c# - 跟踪流式阅读器的行的位置

Question

我需要跟踪从流阅读器读取的行的位置。当我说 时reader.ReadLine()，我需要知道该行在文件中的位置，然后我还希望能够从我之前跟踪的位置读取文件。

这可能吗？

Answer 1

您可以通过以下三种方式之一执行此操作：

1) 编写自己的 StreamReader。这是一个很好的起点：如何知道流阅读器在文本文件中的位置（行号）？

2) StreamReader 类有两个非常重要的私有变量，称为 charPos 和 charLen，它们在定位实际“读取”位置时需要，而不仅仅是流的底层位置。您可以使用反射来获取此处建议的值

Int32 charpos = (Int32) s.GetType().InvokeMember("charPos", 
BindingFlags.DeclaredOnly | 
BindingFlags.Public | BindingFlags.NonPublic | 
BindingFlags.Instance | BindingFlags.GetField
 ,null, s, null); 

Int32 charlen= (Int32) s.GetType().InvokeMember("charLen", 
BindingFlags.DeclaredOnly | 
BindingFlags.Public | BindingFlags.NonPublic | 
BindingFlags.Instance | BindingFlags.GetField
 ,null, s, null);

return (Int32)s.BaseStream.Position-charlen+charpos;

3）只需将整个文件读入一个字符串数组。像这样的东西：

char[] CRLF = new char[2] { '\n', '\r' };
TextReader tr = File.OpenText("some path to file");
string[] fileLines = tr.ReadToEnd().Split(CRLF);

另一种可能性（沿与#3 相同的行）是读取行并将行存储在数组中。当您想阅读前一行时，只需使用数组即可。

Answer 2

跟踪实际 StreamReader 位置（以字节为单位）：

readonly static FieldInfo charPosField = typeof(StreamReader).GetField("charPos", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly);
readonly static FieldInfo charLenField = typeof(StreamReader).GetField("charLen", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly);
readonly static FieldInfo charBufferField = typeof(StreamReader).GetField("charBuffer", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly);

static long ActualPosition(StreamReader reader)
{
    var charBuffer = (char[])charBufferField.GetValue(reader);
    var charLen = (int)charLenField.GetValue(reader);
    var charPos = (int)charPosField.GetValue(reader);

    return reader.BaseStream.Position - reader.CurrentEncoding.GetByteCount(charBuffer, charPos, charLen-charPos);
}

不客气。

Answer 3

也许这可以帮助你

  public class StreamLineReader : IDisposable
    {
        const int BufferLength = 1024;

        Stream _Base;
        int _Read = 0, _Index = 0;
        byte[] _Bff = new byte[BufferLength];

        long _CurrentPosition = 0;
        int _CurrentLine = 0;

        /// <summary>
        /// CurrentLine number
        /// </summary>
        public long CurrentPosition { get { return _CurrentPosition; } }
        /// <summary>
        /// CurrentLine number
        /// </summary>
        public int CurrentLine { get { return _CurrentLine; } }
        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="stream">Stream</param>
        public StreamLineReader(Stream stream) { _Base = stream; }
        /// <summary>
        /// Count lines and goto line number
        /// </summary>
        /// <param name="goToLine">Goto Line number</param>
        /// <returns>Return true if goTo sucessfully</returns>
        public bool GoToLine(int goToLine) { return IGetCount(goToLine, true) == goToLine; }
        /// <summary>
        /// Count lines and goto line number
        /// </summary>
        /// <param name="goToLine">Goto Line number</param>
        /// <returns>Return the Count of lines</returns>
        public int GetCount(int goToLine) { return IGetCount(goToLine, false); }
        /// <summary>
        /// Internal method for goto&Count
        /// </summary>
        /// <param name="goToLine">Goto Line number</param>
        /// <param name="stopWhenLine">Stop when found the selected line number</param>
        /// <returns>Return the Count of lines</returns>
        int IGetCount(int goToLine, bool stopWhenLine)
        {
            _Base.Seek(0, SeekOrigin.Begin);
            _CurrentPosition = 0;
            _CurrentLine = 0;
            _Index = 0;
            _Read = 0;

            long savePosition = _Base.Length;

            do
            {
                if (_CurrentLine == goToLine)
                {
                    savePosition = _CurrentPosition;
                    if (stopWhenLine) return _CurrentLine;
                }
            }
            while (ReadLine() != null);

            // GoToPosition

            int count = _CurrentLine;

            _CurrentLine = goToLine;
            _Base.Seek(savePosition, SeekOrigin.Begin);

            return count;
        }
        /// <summary>
        /// Read Line
        /// </summary>
        /// <returns></returns>
        public string ReadLine()
        {
            bool found = false;

            StringBuilder sb = new StringBuilder();
            while (!found)
            {
                if (_Read <= 0)
                {
                    // Read next block
                    _Index = 0;
                    _Read = _Base.Read(_Bff, 0, BufferLength);
                    if (_Read == 0)
                    {
                        if (sb.Length > 0) break;
                        return null;
                    }
                }

                for (int max = _Index + _Read; _Index < max; )
                {
                    char ch = (char)_Bff[_Index];
                    _Read--; _Index++;
                    _CurrentPosition++;

                    if (ch == '\0' || ch == '\n')
                    {
                        found = true;
                        break;
                    }
                    else if (ch == '\r') continue;
                    else sb.Append(ch);
                }
            }

            _CurrentLine++;
            return sb.ToString();
        }
        /// <summary>
        /// Free resources
        /// </summary>
        public void Dispose()
        {
            if (_Base != null)
            {
                _Base.Close();
                _Base.Dispose();
                _Base = null;
            }
        }
    }

利用：

 using (StreamLineReader st = new StreamLineReader(File.OpenRead("E:\\log.txt")))
        {
            bool ok = st.GoToLine(1);
            int count= st.GetCount(0);

            string w0 = st.ReadLine();
            string w1 = st.ReadLine();
            string w2 = st.ReadLine();
            string w3 = st.ReadLine();
        }

Answer 4

您可以使用的另一种模式是StreamReader在您想要更改位置时使用新模式。这应该适用于任何版本的 .net，并且不需要太多代码。

using var stream = File.Open("file.txt");

using (var reader = new StreamReader(stream, Encoding.UTF8, true, 4096, leaveOpen: true) {
    reader.ReadLine();
}

stream.Seek(0, SeekLocation.Begin);

using (var reader = new StreamReader(stream, Encoding.UTF8, true, 4096, leaveOpen: true) {
    // read the same line again
    reader.ReadLine();
}