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.

Please note: solutions with "input.Replace" are all over Internet and still they are not correct.

See, if you have ONE decoded symbol and then use "replace", ALL symbols in "input" will be replaced, and then all following decoding will be broken.

More correct solution:

public static string DecodeQuotedPrintable(string input, string charSet)
    {

        Encoding enc;

        try
        {
            enc = Encoding.GetEncoding(charSet);
        }
        catch
        {
            enc = new UTF8Encoding();
        }

        input = input.Replace("=\r\n=", "=");
        input = input.Replace("=\r\n ", "\r\n ");
        input = input.Replace("= \r\n", " \r\n");
        var occurences = new Regex(@"(=[0-9A-Z]{2})", RegexOptions.Multiline); //{1,}
        var matches = occurences.Matches(input);

        foreach (Match match in matches)
        {
            try
            {
                byte[] b = new byte[match.Groups[0].Value.Length / 3];
                for (int i = 0; i < match.Groups[0].Value.Length / 3; i++)
                {
                    b[i] = byte.Parse(match.Groups[0].Value.Substring(i * 3 + 1, 2), System.Globalization.NumberStyles.AllowHexSpecifier);
                }
                char[] hexChar = enc.GetChars(b);
                input = input.Replace(match.Groups[0].Value, new String(hexChar));

            }
            catch
            { Console.WriteLine("QP dec err"); }
        }
        input = input.Replace("?=", ""); //.Replace("\r\n", "");

        return input;
    }

I know its old question, but this should help

    private static string GetPrintableCharacter(char character)
    {
        switch (character)
        {
            case '\a':
            {
                return "\\a";
            }
            case '\b':
            {
                return "\\b";
            }
            case '\t':
            {
                return "\\t";
            }
            case '\n':
            {
                return "\\n";
            }
            case '\v':
            {
                return "\\v";
            }
            case '\f':
            {
                return "\\f";
            }
            case '\r':
            {
                return "\\r";
            }
            default:
            {
                if (character == ' ')
                {
                    break;
                }
                else
                {
                    throw new InvalidArgumentException(Resources.NOTSUPPORTCHAR, new object[] { character });
                }
            }
        }
        return "\\x20";
    }

    public static string GetPrintableText(string text)
    {
        StringBuilder stringBuilder = new StringBuilder(1024);
        if (text == null)
        {
            return "[~NULL~]";
        }
        if (text.Length == 0)
        {
            return "[~EMPTY~]";
        }
        stringBuilder.Remove(0, stringBuilder.Length);
        int num = 0;
        for (int i = 0; i < text.Length; i++)
        {
            if (text[i] == '\a' || text[i] == '\b' || text[i] == '\f' || text[i] == '\v' || text[i] == '\t' || text[i] == '\n' || text[i] == '\r' || text[i] == ' ')
            {
                num += 3;
            }
        }
        int length = text.Length + num;
        if (stringBuilder.Capacity < length)
        {
            stringBuilder = new StringBuilder(length);
        }
        string str = text;
        for (int j = 0; j < str.Length; j++)
        {
            char chr = str[j];
            if (chr > ' ')
            {
                stringBuilder.Append(chr);
            }
            else
            {
                stringBuilder.Append(StringHelper.GetPrintableCharacter(chr));
            }
        }
        return stringBuilder.ToString();
    }

A bit improved version of (non-working) code from Martin Murphy:

static Regex reQuotHex = new Regex(@"=[0-9A-H]{2}", RegexOptions.Multiline|RegexOptions.Compiled);

public static string DecodeQuotedPrintable(string input)
{
    var dic = new Dictionary<string, string>();
    foreach (var qp in new HashSet<string>(reQuotHex.Matches(input).Cast<Match>().Select(m => m.Value)))
        dic[qp] = ((char)Convert.ToInt32(qp.Substring(1), 16)).ToString();
        
    foreach (string qp in dic.Keys) {
        input = input.Replace(qp, dic[qp]);
    }
    return input.Replace("=\r\n", "");
}