逐行读取文本文件，并提供精确的偏移/位置报告

Question

我的简单要求：读取一个巨大的（>一百万）行测试文件（对于本示例，假设它是某种 CSV）并保留对该行开头的引用，以便将来更快地查找（读取一行，开始在X）。

我首先尝试了简单而简单的方法，使用 StreamWriter 并访问底层的 BaseStream.Position。不幸的是，这并没有按照我的预期工作：

给定一个包含以下内容的文件

Foo
Bar
Baz
Bla
Fasel

和这个非常简单的代码，

using (var sr = new StreamReader(@"C:\Temp\LineTest.txt")) {
  string line;
  long pos = sr.BaseStream.Position;
  while ((line = sr.ReadLine()) != null) {
    Console.Write("{0:d3} ", pos);
    Console.WriteLine(line);
    pos = sr.BaseStream.Position;
  }
}

输出是：

000 Foo
025 Bar
025 Baz
025 Bla
025 Fasel

我可以想象流正在尝试提供帮助/高效，并且每当有新数据时可能会读取（大）块必要的。对我来说这很糟糕..

最后的问题是：在逐行读取文件时获取（字节，字符）偏移量的任何方法，而不使用基本流并弄乱 \r \n \r\n 和字符串编码等手动？没什么大不了的，真的，我只是不喜欢构建可能已经存在的东西..

Answer 1

您可以创建一个 TextReader 包装器，它将跟踪基本 TextReader 中的当前位置：

public class TrackingTextReader : TextReader
{
    private TextReader _baseReader;
    private int _position;

    public TrackingTextReader(TextReader baseReader)
    {
        _baseReader = baseReader;
    }

    public override int Read()
    {
        _position++;
        return _baseReader.Read();
    }

    public override int Peek()
    {
        return _baseReader.Peek();
    }

    public int Position
    {
        get { return _position; }
    }
}

然后您可以按如下方式使用它：

string text = @"Foo
Bar
Baz
Bla
Fasel";

using (var reader = new StringReader(text))
using (var trackingReader = new TrackingTextReader(reader))
{
    string line;
    while ((line = trackingReader.ReadLine()) != null)
    {
        Console.WriteLine("{0:d3} {1}", trackingReader.Position, line);
    }
}

Answer 2

经过搜索、测试并做了一些疯狂的事情之后，我的代码需要解决（我目前在我的产品中使用此代码）。

public sealed class TextFileReader : IDisposable
{

    FileStream _fileStream = null;
    BinaryReader _binReader = null;
    StreamReader _streamReader = null;
    List<string> _lines = null;
    long _length = -1;

    /// <summary>
    /// Initializes a new instance of the <see cref="TextFileReader"/> class with default encoding (UTF8).
    /// </summary>
    /// <param name="filePath">The path to text file.</param>
    public TextFileReader(string filePath) : this(filePath, Encoding.UTF8) { }

    /// <summary>
    /// Initializes a new instance of the <see cref="TextFileReader"/> class.
    /// </summary>
    /// <param name="filePath">The path to text file.</param>
    /// <param name="encoding">The encoding of text file.</param>
    public TextFileReader(string filePath, Encoding encoding)
    {
        if (!File.Exists(filePath))
            throw new FileNotFoundException("File (" + filePath + ") is not found.");

        _fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);
        _length = _fileStream.Length;
        _binReader = new BinaryReader(_fileStream, encoding);
    }

    /// <summary>
    /// Reads a line of characters from the current stream at the current position and returns the data as a string.
    /// </summary>
    /// <returns>The next line from the input stream, or null if the end of the input stream is reached</returns>
    public string ReadLine()
    {
        if (_binReader.PeekChar() == -1)
            return null;

        string line = "";
        int nextChar = _binReader.Read();
        while (nextChar != -1)
        {
            char current = (char)nextChar;
            if (current.Equals('\n'))
                break;
            else if (current.Equals('\r'))
            {
                int pickChar = _binReader.PeekChar();
                if (pickChar != -1 && ((char)pickChar).Equals('\n'))
                    nextChar = _binReader.Read();
                break;
            }
            else
                line += current;
            nextChar = _binReader.Read();
        }
        return line;
    }

    /// <summary>
    /// Reads some lines of characters from the current stream at the current position and returns the data as a collection of string.
    /// </summary>
    /// <param name="totalLines">The total number of lines to read (set as 0 to read from current position to end of file).</param>
    /// <returns>The next lines from the input stream, or empty collectoin if the end of the input stream is reached</returns>
    public List<string> ReadLines(int totalLines)
    {
        if (totalLines < 1 && this.Position == 0)
            return this.ReadAllLines();

        _lines = new List<string>();
        int counter = 0;
        string line = this.ReadLine();
        while (line != null)
        {
            _lines.Add(line);
            counter++;
            if (totalLines > 0 && counter >= totalLines)
                break;
            line = this.ReadLine();
        }
        return _lines;
    }

