Async method doesn't work as expected

Question

I am making test console app that is to download streaming mp4 using URL.

I used async example posted on MSDN.

Below is the code.

 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;
    }
}

When I used this async method the saved file size is 1,356KB whele the file is suppoed to be 2,409KB.

I could find the breakpoint in IF(read > 0) statement was hit after the breakpoint on responseStream.close() was hit. Is this because it read 0 length and read more after then?

When I used non-async method like below, I could successfully download the file.

  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);


    }

Doesn anyone know why I failed with async method?

Thanks for your help.

Answer 1

There are multiple other ways to download a file that are far more simple , you should choose one of them.

webclient vs httpclient vs httpwebrequest

In your case I would suggest 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);
        }
    }
}

Or you could use the DownloadFileAsync method of the WebClient class, if you need to show some progress, but the example above is simpler.