internal Task RenderAsync(ImageData data, CancellationToken cancellationToken)
{
    return Task.Run(() =>
    {
        var bmp = new Bitmap(data.Width, data.Height);
        for(int y=0; y)
        {
            cancellationToken.ThrowIfCancellationRequested();
            for(int x=0; x)
            {
                // render pixel [x,y] into bmp
            }
        }
        return bmp;
    }, cancellationToken);
}

public static Task Delay(int millisecondsTimeout)
   {
       TaskCompletionSource tcs = null;
       Timer timer = null;

       timer = new Timer(delegate
       {
           timer.Dispose();
           tcs.TrySetResult(DateTimeOffset.UtcNow);
       }, null, Timeout.Infinite, Timeout.Infinite);

       tcs = new TaskCompletionSource(timer);
       timer.Change(millisecondsTimeout, Timeout.Infinite);
       return tcs.Task;
   }

public static async Task Poll(Uri url, CancellationToken cancellationToken,  IProgress<bool> progress)
{
    while(true)
    {
        await Task.Delay(TimeSpan.FromSeconds(10), cancellationToken);
        bool success = false;
        try
        {
            await DownloadStringAsync(url);
            success = true;
        }
        catch { /* ignore errors */ }
        progress.Report(success);
    }
}

// Download File CancellationToken 參數(shù)賦值獲得一個取消請求 progress參數(shù)負責(zé)進度報告
private void DownLoadFile(string url, CancellationToken ct, IProgress<int> progress)
{
    HttpWebRequest request = null;
    HttpWebResponse response = null;
    Stream responseStream = null;
    int bufferSize = 2048;
    byte[] bufferBytes = new byte[bufferSize];
    try
    {
        request = (HttpWebRequest)WebRequest.Create(url);
        if (DownloadSize != 0)
        {
            request.AddRange(DownloadSize);
        }

        response = (HttpWebResponse)request.GetResponse();
        responseStream = response.GetResponseStream();
        int readSize = 0;
        while (true)
        {
            // 收到取消請求則退出異步操作
            if (ct.IsCancellationRequested == true)
            {
                MessageBox.Show(String.Format("下載暫停，下載的文件地址為：{0}\n 已經(jīng)下載的字節(jié)數(shù)為: {1}字節(jié)", downloadPath, DownloadSize));

                response.Close();
                filestream.Close();
                sc.Post((state) =>
                {
                    this.btnStart.Enabled = true;
                    this.btnPause.Enabled = false;
                }, null);

                // 退出異步操作
                break;
            }

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

                // 報告進度
                progress.Report(percentComplete);
            }
            else
            {
                MessageBox.Show(String.Format("下載已完成，下載的文件地址為：{0}，文件的總字節(jié)數(shù)為: {1}字節(jié)", downloadPath, totalSize));

                sc.Post((state) =>
                {
                    this.btnStart.Enabled = false;
                    this.btnPause.Enabled = false;
                }, null);

                response.Close();
                filestream.Close();
                break;
            }
        }
    }
    catch (AggregateException ex)
    {
        // 因為調(diào)用Cancel方法會拋出OperationCanceledException異常 將任何OperationCanceledException對象都視為以處理
        ex.Handle(e => e is OperationCanceledException);
    }
}

#region 字段

private int DownloadSize = 0;
private string downloadPath = null;
private long totalSize = 0;
private FileStream filestream;

private CancellationTokenSource cts = null;
private Task task = null;

private SynchronizationContext sc;

#endregion 字段

#region 構(gòu)造函數(shù)

public FileDownLoadForm()
{
    InitializeComponent();
    string url = "http://download.microsoft.com/download/7/0/3/703455ee-a747-4cc8-bd3e-98a615c3aedb/dotNetFx35setup.exe";
    txbUrl.Text = url;
    this.btnPause.Enabled = false;

    // Get Total Size of the download file
    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;
        if (DownloadSize == totalSize)
        {
            string message = "There is already a file with the same name, do you want to delete it? If not, please change the local path. ";
            var result = MessageBox.Show(message, "File name conflict: " + downloadPath, MessageBoxButtons.OKCancel);

            if (result == System.Windows.Forms.DialogResult.OK)
            {
                File.Delete(downloadPath);
            }
            else
            {
                progressBar1.Value = (int)((float)DownloadSize / (float)totalSize * 100);
                this.btnStart.Enabled = false;
                this.btnPause.Enabled = false;
            }
        }
    }
}

#endregion 構(gòu)造函數(shù)

#region 方法

// Start DownLoad File
private void btnStart_Click(object sender, EventArgs e)
{
    filestream = new FileStream(downloadPath, FileMode.OpenOrCreate);
    this.btnStart.Enabled = false;
    this.btnPause.Enabled = true;

    filestream.Seek(DownloadSize, SeekOrigin.Begin);

    // 捕捉調(diào)用線程的同步上下文派生對象
    sc = SynchronizationContext.Current;

    cts = new CancellationTokenSource();
    // 使用指定的操作初始化新的 Task。
    task = new Task(() => Actionmethod(cts.Token), cts.Token);

    // 啟動 Task，并將它安排到當(dāng)前的 TaskScheduler 中執(zhí)行。
    task.Start();

    //await DownLoadFileAsync(txbUrl.Text.Trim(), cts.Token,new Progress(p => progressBar1.Value = p));
}

// 任務(wù)中執(zhí)行的方法
private void Actionmethod(CancellationToken ct)
{
    // 使用同步上文文的Post方法把更新UI的方法讓主線程執(zhí)行
    DownLoadFile(txbUrl.Text.Trim(), ct, new Progress<int>(p =>
        {
            sc.Post(new SendOrPostCallback((result) => progressBar1.Value = (int)result), p);
        }));
}

// Pause Download
private void btnPause_Click(object sender, EventArgs e)
{
    // 發(fā)出一個取消請求
    cts.Cancel();
}

// Get Total Size of File
private void GetTotalSize()
{
    HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(txbUrl.Text.Trim());
    HttpWebResponse response = (HttpWebResponse)myHttpWebRequest.GetResponse();
    totalSize = response.ContentLength;
    response.Close();
}

public static Task<int> ReadAsync(this Stream stream,  byte[] buffer, int offset,  int count)
{
    if (stream == null) 
       throw new ArgumentNullException("stream");
    
    return Task<int>.Factory.FromAsync(stream.BeginRead,  stream.EndRead, buffer,   offset, count, null);
}

 public static Task<int> ReadAsync(this Stream stream,  byte [] buffer, int offset,   int count)
 {
    if (stream == null) 
        throw new ArgumentNullException("stream");

    var tcs = new TaskCompletionSource<int>();

    stream.BeginRead(buffer, offset, count, iar =>
                     {
                        try { 
                           tcs.TrySetResult(stream.EndRead(iar)); 
                        }
                        catch(OperationCanceledException) { 
                           tcs.TrySetCanceled(); 
                        }
                        catch(Exception exc) { 
                           tcs.TrySetException(exc); 
                        }
                     }, null);
    return tcs.Task;
}

public static Task<string> DownloadStringAsync(Uri url)
 {
     var tcs = new TaskCompletionSource<string>();

     var wc = new WebClient();

     wc.DownloadStringCompleted += (s,e) =>
         {
             if (e.Error != null) 
                tcs.TrySetException(e.Error);
             else if (e.Cancelled) 
                tcs.TrySetCanceled();
             else 
                tcs.TrySetResult(e.Result);
         };
     wc.DownloadStringAsync(url);
     return tcs.Task;
}