file mapping 3 with C#

using System;
using System.IO.MemoryMappedFiles;
using System.IO;

class Program
{
    static void Main(string[] args)
    {
        const uint offset = 16 + 2;
        const uint numOfLine = 20000000;
        const uint filesize = offset * numOfLine;

        var now = DateTime.Now;
        using (var mmf = MemoryMappedFile.CreateFromFile("mapped_file.txt", FileMode.OpenOrCreate, "mapped_file", filesize))
        {
            using (var stream = mmf.CreateViewStream())
            {
                StreamWriter writer = new StreamWriter(stream);
                char[] buf = new char[18];
                buf[16] = '\r';
                buf[17] = '\n';
                var rnd = new Random();
                for (var i = numOfLine; i != 0; --i)
                {
                    for (int j = 16; j != 0; --j)
                        writer.Write("QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm1234567890"[rnd.Next(61)]);
                    writer.WriteLine();
                }
            }
        }
        Console.WriteLine(DateTime.Now - now);
    }
}