How to achieve Base64 URL safe encoding in C#?

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!");

Microsoft.IdentityModel.Tokens is a NuGet package that has to be downloaded.

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.