base64url" according to RFC4648, use System.Web.HttpServerUtility.UrlTokenEncode(bytes) to encode, and System.Web.HttpServerUtility.UrlTokenDecode(bytes) to decode.

Simplest solution: (with no padding)

private static string Base64UrlEncode(string input) {
    var inputBytes = System.Text.Encoding.UTF8.GetBytes(input);
    // Special "url-safe" base64 encode.
    return Convert.ToBase64String(inputBytes)
      .Replace('+', '-') // replace URL unsafe characters with safe ones
      .Replace('/', '_') // replace URL unsafe characters with safe ones
      .Replace("=", ""); // no padding
  }

Credit goes to: Tholle