how to download a file from internet

/*
    HttpDownloader --
       with ability to continue downloading partially downloaded file
    Copyright (C) 2006-2011 by Sergey A Kryukov,
    http://www.sakryukov.org
*/
using System;
using System.Net;
using System.IO;

namespace HttpDownloader {

    class HttpDownloadMain {

        const string NumericFormat = "###,###,###,###,###,###,###"; 
           //more than enough for uint64:
           //18,446,744,073,709,551,615 max

        static string CreateFormat(
          string preFormat, string placeholder) {
            return preFormat.Replace(placeholder, NumericFormat);
        } //CreateFormat

        static string ConvertUrlToFileName(string url) {
            string[] terms = url.Split(
                new string[] { ":", "//" },
                StringSplitOptions.RemoveEmptyEntries);
            string fname = terms[terms.Length - 1];
            fname = fname.Replace('/', '.');
            return fname;
        } //ConvertUrlToFileName

        static long GetExistingFileLength(string filename) {
            if (!File.Exists(filename)) return 0;
            FileInfo info = new FileInfo(filename);
            return info.Length;
        } //GetExistingFileLength

        static void DownloadOne(
          string url, string existingFilename, bool quiet) {
            HttpWebRequest webRequest;
            HttpWebResponse webResponse;
            IWebProxy proxy = null; //SA???
            string fmt = CreateFormat(
                "{0}: {1:#} of {2:#} ({3:g3}%)", "#");
            FileStream fs = null;
            try {
                string fname = existingFilename;
                if (fname == null)
                    fname = ConvertUrlToFileName(url);
                webRequest = (HttpWebRequest)WebRequest.Create(url);
                long preloadedLength = GetExistingFileLength(fname);
                if (preloadedLength > 0)
                    webRequest.AddRange((int)preloadedLength);
                webRequest.Proxy = proxy; //SA??? or DefineProxy
                webResponse = (HttpWebResponse)webRequest.GetResponse();
                fs = new FileStream(
                    fname, FileMode.Append, FileAccess.Write);
                long fileLength = webResponse.ContentLength;
                string todoFormat = CreateFormat(
                    @"Downloading {0}: {1:#} bytes...", "#");
                Console.WriteLine();
                Console.WriteLine(todoFormat, url, fileLength);
                Console.WriteLine(
                    "Pre-loaded: preloaded length {0}",
                    preloadedLength);
                Console.WriteLine("Remaining length: {0}", fileLength);
                Stream strm = webResponse.GetResponseStream();
                int arrSize = 10 * 1024 * 1024; //SA???                 
                byte[] barr = new byte[arrSize];
                long bytesCounter = preloadedLength;
                string fmtPercent = string.Empty;
                while (true) { //SA???
                    int actualBytes = strm.Read(barr, 0, arrSize);
                    if (actualBytes <= 0)
                        break;
                    fs.Write(barr, 0, actualBytes);
                    bytesCounter += actualBytes;
                    double percent = 0d;
                    if (fileLength > 0)
                        percent =
                            100.0d * bytesCounter /
                            (preloadedLength + fileLength);
                    if (!quiet)
                        Console.WriteLine(
                             fmt,
                             fname,
                             bytesCounter,
                             preloadedLength + fileLength,
                             percent);
                } //loop
                Console.WriteLine(@"{0}: complete!", url);
            } catch (Exception e) {
                Console.WriteLine(
                     "{0}: {1} '{2}'",
                     url, e.GetType().FullName,
                     e.Message);
            } finally {
                if (fs != null) {
                    fs.Flush();
                    fs.Close();
                } //if
            } //exception
        } //DownloadOne

        static void Main(string[] args) {
            Console.WriteLine(
               "Usage: >HTTPDownloader <http_url> [<output_file_name>]");
            if (args.Length < 1) return;
            string url = args[0];
            string existingFileName = null;
            if (args.Length > 1)
                existingFileName = args[1];
            DownloadOne(url, existingFileName, false);
        } //Main

    } //class HttpDownloadMain

} //namespace HttpDownloader