学步园

1   多线程下载文件原理

    其实就是把一个文件分成几个部分，不同线程（或者进程）负责下载文件的不同的部分。各种下载工具的底层原理应该和此相同，但由于是复杂的网络环境，实际会远比这复杂。

2 模拟代码实现。

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;

namespace ConsoleTest
{
    class Program
    {
      
        private static int ThreadCount = 20;
        static void Main(string[] args)
        {
            for (int i = 0; i < ThreadCount; i++)
            {
                Thread thread = new Thread(DoWork);
                thread.Name = "Thread"+i;
                Console.WriteLine("Thread" + i + "Start");
                thread.Start(i);
            }          
            System.Console.ReadLine();
        }

        public static void DoWork(object threadIndex)
        {
            Console.WriteLine(Thread.CurrentThread.Name);
            string oldPath = @"F:\Reflector7.3.rar";
            string newPath = @"F:\Reflector7.rar";
            int fileLength = (int)(new FileInfo(oldPath).Length);//需要拷贝的文件大小
            int lengthPerThread = (fileLength/ThreadCount + 1);//计算平均每个线程需要复制的文件内容大小
            int startIndex = (int)threadIndex * lengthPerThread;//计算开始读取文件的位置

            Console.WriteLine(Thread.CurrentThread.Name + "start write startIndex=" + startIndex);
            int hasRead = 0; //
            using (var oldFileReader = File.Open(oldPath, FileMode.Open, FileAccess.Read, FileShare.Read))
            {
                oldFileReader.Seek(startIndex, SeekOrigin.Begin);//移动到需要读取的位置
                byte[] array = new byte[100];
                using (var newFileWriter = File.Open(newPath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite))
                {
                    int read = oldFileReader.Read(array, 0, 100);
                    newFileWriter.Seek(startIndex, SeekOrigin.Begin);//移动到需要写入的位置
                    while (read > 0 && hasRead <= lengthPerThread)//
                    {
                        hasRead += read;
                        newFileWriter.Write(array, 0, read);
                        read = oldFileReader.Read(array, 0, 100);
                        Random random = new Random();
                        int next= random.Next(1, 10);
                        if(next==5)//生成随机数，制造多线程不按生成顺序结束
                        {
                            Console.WriteLine(Thread.CurrentThread.Name + "Sleep");
                            Thread.Sleep(50);
                        }
                    }
                   
                }
            }

            Console.WriteLine(Thread.CurrentThread.Name +" end  write" + (startIndex + hasRead));      
        }
    }  
}