IMAP规范（RFC 2060，5.1.3。邮箱国际命名约定）介绍了如何处理文件夹名称中的非ASCII字符。它定义了修改后的 UTF-7编码：
按惯例，国际邮箱 使用a指定名称 修改版的UTF-7编码 在[UTF-7]中描述。目的 这些修改是纠正的 UTF-7存在以下问题：
UTF-7使用“+”字符进行移位;这与之相冲突 邮箱名称中常用的“+”，特别是USENET 新闻组名称。
UTF-7的编码是BASE64，使用“/”字符;这个 与使用“/”作为流行的层次结构分隔符冲突。
UTF-7禁止未编码使用“\”;这与之相冲突 使用“\”作为流行的层次结构分隔符。
UTF-7禁止未编码使用“〜”;这与之相冲突 在某些服务器中使用“〜”作为主目录指示符。
UTF-7允许多个替代形式表示相同的形式 串;特别是，可打印的US-ASCII字符可以 以编码形式表示。
在修改后的UTF-7中，可打印的US-ASCII字符除“＆amp;”外代表自己; 也就是说，八位字节值为0x20-0x25的字符 和0x27-0x7e。字符“＆amp;” （0x26）由两个八位字节序列“＆amp; - ”表示。
所有其他字符（八位字节值 表示0x00-0x1f，0x7f-0xff和所有Unicode 16位八位字节 在改进的BASE64中，还有一个 从[UTF-7]修改“，”是 用而不是“/” 修改后的BASE64绝不能用于表示 任何打印US-ASCII字符 这可以代表自己。
“＆安培;”用于转移到修改 BASE64和“ - ”转回US-ASCII。所有名称均以US-ASCII开头， 并且必须以US-ASCII结尾（即， 以16位Unicode结尾的名称 八位组必须以“ - ”结尾。
在我开始实现它之前，我的问题是：那里有一些 .NET代码/库（甚至在框架中）吗？我找不到.NET资源（只有implementations for other languages/frameworks）。
谢谢！
答案 0 :(得分：2)
这太专业化，不能出现在框架中。虽然许多不完整的“实现”在我看来根本没有对转换感到麻烦，但很乐意将所有非us-ascii字符传递给IMAP服务器。
但是我过去实现了它，它实际上只有30行代码。你遍历字符串中的所有字符并输出它们，如果它们落在0x20和0x7e之间的范围内（不要忘记在“＆amp;”之后附加“ - ”）否则收集所有非us-ascii并使用它们转换它们UTF7（或UTF8 + base64，我不太确定）将“/”替换为“，”。此外，您需要保持“移位状态”，例如你是否正在编码非us-ascii或输出us-ascii并附加过渡令牌“＆amp;”和“ - ”关于国家改变。
答案 1 :(得分：0)
未经测试，但如果应用了this的错误修正，则Aleksey的MIT许可代码看起来不错：
/// <summary>
/// Takes a UTF-16 encoded string and encodes it as modified UTF-7.
/// </summary>
/// <param name="s">The string to encode.</param>
/// <returns>A UTF-7 encoded string</returns>
/// <remarks>IMAP uses a modified version of UTF-7 for encoding international mailbox names. For
/// details, refer to RFC 3501 section 5.1.3 (Mailbox International Naming Convention).</remarks>
internal static string UTF7Encode(string s) {
StringReader reader = new StringReader(s);
StringBuilder builder = new StringBuilder();
while (reader.Peek() != -1) {
char c = (char)reader.Read();
int codepoint = Convert.ToInt32(c);
// It's a printable ASCII character.
if (codepoint > 0x1F && codepoint < 0x7F) {
builder.Append(c == '&' ? "&-" : c.ToString());
} else {
// The character sequence needs to be encoded.
StringBuilder sequence = new StringBuilder(c.ToString());
while (reader.Peek() != -1) {
codepoint = Convert.ToInt32((char)reader.Peek());
if (codepoint > 0x1F && codepoint < 0x7F)
break;
sequence.Append((char)reader.Read());
}
byte[] buffer = Encoding.BigEndianUnicode.GetBytes(
sequence.ToString());
string encoded = Convert.ToBase64String(buffer).Replace('/', ',').
TrimEnd('=');
builder.Append("&" + encoded + "-");
}
}
return builder.ToString();
}
/// <summary>
/// Takes a modified UTF-7 encoded string and decodes it.
