如何阅读最后一个“n"日志文件行 [英] How to read last "n" lines of log file

问题描述

需要一段代码来读出日志文件的最后n 行".我从网上想出了以下代码.我对 C 锐有点新.由于日志文件可能是相当大，我想避免读取整个文件的开销.有人可以建议任何性能增强.我真的不想读取每个字符并改变位置.

need a snippet of code which would read out last "n lines" of a log file. I came up with the following code from the net.I am kinda new to C sharp. Since the log file might be quite large, I want to avoid overhead of reading the entire file.Can someone suggest any performance enhancement. I do not really want to read each character and change position.

   var reader = new StreamReader(filePath, Encoding.ASCII);
            reader.BaseStream.Seek(0, SeekOrigin.End);
            var count = 0;
            while (count <= tailCount)
            {
                if (reader.BaseStream.Position <= 0) break;
                reader.BaseStream.Position--;
                int c = reader.Read();
                if (reader.BaseStream.Position <= 0) break;
                reader.BaseStream.Position--;
                if (c == '
')
                {
                    ++count;
                }
            }

            var str = reader.ReadToEnd();

推荐答案

您的代码将执行得很差，因为您不允许任何缓存发生.
此外，对于 Unicode，它根本不起作用.

Your code will perform very poorly, since you aren't allowing any caching to happen.
In addition, it will not work at all for Unicode.

我编写了以下实现:

///<summary>Returns the end of a text reader.</summary>
///<param name="reader">The reader to read from.</param>
///<param name="lineCount">The number of lines to return.</param>
///<returns>The last lneCount lines from the reader.</returns>
public static string[] Tail(this TextReader reader, int lineCount) {
    var buffer = new List<string>(lineCount);
    string line;
    for (int i = 0; i < lineCount; i++) {
        line = reader.ReadLine();
        if (line == null) return buffer.ToArray();
        buffer.Add(line);
    }

    int lastLine = lineCount - 1;           //The index of the last line read from the buffer.  Everything > this index was read earlier than everything <= this indes

    while (null != (line = reader.ReadLine())) {
        lastLine++;
        if (lastLine == lineCount) lastLine = 0;
        buffer[lastLine] = line;
    }

    if (lastLine == lineCount - 1) return buffer.ToArray();
    var retVal = new string[lineCount];
    buffer.CopyTo(lastLine + 1, retVal, 0, lineCount - lastLine - 1);
    buffer.CopyTo(0, retVal, lineCount - lastLine - 1, lastLine + 1);
    return retVal;
}

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