Csharp 如何使用具有指定行号的 LINQ 读取文本文件

这是Linq声明.更有效的是，它使用Reverse，因为您提到您要查找的信息在页脚中.

static void Main(string[] args)
{
    var path = Path.Combine(
        Path.GetDirectoryName(Assembly.GetEntryAssembly().Location),
        "TextFile.txt");
    try
    {
        var count =
            int.Parse(
            File.ReadAllLines(path)
            .Reverse()
            .First(line => line.Any() && (line.First() == '7'))
            .Substring(4, 6));
        Console.WriteLine($"Count = {count}");
    }
    catch (Exception ex)
    {
         System.Diagnostics.Debug.Assert(false, ex.Message);
    }
}

EDIT

你问了一个关于表演的好问题.最棒的是，我们不必猜测！有always种方法可以衡量绩效.

这是我刚刚制定的基准.看，我做得很快，所以如果有人发现我遗漏了什么，请指出.但我得到的是:

static void Main(string[] args)
{
    var path = Path.Combine(
        Path.GetDirectoryName(Assembly.GetEntryAssembly().Location),
        "TextFile.txt");
    try
    {
        // 200K lines of random guids
        List<string> builder = 
            Enumerable.Range(0, 200000)
            .Select(n => $"{{{System.Guid.NewGuid().ToString()}}}")
            .ToList();

        var footer =
            File.ReadAllLines(path);

        builder.AddRange(footer);

        var FileLines = builder.ToArray();

        var benchmark = new System.Diagnostics.Stopwatch();
        benchmark.Start();
        int totalCount = int.MinValue;
        foreach (var line in FileLines)
        {
            //// If line length is zero, then do nothing
            if (line.Length == 0)
            {
                continue;
            }
            // Original code from post
            // switch (line.Substring(1, 1))
            // Should be:
            switch (line.Substring(0, 1))
            {
                case "7":
                    totalCount = int.Parse(line.Substring(4, 6));
                    // This is another issue!! Breaking from the switch DOESN'T break from the loop
                    break;
                    // SHOULD BE: goto breakFromInner;
                    // One of the few good reasons to use a goto statement!!
            }
        }
        benchmark.Stop();
        Console.WriteLine($"200K lines using Original code: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {totalCount}");


        benchmark.Restart();
        for (int i = FileLines.Length - 1; i >= 0; i--)
        {
            var line = FileLines[i];
            //// If line length is zero, then do nothing
            if (line.Length == 0)
            {
                continue;
            }
            // Original code from post
            // switch (line.Substring(1, 1))
            // Should be:
            switch (line.Substring(0, 1))
            {
                case "7":
                    totalCount = int.Parse(line.Substring(4, 6));
                    // One of the few good reasons to use a goto statement!!
                    goto breakFromInner;
            }
        }
        // See note
        breakFromInner:
        benchmark.Stop();
        Console.WriteLine($"200K lines using Original code with reverse: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {totalCount}");

        benchmark.Restart();
        var count =
            int.Parse(
                FileLines
                .Reverse()
                .First(line => line.Any() && (line.First() == '7'))
                .Substring(4, 6));
        benchmark.Stop();
        Console.WriteLine($"200K lines using Linq with Reverse: Elapsed = {benchmark.Elapsed}");
        Console.WriteLine($"Count = {count}");
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.Assert(false, ex.Message);
    }
}