Question

我正在开发一个电子邮件查看器，其中一个读取.eml文件并在浏览器控件中显示该消息。我找到了代码片段，它可以显示7位和可引用的可打印消息（Content-Transfer-Encoding：quoted-printable / Content-Transfer-Encoding：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;
    }

我用谷歌搜索了一个8位解码器，却找不到任何解码器。我真的需要一个8位解码器，你知道一个好的工作吗？

更新

相关标题：

 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

我想我不需要案例“8bit”：部分。正如SLaks所提到的，我需要在进程的最开始将邮件源加载到字节数组而不是字符串。从字节数组中检查邮件头中的 charset = 将给出相应的代码页。

Answer 1

这就是我解决问题的方法：

// 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;

}

Answer 2

由于StringReader()，您可能会遇到执行问题。在某个地方有人需要将原始字节转换为字符串。除非你在此之前做了一些特别的事情，否则.Net会为你做这件事，并且通常会使用计算机默认设置。

8位时代的问题是第8位有几十个实现（如果不是更多），并且没有真正的方法可以告诉字节使用哪个实现。如果您使用的是ASCII，那么第8位设置的任何内容都将转换为ASCII 63 - ?。如果你使用的是UTF-8，那么设置了第8位的任何东西都会尝试读取下一个到五个字符（see Wikipedia for more info），如果这不起作用，它将被转换为UTF-8 65533 {{ 1}}这就是你所看到的。如果您手动指定编码，例如您正在给出的编码�，那么将正确解析该第8位。下面是显示此关闭的示例代码。而不是转储到koi8-r我是消息传递拳击，但只要你记得change your console's encoding，你就可以切换它。

Console

总而言之，如果你获得了UTF-8替换字符（你是），这意味着你已经丢失了这些字节的原始值，你需要先修复它。无论将字节转换为字符串是什么都需要考虑var bytes = new byte[] { 226 }; var s1 = System.Text.Encoding.ASCII.GetString(bytes);//Invalid var s2 = System.Text.Encoding.UTF8.GetString(bytes);//Invalid var s3 = System.Text.Encoding.GetEncoding("koi8-r").GetString(bytes); //Б MessageBox.Show(String.Format("{0} {1} {2}", s1, s2, s3));，事后你就无法做到。

解码8位邮件消息：内容传输编码：8位

2 个答案: