c# - 解码 8 位邮件消息 : Content-Transfer-Encoding: 8bit

Question

我正在开发一个电子邮件查看器，它可以读取 .eml 文件并在浏览器控件中显示消息。我找到了代码片段，它可以显示 7 位和引用打印消息(内容传输编码:引用打印/内容传输编码:base64)。 我需要的是解码 8 位消息。

    private static AlternateView ImportText(StringReader r, string encoding, System.Net.Mime.ContentType contentType)
    {
        string line = string.Empty;
        StringBuilder b = new StringBuilder();
        while ((line = r.ReadLine())!= null)
        {
            switch (encoding)
            {
                case "quoted-printable":
                    if (line.EndsWith("="))
                    {
                        b.Append(DecodeQuotedPrintables(line.TrimEnd('='), contentType.CharSet));
                    }
                    else
                    {
                        b.Append(DecodeQuotedPrintables(line, contentType.CharSet) + "\n");
                    }
                    break;
                case "base64":
                    b.Append(DecodeBase64(line, contentType.CharSet));
                    break;

                case "8bit": // I need an 8bit decoder here!!!
                    b.Append(IneedAn8bitDecoderHere(line, contentType.CharSet));
                    break;
                default:
                    b.Append(line);
                    break;
            }
        }

        AlternateView returnValue = AlternateView.CreateAlternateViewFromString(b.ToString(), null, contentType.MediaType);
        returnValue.TransferEncoding = TransferEncoding.QuotedPrintable;
        return returnValue;
    }

我在谷歌上搜索了一个 8 位解码器，但没有找到。我真的需要一个 8 位解码器吗？你知道一个很好用的解码器吗？

更新:

相关 header :

 MIME-Version: 1.0
 Content-Type: text/plain; charset="koi8-r";
 Content-Transfer-Encoding: 8bit

我的代码中的正文消息(字符串行):

 ����������� �� ����, � �����  ��� � ������        ��������� �������  �   ��������  �������� ��   ������� 

Outlook 在现实世界中显示的内容:

 Фантастично но факт, я снова  как и раньше сделалась статной  и   красивой  примерно за  месяцок 

我认为我不需要case "8bit": 部分。正如 SLaks 所提到的，我需要在流程的最开始将邮件源加载到字节数组而不是字符串中。从字节数组检查邮件 header 中的 charset= 将提供适当的代码页。

Answer 1

我是这样解决问题的:

// My previous method:
string file = File.ReadAllText("koi8-r.eml");

// Correct method:    
Encoding efile = detectTextEncoding("koi8-r.eml", out file);

txtRaw.Text = output;

链接:detectEncoding()

// Function to detect the encoding for UTF-7, UTF-8/16/32 (bom, no bom, little
// & big endian), and local default codepage, and potentially other codepages.
// 'taster' = number of bytes to check of the file (to save processing). Higher
// value is slower, but more reliable (especially UTF-8 with special characters
// later on may appear to be ASCII initially). If taster = 0, then taster
// becomes the length of the file (for maximum reliability). 'text' is simply
// the string with the discovered encoding applied to the file.
public Encoding detectTextEncoding(string filename, out String text, int taster = 1000)
{
byte[] b = File.ReadAllBytes(filename);

//////////////// First check the low hanging fruit by checking if a
//////////////// BOM/signature exists (sourced from http://www.unicode.org/faq/utf_bom.html#bom4)
if (b.Length >= 4 && b[0] == 0x00 && b[1] == 0x00 && b[2] == 0xFE && b[3] == 0xFF) { text = Encoding.GetEncoding("utf-32BE").GetString(b, 4, b.Length - 4); return Encoding.GetEncoding("utf-32BE"); }  // UTF-32, big-endian 
else if (b.Length >= 4 && b[0] == 0xFF && b[1] == 0xFE && b[2] == 0x00 && b[3] == 0x00) { text = Encoding.UTF32.GetString(b, 4, b.Length - 4); return Encoding.UTF32; }    // UTF-32, little-endian
else if (b.Length >= 2 && b[0] == 0xFE && b[1] == 0xFF) { text = Encoding.BigEndianUnicode.GetString(b, 2, b.Length - 2); return Encoding.BigEndianUnicode; }     // UTF-16, big-endian
else if (b.Length >= 2 && b[0] == 0xFF && b[1] == 0xFE) { text = Encoding.Unicode.GetString(b, 2, b.Length - 2); return Encoding.Unicode; }              // UTF-16, little-endian
else if (b.Length >= 3 && b[0] == 0xEF && b[1] == 0xBB && b[2] == 0xBF) { text = Encoding.UTF8.GetString(b, 3, b.Length - 3); return Encoding.UTF8; } // UTF-8
else if (b.Length >= 3 && b[0] == 0x2b && b[1] == 0x2f && b[2] == 0x76) { text = Encoding.UTF7.GetString(b,3,b.Length-3); return Encoding.UTF7; } // UTF-7


//////////// If the code reaches here, no BOM/signature was found, so now
//////////// we need to 'taste' the file to see if can manually discover
//////////// the encoding. A high taster value is desired for UTF-8
if (taster == 0 || taster > b.Length) taster = b.Length;    // Taster size can't be bigger than the filesize obviously.


