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

namespace AsynSample
{
  class FileReader
  {
    /// <summary>
    /// 缓存池
    /// </summary>
    private byte[] Buffer { get; set; }
    /// <summary>
    /// 缓存区大小
    /// </summary>
    public int BufferSize { get; set; }

    public FileReader(int bufferSize)
    {
      this.BufferSize = bufferSize;
      this.Buffer = new byte[BufferSize];
    }

    /// <summary>
    /// 同步读取文件
    /// </summary>
    /// <param name="path">文件路径</param>
    public void SynsReadFile(string path)
    {
      Console.WriteLine("同步读取文件 begin");
      using (FileStream fs = new FileStream(path, FileMode.Open))
      {        
        fs.Read(Buffer, 0, BufferSize);
        string output = System.Text.Encoding.UTF8.GetString(Buffer);
        Console.WriteLine("读取的文件信息：{0}",output);
      }
      
      Console.WriteLine("同步读取文件 end");
    }
    /// <summary>
    /// 异步读取文件
    /// </summary>
    /// <param name="path"></param>
    public void AsynReadFile(string path)
    {
      Console.WriteLine("异步读取文件 begin");
      //执行Endread时报错，fs已经释放，注意在异步中不能使用释放需要的资源
      //using (FileStream fs = new FileStream(path, FileMode.Open))
      //{
      //  Buffer = new byte[BufferSize];
      //  fs.BeginRead(Buffer, 0, BufferSize, AsyncReadCallback, fs);
      //} 
      if (File.Exists(path))
      {
        FileStream fs = new FileStream(path, FileMode.Open);
        fs.BeginRead(Buffer, 0, BufferSize, AsyncReadCallback, fs);
      }
      else
      {
        Console.WriteLine("该文件不存在");
      }

    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="ar"></param>
    void AsyncReadCallback(IAsyncResult ar)
    {
      FileStream stream = ar.AsyncState as FileStream;
      if (stream != null)
      {
        Thread.Sleep(1000);
        //读取结束
        stream.EndRead(ar);
        stream.Close();

        string output = System.Text.Encoding.UTF8.GetString(this.Buffer);
        Console.WriteLine("读取的文件信息：{0}", output);
      }
    }
  }
}

using System;
using System.Threading;

namespace AsynSample
{
  class Program
  {
    static void Main(string[] args)
    {
      FileReader reader = new FileReader(1024);

      //改为自己的文件路径
      string path = "C:\\Windows\\DAI.log";

      Console.WriteLine("开始读取文件了...");
      //reader.SynsReadFile(path);

      reader.AsynReadFile(path);

      Console.WriteLine("我这里还有一大滩事呢.");
      DoSomething();
      Console.WriteLine("终于完事了，输入任意键，歇着！");
      Console.ReadKey();      
    }
    /// <summary>
    /// 
    /// </summary>
    static void DoSomething()
    {
      Thread.Sleep(1000);
      for (int i = 0; i < 10000; i++)
      {
        if (i % 888 == 0)
        {
          Console.WriteLine("888的倍数：{0}",i);
        }
      }
    }
  }
}

public delegate string MyFunc(int num, DateTime dt);

using System;
using System.Text;

namespace AsynSample
{
  /// <summary>
  /// 下载委托
  /// </summary>
  /// <param name="fileName"></param>
  public delegate string AysnDownloadDelegate(string fileName);
  /// <summary>
  /// 通过委托实现异步调用
  /// </summary>
  class DownloadFile
  {
    /// <summary>
    /// 同步下载
    /// </summary>
    /// <param name="fileName"></param>
    public string Downloading(string fileName)
    {
      string filestr = string.Empty;
      Console.WriteLine("下载事件开始执行");
      System.Threading.Thread.Sleep(3000);
      Random rand = new Random();
      StringBuilder builder =new StringBuilder();
      int num;
      for(int i=0;i<100;i++)
      {
        num = rand.Next(1000);
        builder.Append(i);
      }
      filestr = builder.ToString();
      Console.WriteLine("下载事件执行结束");

      return filestr;
    }
    /// <summary>
    /// 异步下载
    /// </summary>
    public IAsyncResult BeginDownloading(string fileName)
    {
      string fileStr = string.Empty;
      AysnDownloadDelegate downloadDelegate = new AysnDownloadDelegate(Downloading);
      return downloadDelegate.BeginInvoke(fileName, Downloaded, downloadDelegate);
    }
    /// <summary>
    /// 异步下载完成后事件
    /// </summary>
    /// <param name="result"></param>
    private void Downloaded(IAsyncResult result)
    {
      AysnDownloadDelegate aysnDelegate = result.AsyncState as AysnDownloadDelegate;
      if (aysnDelegate != null)
      {
        string fileStr = aysnDelegate.EndInvoke(result);
        if (!string.IsNullOrEmpty(fileStr))
        {
          Console.WriteLine("下载文件：{0}", fileStr);
        }
        else
        {
          Console.WriteLine("下载数据为空!");
        }
      }
      else
      {
        Console.WriteLine("下载数据为空!");
      }
    }
  }
}