DouKaku?

using System;
using System.Text;
using System.Threading;
using System.IO;

class Tail
{
    private const int SLEEP_TIME = 1000;
    private static bool fOption = false;
    private static int outputLines = 10;
    private static string filename = string.Empty;
    private static string usage = "Usage: Tail.exe [-n number] [-f] filename";

    static void Main(string[] args)
    {
        try {
            for (int i = 0; i < args.Length; i++) {
                if (args[i][0] == '-') {
                    switch (args[i][1]) {
                        case 'n':
                            outputLines = int.Parse(args[++i]);
                            break;
                        case 'f':
                            fOption = true;
                            break;
                        default:
                            Console.Error.WriteLine("Invalid Option: {0}", args[i]);
                            Console.Error.WriteLine(usage);
                            return;
                    }
                }
                else
                    filename = args[i];
            }

            string buffer = string.Empty;
            int allLines = 0;
            string line = string.Empty;
            using (FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)) {
                using (StreamReader sr = new StreamReader(fs, Encoding.Default)) {
                    while (sr.ReadLine() != null)
                        allLines++;
                    fs.Seek(0, SeekOrigin.Begin);
                    if (allLines <= outputLines)
                        buffer = sr.ReadToEnd();
                    else {
                        for (int i = 0; i < allLines; i++) {
                            line = sr.ReadLine();
                            if (i > allLines - outputLines - 1)
                                buffer += line + "\n";
                        }
                    }

                    Console.WriteLine(buffer);

                    while (fOption) {
                        Thread.Sleep(SLEEP_TIME);
                        while ((line = sr.ReadLine()) != null)
                            Console.WriteLine(line);
                    }
                }
            }
        }
        catch (Exception ex) {
            Console.Error.WriteLine(ex.Message.ToString());
            Console.Error.WriteLine(usage);
        }
    }
}