原文網址：http://www.blogwind.com/Wuvist/42999.shtml
在.Net framework中StreamReader的使用encoding必須在構造器中指定，而且中途完全不可以更改。
在一般的情況下，這不會造成什么問題。一般若是從硬盤讀取文件，單一文件內的編碼一般都是統一的。即便是發現讀錯，亦可以關閉StreamReader，重啟使用新的編碼讀取。
偏偏偶最近遇到了需要修改編碼的需求，而且，我的程序沒有關閉重讀的機會。因為偶使用的StreamReader的BaseStream是一個Network Stream，我不可以關閉它……但是Network Stream傳過來的東西很可能包涵不同的編碼……GB2312，Big5，UTF8，ISO-8859-1等等……雖然是先得到編碼信息，然后再讀具體內容，但是，一開始使用的Stream Reader編碼一旦錯了，讀出來的東西便再也無法恢復……會丟字之類的……
我也不可以在獲得編碼信息之后，重新建立一個新的Stream Reader，因為具體內容已經被原來的Stream Reader給緩沖掉了……
唯一的解決方法，便是自己實現一個可以改變CurrentEncoding屬性的Stream Reader了……
全部從頭寫起非常不實際，偶是先當了mono的源碼，從mono的Stream Reader實現代碼做修改。
Stream Reader其實很簡單，它內部有兩個Buffer，一個是input buffer，一個是decoded buffer，前者用于緩存從base stream讀過來的原始數據，后者用于緩存根據原始數據解碼出來后的東西……只要看明白mono的實現中ReadBuffer這個方法，要動態修改CurrentEncoding也就不是太難了……
我需要處理的網絡協議是一個行協議……偶在程序中只調用了StreamReader的Readline方法，而完全沒有使用Read的兩個方法，這也使得偶動態修改編碼容易了許多……
偶的做法是每次調用Readline的時候，不僅移動decoded buffer的游標（pos），同時也移動input buffer一個新的游標（pos_input），做法很簡單，Readline方法需要調用FindNextEOL移動游標查找換行符號……我在FindNextEOL方法添加多一行：
int FindNextEOL ()
{
FindNextInputEOL();
....
而FindNextInputEOL這個新的函數，完全是FindNextEOL的翻版，只是前者處理input buffer，而后者處理decoded buffer……
如此一來，我便可以知道每次Readline之后，input buffer中還沒有被上層讀到的原始數據有哪些了……
然后，再把CurrentEncoding屬性添加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);
}
設定新編碼時，程序便根據input buffer的游標（pos_input）把沒有被讀到的原始數據重新decode一次，并且替換掉decoded buffer中的內容。
然后，事情就搞定了……甚至不需要對Readline方法做任何修改……除了把cbEncoded這個變量放到全局里面外……
但是，偶這個修改使得Read的兩個方法變得完全不可以用……一旦調用了……便會使得input buffer與decoded buffer里面兩個游標不同步……下面附上完整的代碼，還望有大俠可以幫忙把Read的兩個方法也給搞定了…… 先謝過……
/
// 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 // 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 ();
}
}
}
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