.NET2.0版本中基于事件的异步编程模式(EAP)

public static void GetInfomation()
{
    WebClient client = new WebClient();
    Uri uri = new Uri("http://www.baidu.com");
    client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressCallback);
    client.DownloadFileAsync(uri, "serverdata.txt");
}

static void client_DownloadProgressCallback(object sender, DownloadProgressChangedEventArgs e)
{
    // Displays the operation identifier, and the transfer progress.
    Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...", (string)e.UserState, e.BytesReceived, e.TotalBytesToReceive, e.ProgressPercentage);
}

static void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
{
    string RetStr = (e.Error == null ? "成功" : e.Error.Message);
    Console.Write(RetStr);  //显示操作结果信息
}

public int DownloadSize = 0;
public string downloadPath = null;
private RequestState requestState = null;
private long totalSize = 0;

public FileDownloader()
{
    InitializeComponent();

    string url = "http://download.microsoft.com/download/7/0/3/703455ee-a747-4cc8-bd3e-98a615c3aedb/dotNetFx35setup.exe";
    //string url = "http://download.microsoft.com/download/9/5/A/95A9616B-7A37-4AF6-BC36-D6EA96C8DAAE/dotNetFx40_Full_x86_x64.exe";
    txbUrl.Text = url;
    this.btnPause.Enabled = false;
    //this.status = DownloadStatus.Initialized;
    // Get download Path

    GetTotalSize();
    downloadPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop) + "\\" + Path.GetFileName(this.txbUrl.Text.Trim());
    if (File.Exists(downloadPath))
    {
        FileInfo fileInfo = new FileInfo(downloadPath);
        DownloadSize = (int)fileInfo.Length;
        progressBar1.Value = (int)((float)DownloadSize / (float)totalSize * 100);
    }

    // 启用支持 ReportProgress and Cancellation
    bgWorkerFileDownload.WorkerReportsProgress = true;
    bgWorkerFileDownload.WorkerSupportsCancellation = true;
}

//开始或恢复下载
private void btnDownload_Click(object sender, EventArgs e)
{
    if (bgWorkerFileDownload.IsBusy != true)
    {
        // 启动异步操作触发DoWork事件
        bgWorkerFileDownload.RunWorkerAsync();

        // 创建RequestState 实例
        requestState = new RequestState(downloadPath);
        requestState.filestream.Seek(DownloadSize, SeekOrigin.Begin);
        this.btnDownload.Enabled = false;
        this.btnPause.Enabled = true;
    }
    else
    {
        MessageBox.Show("正在执行操作，请稍后");
    }
}

// 暂停下载
private void btnPause_Click(object sender, EventArgs e)
{
    if (bgWorkerFileDownload.IsBusy && bgWorkerFileDownload.WorkerSupportsCancellation == true)
    {
        // 终止异步操作触发RunWorkerCompleted事件
        bgWorkerFileDownload.CancelAsync();
    }
}

#region BackGroundWorker Event

// 1、当调用RunWorkerAsync方法时触发
private void bgWorkerFileDownload_DoWork(object sender, DoWorkEventArgs e)
{
    //获取事件源
    BackgroundWorker bgworker = sender as BackgroundWorker;
    try
    {
        // 执行下载操作
        // 初始化一个HttpWebRequest对象
        HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(txbUrl.Text.Trim());

        // 如果文件已下载了一部分，
        // 服务器应该从DownloadSize开始发送数据到HTTP实体中数据的末尾。
        if (DownloadSize != 0)
        {
            myHttpWebRequest.AddRange(DownloadSize);
        }

        // 指定一个 HttpWebRequest 实例到request字段.
        requestState.request = myHttpWebRequest;
        requestState.response = (HttpWebResponse)myHttpWebRequest.GetResponse();
        requestState.streamResponse = requestState.response.GetResponseStream();
        int readSize = 0;
        while (true)
        {
            if (bgworker.CancellationPending == true)
            {
                e.Cancel = true;
                break;
            }

            readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);
            if (readSize > 0)
            {
                DownloadSize += readSize;
                int percentComplete = (int)((float)DownloadSize / (float)totalSize * 100);
                requestState.filestream.Write(requestState.BufferRead, 0, readSize);

                // 报告进度，引发ProgressChanged事件的发生
                bgworker.ReportProgress(percentComplete);
            }
            else
            {
                break;
            }
        }
    }
    catch
    {
        throw;
    }
}

//  2、当调用ReportProgress方法时触发
private void bgWorkerFileDownload_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    this.progressBar1.Value = e.ProgressPercentage;
}

// 3、当后台操作完成，取消或者有错误时触发
private void bgWorkerFileDownload_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    if (e.Error != null)
    {
        MessageBox.Show(e.Error.Message);
        requestState.response.Close();
    }
    else if (e.Cancelled)
    {
        MessageBox.Show(String.Format("下载暂停，下载的文件地址为：{0}\n 已经下载的字节数为: {1}字节", downloadPath, DownloadSize));
        requestState.response.Close();
        requestState.filestream.Close();

        this.btnDownload.Enabled = true;
        this.btnPause.Enabled = false;
    }
    else
    {
        MessageBox.Show(String.Format("下载已完成，下载的文件地址为：{0}，文件的总字节数为: {1}字节", downloadPath, totalSize));

        this.btnDownload.Enabled = false;
        this.btnPause.Enabled = false;
        requestState.response.Close();
        requestState.filestream.Close();
    }
}

#endregion BackGroundWorker Event

// 获取文件总大小
private void GetTotalSize()
{
    HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(txbUrl.Text.Trim());
    HttpWebResponse response = (HttpWebResponse)myHttpWebRequest.GetResponse();
    totalSize = response.ContentLength;
    response.Close();
}
}

// 此类存储请求的状态
public class RequestState
{
 public int BufferSize = 2048;

 public byte[] BufferRead;
 public HttpWebRequest request;
 public HttpWebResponse response;
 public Stream streamResponse;

 public FileStream filestream;

 public RequestState(string downloadPath)
 {
    BufferRead = new byte[BufferSize];
    request = null;
    streamResponse = null;
    filestream = new FileStream(downloadPath, FileMode.OpenOrCreate);
 }