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) // don't 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 ();
}
}
}