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;
         &

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