    /// <summary>
    /// Reads all lines of characters from the current stream (from the begin to end) and returns the data as a collection of string.
    /// </summary>
    /// <returns>The next lines from the input stream, or empty collectoin if the end of the input stream is reached</returns>
    public List<string> ReadAllLines()
    {
        if (_streamReader == null)
            _streamReader = new StreamReader(_fileStream);
        _streamReader.BaseStream.Seek(0, SeekOrigin.Begin);
        _lines = new List<string>();
        string line = _streamReader.ReadLine();
        while (line != null)
        {
            _lines.Add(line);
            line = _streamReader.ReadLine();
        }
        return _lines;
    }

    /// <summary>
    /// Gets the length of text file (in bytes).
    /// </summary>
    public long Length
    {
        get { return _length; }
    }

    /// <summary>
    /// Gets or sets the current reading position.
    /// </summary>
    public long Position
    {
        get
        {
            if (_binReader == null)
                return -1;
            else
                return _binReader.BaseStream.Position;
        }
        set
        {
            if (_binReader == null)
                return;
            else if (value >= this.Length)
                this.SetPosition(this.Length);
            else
                this.SetPosition(value);
        }
    }

    void SetPosition(long position)
    {
        _binReader.BaseStream.Seek(position, SeekOrigin.Begin);
    }

    /// <summary>
    /// Gets the lines after reading.
    /// </summary>
    public List<string> Lines
    {
        get
        {
            return _lines;
        }
    }

    /// <summary>
    /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
    /// </summary>
    public void Dispose()
    {
        if (_binReader != null)
            _binReader.Close();
        if (_streamReader != null)
        {
            _streamReader.Close();
            _streamReader.Dispose();
        }
        if (_fileStream != null)
        {
            _fileStream.Close();
            _fileStream.Dispose();
        }
    }

    ~TextFileReader()
    {
        this.Dispose();
    }
}

Answer 3

这确实是一个棘手的问题。
在互联网上对不同解决方案进行了漫长而疲惫的枚举（包括此线程中的解决方案，谢谢！）之后，我不得不创建自己的自行车。

我有以下要求：

• 性能 - 读取必须非常快，因此一次读取一个字符或使用反射是不可接受的，因此需要缓冲
• 流式处理 - 文件可以是巨大，因此将其完全读入内存是不可接受的
• 拖尾 - 文件尾部应该可用
• 长行 - 行可以很长，因此不能限制缓冲区

• <强>稳定 - 在使用过程中单字节错误立即可见。对我来说不幸的是，我发现的几个实现都存在稳定性问题

  公共类OffsetStreamReader
  {
      私有常量 int InitialBufferSize = 4096;    
      私有只读 char _bom；
      私有只读字节_end；
      私有只读编码_encoding；
      私有只读流_stream；
      私有只读布尔_tail；
  
      私有字节[] _buffer；
      私有 int _processedInBuffer;
      私有 int _informationInBuffer;
  
      公共OffsetStreamReader（流流，布尔尾）
      {
          _buffer = 新字节[InitialBufferSize];
          _processedInBuffer = 初始缓冲区大小；
  
          if (stream == null || !stream.CanRead)
              抛出新的 ArgumentException(“流”);
  
          _stream = 流；
          _tail = 尾巴；
          _encoding = 编码.UTF8;
  
          _bom = '\uFEFF';
          _end = _encoding.GetBytes(new [] {'\n'})[0];
      }
  
      公共长偏移量{获取;私人套装； }
  
      公共字符串 ReadLine()
      {
          // 底层流关闭
          if (!_stream.CanRead)
              返回空值；
  
          // 结束符
          if (_processedInBuffer == _informationInBuffer)
          {
              如果（_尾）
              {
                  _processedInBuffer = _buffer.Length;
                  _信息缓冲区 = 0;
                  读取缓冲区（）；
              }
  
              返回空值；
          }
  
          var lineEnd = Search(_buffer, _end, _processedInBuffer);
          var haveEnd = true;
  
          // 文件结束但未完成换行符
          if (lineEnd.HasValue == false && _informationInBuffer + _processedInBuffer < _buffer.Length)
          {
              如果（_尾）
                  返回空值；
              别的
              {
                  lineEnd = _informationInBuffer;
                  有结束=假；
              }
          }
  
          // 当前缓冲区没有结束
          if (!lineEnd.HasValue)
          {
              读取缓冲区（）；
              if (_informationInBuffer != 0)
                  返回 ReadLine();
  
              返回空值；
          }
  
          var arr = new byte[lineEnd.Value - _processedInBuffer];
          Array.Copy(_buffer, _processedInBuffer, arr, 0, arr.Length);
  
          偏移量 = 偏移量 + lineEnd.Value - _processedInBuffer + (haveEnd ? 1 : 0);
          _processedInBuffer = lineEnd.Value + (haveEnd ? 1 : 0);
  
          return _encoding.GetString(arr).TrimStart(_bom).TrimEnd('\r', '\n');
      }
  
      私有无效 ReadBuffer()
      {
          var notProcessedPartLength = _buffer.Length - _processedInBuffer;
  
