C#: Class for decoding Quoted-Printable encoding?

There is functionality in the framework libraries to do this, but it doesn't appear to be cleanly exposed. The implementation is in the internal class System.Net.Mime.QuotedPrintableStream. This class defines a method called DecodeBytes which does what you want. The method appears to be used by only one method which is used to decode MIME headers. This method is also internal, but is called fairly directly in a couple of places, e.g., the Attachment.Name setter. A demonstration:

using System;
using System.Net.Mail;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Attachment attachment = Attachment.CreateAttachmentFromString("", "=?iso-8859-1?Q?=A1Hola,_se=F1or!?=");
            Console.WriteLine(attachment.Name);
        }
    }
}

Produces the output:

¡Hola,_señor!

You may have to do some testing to ensure carriage returns, etc are treated correctly although in a quick test I did they seem to be. However, it may not be wise to rely on this functionality unless your use-case is close enough to decoding of a MIME header string that you don't think it will be broken by any changes made to the library. You might be better off writing your own quoted-printable decoder.

I extended the solution of Martin Murphy and I hope it will work in every case.

private static string DecodeQuotedPrintables(string input, string charSet)
{           
    if (string.IsNullOrEmpty(charSet))
    {
        var charSetOccurences = new Regex(@"=\?.*\?Q\?", RegexOptions.IgnoreCase);
        var charSetMatches = charSetOccurences.Matches(input);
        foreach (Match match in charSetMatches)
        {
            charSet = match.Groups[0].Value.Replace("=?", "").Replace("?Q?", "");
            input = input.Replace(match.Groups[0].Value, "").Replace("?=", "");
        }
    }

    Encoding enc = new ASCIIEncoding();
    if (!string.IsNullOrEmpty(charSet))
    {
        try
        {
            enc = Encoding.GetEncoding(charSet);
        }
        catch
        {
            enc = new ASCIIEncoding();
        }
    }

    //decode iso-8859-[0-9]
    var occurences = new Regex(@"=[0-9A-Z]{2}", RegexOptions.Multiline);
    var matches = occurences.Matches(input);
    foreach (Match match in matches)
    {
        try
        {
            byte[] b = new byte[] { byte.Parse(match.Groups[0].Value.Substring(1), System.Globalization.NumberStyles.AllowHexSpecifier) };
            char[] hexChar = enc.GetChars(b);
            input = input.Replace(match.Groups[0].Value, hexChar[0].ToString());
        }
        catch { }
    }

    //decode base64String (utf-8?B?)
    occurences = new Regex(@"\?utf-8\?B\?.*\?", RegexOptions.IgnoreCase);
    matches = occurences.Matches(input);
    foreach (Match match in matches)
    {
        byte[] b = Convert.FromBase64String(match.Groups[0].Value.Replace("?utf-8?B?", "").Replace("?UTF-8?B?", "").Replace("?", ""));
        string temp = Encoding.UTF8.GetString(b);
        input = input.Replace(match.Groups[0].Value, temp);
    }

    input = input.Replace("=\r\n", "");
    return input;
}