異步方法無法正常工作

Question

我正在制作測試控制台應用程序，該應用程序將使用URL下載流mp4。

我使用了MSDN上發布的異步示例。

下面是代碼。

 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;
    }
}

當我使用這種異步方法時，保存的文件大小為1356KB，而該文件的大小應為2409KB。

在responseStream.close（）上的斷點被擊中之后，我可以在IF（read> 0）語句中找到斷點。 這是因為它讀取0長度，然后讀取更多嗎？

當我使用如下所示的非異步方法時，我可以成功下載文件。

  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);


    }

有誰知道我為什么用異步方法失敗了？

謝謝你的幫助。

Answer 1

有多種其他下載文件的方法，它們更為簡單 ，您應該選擇其中一種。

webclient vs httpclient vs httpwebrequest

在您的情況下，我建議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);
        }
    }
}

或者，如果需要顯示一些進度，則可以使用WebClient類的DownloadFileAsync方法，但是上面的示例更簡單。