Read previous line using File.ReadLines

Question

I want to read a text file from last line because it could be big size and just want to check today's log content.

I'm using File.ReadLines() and success to read last line. How can I read the previous line? Or is there any way to get the current line number so that I can subtract the number?

foreach (string f in Directory.GetFiles(sDir, "*.log", SearchOption.AllDirectories))
{
    string last = File.ReadLines(f, Encoding.Default).Last(); //read Last Line here

    if (last.IndexOf("error", StringComparison.OrdinalIgnoreCase) > 0)
    {
        WriteError(last);
    }
}

Answer 1

It's not really clear to me whether you are asking to avoid reading the whole file (in which case the duplicate answer proposed is the best way to approach this), or you just want to be able to figure out the next-to-the-last line in the file.

If the latter, then this would work:

string[] Tail(string file, int maxLinesToReturn, Encoding encoding)
{
    Queue<string> mostRecentLines = new Queue<string>();

    foreach (string line in File.ReadLines(file, encoding))
    {
        if (mostRecentLines.Count >= maxLinesToReturn)
        {
            mostRecentLines.Dequeue();
        }
        mostRecentLines.Enqueue(line);
    }

    return mostRecentLines.ToArray();
}

Use like this:

foreach (string f in Directory.GetFiles(sDir, "*.log", SearchOption.AllDirectories))
{
    foreach (string line in Tail(f, 2, Encoding.Default)
        .Where(line => line.IndexOf("error", StringComparison.OrdinalIgnoreCase) > 0)))
    {
        WriteError(line);
    }
}

Answer 2

foreach (string f in Directory.GetFiles(sDir, "*.log", SearchOption.AllDirectories))
{
    string last="", prev="";
    foreach (string cur in  File.ReadLines(f, Encoding.Default) )
    {
        prev = last;
        last = cur;
    }

    //not sure what you want to do with "prev" here.
    if (last.IndexOf("error", StringComparison.OrdinalIgnoreCase) > 0)
    {
        WriteError(last);
    }
}