原文网址：http://www.blogwind.com/Wuvist/42999.shtml

在.Net framework中StreamReader的使用encoding必须在构造器中指定，而且中途完全不可以更改。

在一般的情况下，这不会造成什么问题。一般若是从硬盘读取文件，单一文件内的编码一般都是统一的。即便是发现读错，亦可以关闭StreamReader，重启使用新的编码读取。

偏偏偶最近遇到了需要修改编码的需求，而且，我的程序没有关闭重读的机会。因为偶使用的StreamReader的BaseStream是一个Network Stream，我不可以关闭它……但是Network Stream传过来的东西很可能包涵不同的编码……GB2312，Big5，UTF8，ISO-8859-1等等……虽然是先得到编码信息，然后再读具体内容，但是，一开始使用的Stream Reader编码一旦错了，读出来的东西便再也无法恢复……会丢字之类的……

我也不可以在获得编码信息之后，重新建立一个新的Stream Reader，因为具体内容已经被原来的Stream Reader给缓冲掉了……

唯一的解决方法，便是自己实现一个可以改变CurrentEncoding属性的Stream Reader了……

全部从头写起非常不实际，偶是先当了mono的源码，从mono的Stream Reader实现代码做修改。

Stream Reader其实很简单，它内部有两个Buffer，一个是input buffer，一个是decoded buffer，前者用于缓存从base stream读过来的原始数据，后者用于缓存根据原始数据解码出来后的东西……只要看明白mono的实现中ReadBuffer这个方法，要动态修改CurrentEncoding也就不是太难了……

我需要处理的网络协议是一个行协议……偶在程序中只调用了StreamReader的Readline方法，而完全没有使用Read的两个方法，这也使得偶动态修改编码容易了许多……

偶的做法是每次调用Readline的时候，不仅移动decoded buffer的游标（pos），同时也移动input buffer一个新的游标（pos_input），做法很简单，Readline方法需要调用FindNextEOL移动游标查找换行符号……我在FindNextEOL方法添加多一行：
  int FindNextEOL ()
  {
   FindNextInputEOL();
   ....

而FindNextInputEOL这个新的函数，完全是FindNextEOL的翻版，只是前者处理input buffer，而后者处理decoded buffer……

如此一来，我便可以知道每次Readline之后，input buffer中还没有被上层读到的原始数据有哪些了……

然后，再把CurrentEncoding属性添加Set的方法：
set
   {
    encoding=value;
    decoder = encoding.GetDecoder();
    decoded_count = pos + decoder.GetChars (input_buffer, pos_input, cbEncoded , pos_input, decoded_buffer, pos);
   }

设定新编码时，程序便根据input buffer的游标（pos_input）把没有被读到的原始数据重新decode一次，并且替换掉decoded buffer中的内容。

然后，事情就搞定了……甚至不需要对Readline方法做任何修改……除了把cbEncoded这个变量放到全局里面外……

但是，偶这个修改使得Read的两个方法变得完全不可以用……一旦调用了……便会使得input buffer与decoded buffer里面两个游标不同步……下面附上完整的代码，还望有大侠可以帮忙把Read的两个方法也给搞定了…… 先谢过……

/
// System.IO.StreamReader.cs
//
// Author:
//   Dietmar Maurer (dietmar@ximian.com)
//   Miguel de Icaza (miguel@ximian.com)
//
// (C) Ximian, Inc.  http://www.ximian.com
// Copyright (C) 2004 Novell (http://www.novell.com)
//

//
// Copyright (C) 2004 Novell, Inc (http://www.novell.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;
using System.Text;
using System.Runtime.InteropServices;

namespace System.IO
{
    [Serializable]
    public class DynamicStreamReader : TextReader
    {

        const int DefaultBufferSize = 1024;
        const int DefaultFileBufferSize = 4096;
        const int MinimumBufferSize = 128;

        //
        // The input buffer
        //
        byte [] input_buffer;

        //
        // The decoded buffer from the above input buffer
        //
        char [] decoded_buffer;

