异步方法无法按预期工作

Posted 2023-04-12

【中文标题】异步方法无法按预期工作

【英文标题】：Async method doesn't work as expected

【发布时间】：2017-04-17 23:11:39

【问题描述】：

我正在制作使用 URL 下载流式 mp4 的测试控制台应用程序。

我使用了 MSDN 上发布的异步示例。

下面是代码。

 class Program

    public static ManualResetEvent allDone = new ManualResetEvent(false);
    const int BUFFER_SIZE = 1024;
    public static int total = 0;

    static void Main(string[] args)
    

        Uri httpSite = new Uri("URI");

        WebRequest wreq = WebRequest.Create(httpSite);

        RequestState rs = new RequestState();

        rs.Request = wreq;

        IAsyncResult r = (IAsyncResult)wreq.BeginGetResponse(
           new AsyncCallback(RespCallback), rs);

        allDone.WaitOne();
        FileStream file = new FileStream("test.mp4", FileMode.Append);
        file.Write(rs.data.ToArray(), 0, rs.data.Count());
        Console.ReadKey();

    

    private static void RespCallback(IAsyncResult ar)
    
        // Get the RequestState object from the async result.
        RequestState rs = (RequestState)ar.AsyncState;

        // Get the WebRequest from RequestState.
        WebRequest req = rs.Request;

        // Call EndGetResponse, which produces the WebResponse object
        //  that came from the request issued above.
        WebResponse resp = req.EndGetResponse(ar);

        //  Start reading data from the response stream.
        Stream ResponseStream = resp.GetResponseStream();

        // Store the response stream in RequestState to read 
        // the stream asynchronously.
        rs.ResponseStream = ResponseStream;

        //  Pass rs.BufferRead to BeginRead. Read data into rs.BufferRead
        IAsyncResult iarRead = ResponseStream.BeginRead(rs.BufferRead, 0,
           BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs);
    
    private static void ReadCallBack(IAsyncResult asyncResult)
    
        // Get the RequestState object from AsyncResult.
        RequestState rs = (RequestState)asyncResult.AsyncState;

        // Retrieve the ResponseStream that was set in RespCallback. 
        Stream responseStream = rs.ResponseStream;

        // Read rs.BufferRead to verify that it contains data. 
        int read = responseStream.EndRead(asyncResult);
        if (read > 0)
        
            // Prepare a Char array buffer for converting to Unicode.
            byte[] charBuffer = new byte[BUFFER_SIZE];             


            responseStream.Read(charBuffer, 0, read);
            rs.data.AddRange(charBuffer);


            IAsyncResult ar = responseStream.BeginRead(
               rs.BufferRead, 0, BUFFER_SIZE,
               new AsyncCallback(ReadCallBack), rs);
        
        else
                       
            responseStream.Close();
            // Set the ManualResetEvent so the main thread can exit.
            allDone.Set();
        
        return;
    

当我使用这种异步方法时，保存的文件大小是 1,356KB，而文件应该是 2,409KB。

我可以在 responseStream.close() 上的断点被击中后找到 IF(read > 0) 语句中的断点。这是因为它读取的长度为 0，然后读取更多吗？

当我使用如下非异步方法时，我可以成功下载文件。

  public static void StreamDownload(string url)
    
        int dataLength;
        int bytesRead;

        WebRequest req = WebRequest.Create(url);
        WebResponse response = req.GetResponse();
        string fileName = System.IO.Path.GetFileName("test.mp4");

        Stream dataStream = response.GetResponseStream();

        byte[] buffer = new byte[1024];

        FileStream file = new FileStream(fileName, FileMode.Append);

        dataLength = (int)response.ContentLength;

        do
        
            bytesRead = dataStream.Read(buffer, 0, buffer.Length);
            file.Write(buffer, 0, bytesRead);

         while (bytesRead != 0);


    

有人知道为什么我使用异步方法失败了吗？

感谢您的帮助。

【参考方案1】：

还有多种其他方法可以简单得多下载文件，您应该选择其中一种。

webclient vs httpclient vs httpwebrequest

在你的情况下，我建议 HttpClient：

public static async Task DownloadAsync(string requestUri, string filename)

    using (var httpClient = new HttpClient())
    
        using (var fs = File.OpenWrite(filename))
        
            await (await httpClient.GetStreamAsync(requestUri)).CopyToAsync(fs);
        
    

如果您需要显示一些进度，或者您可以使用 WebClient 类的DownloadFileAsync 方法，但上面的示例更简单。