pat long munkii yebee - Page 91

How to add correct cancellation when downloading a file with the example in the samples of the new P

- by Mike

Hello everybody, I have downloaded the last samples of the Parallel Programming team, and I don't succeed in adding correctly the possibility to cancel the download of a file. Here is the code I ended to have: var wreq = (HttpWebRequest)WebRequest.Create(uri); // Fire start event DownloadStarted(this, new DownloadStartedEventArgs(remoteFilePath)); long totalBytes = 0; wreq.DownloadDataInFileAsync(tmpLocalFile, cancellationTokenSource.Token, allowResume, totalBytesAction => { totalBytes = totalBytesAction; }, readBytes => { Log.Debug("Progression : {0} / {1} => {2}%", readBytes, totalBytes, 100 * (double)readBytes / totalBytes); DownloadProgress(this, new DownloadProgressEventArgs(remoteFilePath, readBytes, totalBytes, (int)(100 * readBytes / totalBytes))); }) .ContinueWith( (antecedent ) => { if (antecedent.IsFaulted) Log.Debug(antecedent.Exception.Message); //Fire end event SetEndDownload(antecedent.IsCanceled, antecedent.Exception, tmpLocalFile, 0); }, cancellationTokenSource.Token); I want to fire an end event after the download is finished, hence the ContinueWith. I slightly changed the code of the samples to add the CancellationToken and the 2 delegates to get the size of the file to download, and the progression of the download: return webRequest.GetResponseAsync() .ContinueWith(response => { if (totalBytesAction != null) totalBytesAction(response.Result.ContentLength); response.Result.GetResponseStream().WriteAllBytesAsync(filePath, ct, resumeDownload, progressAction).Wait(ct); }, ct); I had to add the call to the Wait function, because if I don't, the method exits and the end event is fired too early. Here are the modified method extensions (lot of code, apologies :p) public static Task WriteAllBytesAsync(this Stream stream, string filePath, CancellationToken ct, bool resumeDownload = false, Action<long> progressAction = null) { if (stream == null) throw new ArgumentNullException("stream"); // Copy from the source stream to the memory stream and return the copied data return stream.CopyStreamToFileAsync(filePath, ct, resumeDownload, progressAction); } public static Task CopyStreamToFileAsync(this Stream source, string destinationPath, CancellationToken ct, bool resumeDownload = false, Action<long> progressAction = null) { if (source == null) throw new ArgumentNullException("source"); if (destinationPath == null) throw new ArgumentNullException("destinationPath"); // Open the output file for writing var destinationStream = FileAsync.OpenWrite(destinationPath); // Copy the source to the destination stream, then close the output file. return CopyStreamToStreamAsync(source, destinationStream, ct, progressAction).ContinueWith(t => { var e = t.Exception; destinationStream.Close(); if (e != null) throw e; }, ct, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Current); } public static Task CopyStreamToStreamAsync(this Stream source, Stream destination, CancellationToken ct, Action<long> progressAction = null) { if (source == null) throw new ArgumentNullException("source"); if (destination == null) throw new ArgumentNullException("destination"); return Task.Factory.Iterate(CopyStreamIterator(source, destination, ct, progressAction)); } private static IEnumerable<Task> CopyStreamIterator(Stream input, Stream output, CancellationToken ct, Action<long> progressAction = null) { // Create two buffers. One will be used for the current read operation and one for the current // write operation. We'll continually swap back and forth between them. byte[][] buffers = new byte[2][] { new byte[BUFFER_SIZE], new byte[BUFFER_SIZE] }; int filledBufferNum = 0; Task writeTask = null; int readBytes = 0; // Until there's no more data to be read or cancellation while (true) { ct.ThrowIfCancellationRequested(); // Read from the input asynchronously var readTask = input.ReadAsync(buffers[filledBufferNum], 0, buffers[filledBufferNum].Length); // If we have no pending write operations, just yield until the read operation has // completed. If we have both a pending read and a pending write, yield until both the read // and the write have completed. yield return writeTask == null ? readTask : Task.Factory.ContinueWhenAll(new[] { readTask, writeTask }, tasks => tasks.PropagateExceptions()); // If no data was read, nothing more to do. if (readTask.Result <= 0) break; readBytes += readTask.Result; if (progressAction != null) progressAction(readBytes); // Otherwise, write the written data out to the file writeTask = output.WriteAsync(buffers[filledBufferNum], 0, readTask.Result); // Swap buffers filledBufferNum ^= 1; } } So basically, at the end of the chain of called methods, I let the CancellationToken throw an OperationCanceledException if a Cancel has been requested. What I hoped was to get IsFaulted == true in the appealing code and to fire the end event with the canceled flags and the correct exception. But what I get is an unhandled exception on the line response.Result.GetResponseStream().WriteAllBytesAsync(filePath, ct, resumeDownload, progressAction).Wait(ct); telling me that I don't catch an AggregateException. I've tried various things, but I don't succeed to make the whole thing work properly. Does anyone of you have played enough with that library and may help me? Thanks in advance Mike

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