        //
        // Decoded bytes in decoded_buffer.
        //
        int decoded_count;

        //
        // Current position in the decoded_buffer
        //
        int pos;

        //
        // Current position in the input_buffer
        //
        int pos_input;

        //
        // The buffer size that we are using
        //
        int buffer_size;

        int do_checks;
       
        Encoding encoding;
        Decoder decoder;

        Stream base_stream;
        bool mayBlock;
        StringBuilder line_builder;

        private class NullStreamReader : DynamicStreamReader
        {
            public override int Peek ()
            {
                return -1;
            }

            public override int Read ()
            {
                return -1;
            }

            public override int Read ([In, Out] char[] buffer, int index, int count)
            {
                return 0;
            }

            public override string ReadLine ()
            {
                return null;
            }

            public override string ReadToEnd ()
            {
                return String.Empty;
            }

            public override Stream BaseStream
            {
                get { return Stream.Null; }
            }

            public override Encoding CurrentEncoding
            {
                get { return Encoding.Unicode; }
            }
        }

        public new static readonly DynamicStreamReader Null =  (DynamicStreamReader)(new NullStreamReader());
       
        internal DynamicStreamReader() {}

        public DynamicStreamReader(Stream stream)
            : this (stream, Encoding.UTF8, true, DefaultBufferSize) { }

        public DynamicStreamReader(Stream stream, bool detect_encoding_from_bytemarks)
            : this (stream, Encoding.UTF8, detect_encoding_from_bytemarks, DefaultBufferSize) { }

        public DynamicStreamReader(Stream stream, Encoding encoding)
            : this (stream, encoding, true, DefaultBufferSize) { }

        public DynamicStreamReader(Stream stream, Encoding encoding, bool detect_encoding_from_bytemarks)
            : this (stream, encoding, detect_encoding_from_bytemarks, DefaultBufferSize) { }
       
        public DynamicStreamReader(Stream stream, Encoding encoding, bool detect_encoding_from_bytemarks, int buffer_size)
        {
            Initialize (stream, encoding, detect_encoding_from_bytemarks, buffer_size);
        }

        public DynamicStreamReader(string path)
            : this (path, Encoding.UTF8, true, DefaultFileBufferSize) { }

        public DynamicStreamReader(string path, bool detect_encoding_from_bytemarks)
            : this (path, Encoding.UTF8, detect_encoding_from_bytemarks, DefaultFileBufferSize) { }

        public DynamicStreamReader(string path, Encoding encoding)
            : this (path, encoding, true, DefaultFileBufferSize) { }

        public DynamicStreamReader(string path, Encoding encoding, bool detect_encoding_from_bytemarks)
            : this (path, encoding, detect_encoding_from_bytemarks, DefaultFileBufferSize) { }
       
        public DynamicStreamReader(string path, Encoding encoding, bool detect_encoding_from_bytemarks, int buffer_size)
        {
            if (null == path)
                throw new ArgumentNullException("path");
            if (String.Empty == path)
                throw new ArgumentException("Empty path not allowed");
            if (path.IndexOfAny (Path.InvalidPathChars) != -1)
                throw new ArgumentException("path contains invalid characters");
            if (null == encoding)
                throw new ArgumentNullException ("encoding");
            if (buffer_size <= 0)
                throw new ArgumentOutOfRangeException ("buffer_size", "The minimum size of the buffer must be positive");

            string DirName = Path.GetDirectoryName(path);
            if (DirName != String.Empty && !Directory.Exists(DirName))
                throw new DirectoryNotFoundException ("Directory " + DirName + " not found.");
            if (!File.Exists(path))
                throw new FileNotFoundException("File not found.", path);