          // 扩展缓冲区以便能够将整行放入缓冲区
          // 是[NOT_PROCESSED]
          // 变成[NOT_PROCESSED]
          if (notProcessedPartLength == _buffer.Length)
          {
              var ExtendedBuffer = new byte[_buffer.Length + _buffer.Length/2];
              Array.Copy(_buffer, ExtendedBuffer, _buffer.Length);
              _buffer = 扩展缓冲区；
          }
  
          // 将未处理的信息复制到开头
          // 是[PROCESSED NOT_PROCESSED]
          // 变成[NOT_PROCESSED]
          Array.Copy(_buffer, (long) _processedInBuffer, _buffer, 0, notProcessedPartLength);
  
          // 读取更多信息到缓冲区的空部分
          // 是 [ NOT_PROCESSED ]
          // 变成 [ NOT_PROCESSED NEW_NOT_PROCESSED ]
          _informationInBuffer = notProcessedPartLength + _stream.Read(_buffer, notProcessedPartLength, _buffer.Length - notProcessedPartLength);
  
          _processedInBuffer = 0;
      }
  
      私人整数？搜索（字节[]缓冲区，字节byteToSearch，int bufferOffset）
      {
          for (int i = bufferOffset; i < buffer.Length - 1; i++)
          {
              if (buffer[i] == byteToSearch)
                  返回我；
          }
          返回空值；
      }
  }
  

Answer 4

尽管托马斯·莱维斯克的解决方案效果很好，但这是我的。它使用反射，因此速度会较慢，但它与编码无关。另外我也添加了 Seek 扩展。

/// <summary>Useful <see cref="StreamReader"/> extentions.</summary>
public static class StreamReaderExtentions
{
    /// <summary>Gets the position within the <see cref="StreamReader.BaseStream"/> of the <see cref="StreamReader"/>.</summary>
    /// <remarks><para>This method is quite slow. It uses reflection to access private <see cref="StreamReader"/> fields. Don't use it too often.</para></remarks>
    /// <param name="streamReader">Source <see cref="StreamReader"/>.</param>
    /// <exception cref="ArgumentNullException">Occurs when passed <see cref="StreamReader"/> is null.</exception>
    /// <returns>The current position of this stream.</returns>
    public static long GetPosition(this StreamReader streamReader)
    {
        if (streamReader == null)
            throw new ArgumentNullException("streamReader");

        var charBuffer = (char[])streamReader.GetType().InvokeMember("charBuffer", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        var charPos = (int)streamReader.GetType().InvokeMember("charPos", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        var charLen = (int)streamReader.GetType().InvokeMember("charLen", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);

        var offsetLength = streamReader.CurrentEncoding.GetByteCount(charBuffer, charPos, charLen - charPos);

        return streamReader.BaseStream.Position - offsetLength;
    }

    /// <summary>Sets the position within the <see cref="StreamReader.BaseStream"/> of the <see cref="StreamReader"/>.</summary>
    /// <remarks>
    /// <para><see cref="StreamReader.BaseStream"/> should be seekable.</para>
    /// <para>This method is quite slow. It uses reflection and flushes the charBuffer of the <see cref="StreamReader.BaseStream"/>. Don't use it too often.</para>
    /// </remarks>
    /// <param name="streamReader">Source <see cref="StreamReader"/>.</param>
    /// <param name="position">The point relative to origin from which to begin seeking.</param>
    /// <param name="origin">Specifies the beginning, the end, or the current position as a reference point for origin, using a value of type <see cref="SeekOrigin"/>. </param>
    /// <exception cref="ArgumentNullException">Occurs when passed <see cref="StreamReader"/> is null.</exception>
    /// <exception cref="ArgumentException">Occurs when <see cref="StreamReader.BaseStream"/> is not seekable.</exception>
    /// <returns>The new position in the stream. This position can be different to the <see cref="position"/> because of the preamble.</returns>
    public static long Seek(this StreamReader streamReader, long position, SeekOrigin origin)
    {
        if (streamReader == null)
            throw new ArgumentNullException("streamReader");

        if (!streamReader.BaseStream.CanSeek)
            throw new ArgumentException("Underlying stream should be seekable.", "streamReader");

        var preamble = (byte[])streamReader.GetType().InvokeMember("_preamble", BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField, null, streamReader, null);
        if (preamble.Length > 0 && position < preamble.Length) // preamble or BOM must be skipped
            position += preamble.Length;

        var newPosition = streamReader.BaseStream.Seek(position, origin); // seek
        streamReader.DiscardBufferedData(); // this updates the buffer

        return newPosition;
    }
}

Answer 5

这行得通吗：

using (var sr = new StreamReader(@"C:\Temp\LineTest.txt")) {
  string line;
  long pos = 0;
  while ((line = sr.ReadLine()) != null) {
    Console.Write("{0:d3} ", pos);
    Console.WriteLine(line);
    pos += line.Length;
  }
}