// Some text files are encoded in UTF8, but have no BOM/signature. Hence
// the below manually checks for a UTF8 pattern. This code is based off
// the top answer at: https://stackoverflow.com/questions/6555015/check-for-invalid-utf8
// For our purposes, an unnecessarily strict (and terser/slower)
// implementation is shown at: https://stackoverflow.com/questions/1031645/how-to-detect-utf-8-in-plain-c
// For the below, false positives should be exceedingly rare (and would
// be either slightly malformed UTF-8 (which would suit our purposes
// anyway) or 8-bit extended ASCII/UTF-16/32 at a vanishingly long shot).
int i = 0;
bool utf8 = false;
while (i < taster - 4)
{
    if (b[i] <= 0x7F) { i += 1; continue; }     // If all characters are below 0x80, then it is valid UTF8, but UTF8 is not 'required' (and therefore the text is more desirable to be treated as the default codepage of the computer). Hence, there's no "utf8 = true;" code unlike the next three checks.
    if (b[i] >= 0xC2 && b[i] <= 0xDF && b[i + 1] >= 0x80 && b[i + 1] < 0xC0) { i += 2; utf8 = true; continue; }
    if (b[i] >= 0xE0 && b[i] <= 0xF0 && b[i + 1] >= 0x80 && b[i + 1] < 0xC0 && b[i + 2] >= 0x80 && b[i + 2] < 0xC0) { i += 3; utf8 = true; continue; }
    if (b[i] >= 0xF0 && b[i] <= 0xF4 && b[i + 1] >= 0x80 && b[i + 1] < 0xC0 && b[i + 2] >= 0x80 && b[i + 2] < 0xC0 && b[i + 3] >= 0x80 && b[i + 3] < 0xC0) { i += 4; utf8 = true; continue; }
    utf8 = false; break;
}
if (utf8 == true) {
    text = Encoding.UTF8.GetString(b);
    return Encoding.UTF8;
}


// The next check is a heuristic attempt to detect UTF-16 without a BOM.
// We simply look for zeroes in odd or even byte places, and if a certain
// threshold is reached, the code is 'probably' UF-16.          
double threshold = 0.1; // proportion of chars step 2 which must be zeroed to be diagnosed as utf-16. 0.1 = 10%
int count = 0;
for (int n = 0; n < taster; n += 2) if (b[n] == 0) count++;
if (((double)count) / taster > threshold) { text = Encoding.BigEndianUnicode.GetString(b); return Encoding.BigEndianUnicode; }
count = 0;
for (int n = 1; n < taster; n += 2) if (b[n] == 0) count++;
if (((double)count) / taster > threshold) { text = Encoding.Unicode.GetString(b); return Encoding.Unicode; } // (little-endian)


// Finally, a long shot - let's see if we can find "charset=xyz" or
// "encoding=xyz" to identify the encoding:
for (int n = 0; n < taster-9; n++)
{
    if (
        ((b[n + 0] == 'c' || b[n + 0] == 'C') && (b[n + 1] == 'h' || b[n + 1] == 'H') && (b[n + 2] == 'a' || b[n + 2] == 'A') && (b[n + 3] == 'r' || b[n + 3] == 'R') && (b[n + 4] == 's' || b[n + 4] == 'S') && (b[n + 5] == 'e' || b[n + 5] == 'E') && (b[n + 6] == 't' || b[n + 6] == 'T') && (b[n + 7] == '=')) ||
        ((b[n + 0] == 'e' || b[n + 0] == 'E') && (b[n + 1] == 'n' || b[n + 1] == 'N') && (b[n + 2] == 'c' || b[n + 2] == 'C') && (b[n + 3] == 'o' || b[n + 3] == 'O') && (b[n + 4] == 'd' || b[n + 4] == 'D') && (b[n + 5] == 'i' || b[n + 5] == 'I') && (b[n + 6] == 'n' || b[n + 6] == 'N') && (b[n + 7] == 'g' || b[n + 7] == 'G') && (b[n + 8] == '='))
        )
    {
        if (b[n + 0] == 'c' || b[n + 0] == 'C') n += 8; else n += 9;
        if (b[n] == '"' || b[n] == '\'') n++;
        int oldn = n;
        while (n < taster && (b[n] == '_' || b[n] == '-' || (b[n] >= '0' && b[n] <= '9') || (b[n] >= 'a' && b[n] <= 'z') || (b[n] >= 'A' && b[n] <= 'Z')))
        { n++; }
        byte[] nb = new byte[n-oldn];
        Array.Copy(b, oldn, nb, 0, n-oldn);
        try {
            string internalEnc = Encoding.ASCII.GetString(nb);
            text = Encoding.GetEncoding(internalEnc).GetString(b);
            return Encoding.GetEncoding(internalEnc);
        }
        catch { break; }    // If C# doesn't recognize the name of the encoding, break.
    }
}


// If all else fails, the encoding is probably (though certainly not
// definitely) the user's local codepage! One might present to the user a
// list of alternative encodings as shown here: https://stackoverflow.com/questions/8509339/what-is-the-most-common-encoding-of-each-language
// A full list can be found using Encoding.GetEncodings();
text = Encoding.Default.GetString(b);
return Encoding.Default;