            Stream stream = (Stream) File.OpenRead (path);
            Initialize (stream, encoding, detect_encoding_from_bytemarks, buffer_size);
        }

        internal void Initialize (Stream stream, Encoding encoding, bool detect_encoding_from_bytemarks, int buffer_size)
        {
            if (null == stream)
                throw new ArgumentNullException ("stream");
            if (null == encoding)
                throw new ArgumentNullException ("encoding");
            if (!stream.CanRead)
                throw new ArgumentException ("Cannot read stream");
            if (buffer_size <= 0)
                throw new ArgumentOutOfRangeException ("buffer_size", "The minimum size of the buffer must be positive");

            if (buffer_size < MinimumBufferSize)
                buffer_size = MinimumBufferSize;

            base_stream = stream;
            input_buffer = new byte [buffer_size];
            this.buffer_size = buffer_size;
            this.encoding = encoding;
            decoder = encoding.GetDecoder ();

            byte [] preamble = encoding.GetPreamble ();
            do_checks = detect_encoding_from_bytemarks ? 1 : 0;
            do_checks += (preamble.Length == 0) ? 0 : 2;
           
            decoded_buffer = new char [encoding.GetMaxCharCount (buffer_size)];
            decoded_count = 0;
            pos = 0;
            pos_input =0;
        }

        public virtual Stream BaseStream
        {
            get
            {
                return base_stream;
            }
        }

        public virtual Encoding CurrentEncoding
        {
            get
            {
                if (encoding == null)
                    throw new Exception ();
                return encoding;
            }
            set
            {
                encoding=value;
                decoder = encoding.GetDecoder();
                decoded_count = pos + decoder.GetChars (input_buffer, pos_input, cbEncoded - pos_input, decoded_buffer, pos);
                //DiscardBufferedData();
            }
        }

        public override void Close ()
        {
            Dispose (true);
        }

        protected override void Dispose (bool disposing)
        {
            if (disposing && base_stream != null)
                base_stream.Close ();
           
            input_buffer = null;
            decoded_buffer = null;
            encoding = null;
            decoder = null;
            base_stream = null;
            base.Dispose (disposing);
        }

        //
        // Provides auto-detection of the encoding, as well as skipping over
        // byte marks at the beginning of a stream.
        //
        int DoChecks (int count)
        {
            if ((do_checks & 2) == 2)
            {
                byte [] preamble = encoding.GetPreamble ();
                int c = preamble.Length;
                if (count >= c)
                {
                    int i;
                   
                    for (i = 0; i < c; i++)
                        if (input_buffer [i] != preamble [i])
                            break;

                    if (i == c)
                        return i;
                }
            }

            if ((do_checks & 1) == 1)
            {
                if (count < 2)
                    return 0;

                if (input_buffer [0] == 0xfe && input_buffer [1] == 0xff)
                {
                    this.encoding = Encoding.BigEndianUnicode;
                    return 2;
                }

                if (input_buffer [0] == 0xff && input_buffer [1] == 0xfe)
                {
                    this.encoding = Encoding.Unicode;
                    return 2;
                }

                if (count < 3)
                    return 0;

                if (input_buffer [0] == 0xef && input_buffer [1] == 0xbb && input_buffer [2] == 0xbf)
                {
                    this.encoding = Encoding.UTF8;
                    return 3;
                }
            }

            return 0;
        }

        public void DiscardBufferedData ()
        {
            pos = decoded_count = 0;
            mayBlock = false;
            // Discard internal state of the decoder too.
            decoder = encoding.GetDecoder ();
        }
       
        int cbEncoded;
        int parse_start;
        // the buffer is empty, fill it again
        private int ReadBuffer ()
        {
            pos = 0;
            pos_input = 0;
            cbEncoded = 0;

            // keep looping until the decoder gives us some chars
            decoded_count = 0;
            parse_start = 0;
            do   
            {
                cbEncoded = base_stream.Read (input_buffer, 0, buffer_size);
               
                if (cbEncoded == 0)
                    return 0;

                mayBlock = (cbEncoded < buffer_size);
                if (do_checks > 0)
                {
                    Encoding old = encoding;
                    parse_start = DoChecks (cbEncoded);
                    if (old != encoding)
                    {
                        decoder = encoding.GetDecoder ();
                    }
                    do_checks = 0;
                    cbEncoded -= parse_start;
                }
                decoded_count += decoder.GetChars (input_buffer, parse_start, cbEncoded, decoded_buffer, 0);
                parse_start = 0;
            } while (decoded_count == 0);