/// </summary>
/// <param name="s">The UTF-7 encoded string to decode.</param>
/// <returns>A UTF-16 encoded "standard" C# string</returns>
/// <exception cref="FormatException">The input string is not a properly UTF-7 encoded
/// string.</exception>
/// <remarks>IMAP uses a modified version of UTF-7 for encoding international mailbox names. For
/// details, refer to RFC 3501 section 5.1.3 (Mailbox International Naming Convention).</remarks>
internal static string UTF7Decode(string s) {
StringReader reader = new StringReader(s);
StringBuilder builder = new StringBuilder();
while (reader.Peek() != -1) {
char c = (char)reader.Read();
if (c == '&' && reader.Peek() != '-') {
// The character sequence needs to be decoded.
StringBuilder sequence = new StringBuilder();
while (reader.Peek() != -1) {
if ((c = (char)reader.Read()) == '-')
break;
sequence.Append(c);
}
string encoded = sequence.ToString().Replace(',', '/');
int pad = encoded.Length % 4;
if (pad > 0)
encoded = encoded.PadRight(encoded.Length + (4 - pad), '=');
try {
byte[] buffer = Convert.FromBase64String(encoded);
builder.Append(Encoding.BigEndianUnicode.GetString(buffer));
} catch (Exception e) {
throw new FormatException(
"The input string is not in the correct Format.", e);
}
} else {
if (c == '&' && reader.Peek() == '-')
reader.Read();
builder.Append(c);
}
}
return builder.ToString();
}
在当前状态下不使用this代码，它包含[...] UTF7.GetBytes([...]) [...] .Replace('+', '&')-它使用现有的.Net UTF-7编码例程，并且（其中包括）将+替换为结果中的&。这是错误，因为它不仅将“移位字符”从+更改为&（有意且正确），而且还将所有+个字符在以base64编码的区域内（不得更改为&）。
答案 2 :(得分：0)
//
// ImapEncoding.cs
//
// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
//
// Copyright (c) 2013-2019 Microsoft Corp. (www.microsoft.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
using System.Text;
namespace MailKit.Net.Imap {
static class ImapEncoding
{
const string utf7_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";
static readonly byte[] utf7_rank = {
255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255, 62, 63,255,255,255,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61,255,255,255,255,255,255,
255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,255,255,255,255,255,
255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,255,255,255,255,255,
};
public static string Decode (string text)
{
var decoded = new StringBuilder ();
bool shifted = false;
int bits = 0, v = 0;
int index = 0;
char c;
while (index < text.Length) {
c = text[index++];
if (shifted) {
if (c == '-') {
// shifted back out of modified UTF-7
shifted = false;
bits = v = 0;
} else if (c > 127) {
// invalid UTF-7
return text;
} else {
byte rank = utf7_rank[(byte) c];
if (rank == 0xff) {
// invalid UTF-7
return text;
}
v = (v << 6) | rank;
bits += 6;
if (bits >= 16) {
char u = (char) ((v >> (bits - 16)) & 0xffff);
decoded.Append (u);
bits -= 16;
}
}
} else if (c == '&' && index < text.Length) {
if (text[index] == '-') {
decoded.Append ('&');
index++;
} else {
// shifted into modified UTF-7
shifted = true;
}
} else {
decoded.Append (c);
}
}
return decoded.ToString ();
}
static void Utf7ShiftOut (StringBuilder output, int u, int bits)
{
if (bits > 0) {
int x = (u << (6 - bits)) & 0x3f;
output.Append (utf7_alphabet[x]);
}
output.Append ('-');
}
public static string Encode (string text)
{
var encoded = new StringBuilder ();
bool shifted = false;
int bits = 0, u = 0;
for (int index = 0; index < text.Length; index++) {
char c = text[index];
if (c >= 0x20 && c < 0x7f) {
// characters with octet values 0x20-0x25 and 0x27-0x7e
// represent themselves while 0x26 ("&") is represented
// by the two-octet sequence "&-"
if (shifted) {
Utf7ShiftOut (encoded, u, bits);
shifted = false;
bits = 0;
}
if (c == 0x26)
encoded.Append ("&-");
else
encoded.Append (c);
} else {
// base64 encode
if (!shifted) {
encoded.Append ('&');
shifted = true;
}
u = (u << 16) | (c & 0xffff);
bits += 16;
while (bits >= 6) {
int x = (u >> (bits - 6)) & 0x3f;
encoded.Append (utf7_alphabet[x]);
bits -= 6;
}
}
}
if (shifted)
Utf7ShiftOut (encoded, u, bits);
return encoded.ToString ();
}
}
}