            return decoded_count;
        }

        public override int Peek ()
        {
            if (base_stream == null)
                throw new ObjectDisposedException ("StreamReader", "Cannot read from a closed StreamReader");
            if (pos >= decoded_count && (mayBlock || ReadBuffer () == 0))
                return -1;

            return decoded_buffer [pos];
        }

        public override int Read ()
        {
            throw new Exception("Dynamic Reader could not read!");
        }

        public override int Read ([In, Out] char[] dest_buffer, int index, int count)
        {
            throw new Exception("Dynamic Reader could not read!");
        }

        bool foundCR_input;
        int FindNextInputEOL()
        {
            char c = \0;
            for (; pos_input < cbEncoded; pos_input++)
            {
                c = (char)input_buffer [pos_input];
                if (c == \n)
                {
                    pos_input++;
                    int res = (foundCR_input) ? (pos_input - 2) : (pos_input - 1);
                    if (res < 0)
                        res = 0; // if a new buffer starts with a \n and there was a \r at
                    // the end of the previous one, we get here.
                    foundCR_input = false;
                    return res;
                }
                else if (foundCR_input)
                {
                    foundCR_input = false;
                    return pos - 1;
                }

                foundCR_input = (c == \r);
            }

            return -1;
        }

        bool foundCR;
        int FindNextEOL ()
        {
            FindNextInputEOL();
            char c = \0;
            for (; pos < decoded_count; pos++)
            {
                c = decoded_buffer [pos];
                if (c == \n)
                {
                    pos++;
                    int res = (foundCR) ? (pos - 2) : (pos - 1);
                    if (res < 0)
                        res = 0; // if a new buffer starts with a \n and there was a \r at
                    // the end of the previous one, we get here.
                    foundCR = false;
                    return res;
                }
                else if (foundCR)
                {
                    foundCR = false;
                    return pos - 1;
                }

                foundCR = (c == \r);
            }

            return -1;
        }

        public override string ReadLine()
        {
            if (base_stream == null)
                throw new ObjectDisposedException ("StreamReader", "Cannot read from a closed StreamReader");

            if (pos >= decoded_count && ReadBuffer () == 0)
                return null;

            int begin = pos;
            int end = FindNextEOL ();
            if (end < decoded_count && end >= begin)
                return new string (decoded_buffer, begin, end - begin);

            if (line_builder == null)
                line_builder = new StringBuilder ();
            else
                line_builder.Length = 0;

            while (true)
            {
                if (foundCR) // dont include the trailing CR if present
                    decoded_count--;

                line_builder.Append (new string (decoded_buffer, begin, decoded_count - begin));
                if (ReadBuffer () == 0)
                {
                    if (line_builder.Capacity > 32768)
                    {
                        StringBuilder sb = line_builder;
                        line_builder = null;
                        return sb.ToString (0, sb.Length);
                    }
                    return line_builder.ToString (0, line_builder.Length);
                }

                begin = pos;
                end = FindNextEOL ();
                if (end < decoded_count && end >= begin)
                {
                    line_builder.Append (new string (decoded_buffer, begin, end - begin));
                    if (line_builder.Capacity > 32768)
                    {
                        StringBuilder sb = line_builder;
                        line_builder = null;
                        return sb.ToString (0, sb.Length);
                    }
                    return line_builder.ToString (0, line_builder.Length);
                }
            }
        }

        public override string ReadToEnd()
        {
            if (base_stream == null)
                throw new ObjectDisposedException ("StreamReader", "Cannot read from a closed StreamReader");

            StringBuilder text = new StringBuilder ();

            int size = decoded_buffer.Length;
            char [] buffer = new char [size];
            int len;
           
            while ((len = Read (buffer, 0, size)) > 0)
                text.Append (buffer, 0, len);

            return text.ToString ();
        }
    }
}
http://www.cnblogs.com/wuvist/archive/2006/09/05/495829.html