Task cancellation: while loop not exited properly when ThrowIfCancellationRequested() is called - wpf

Question

I've developed a small project (using MVVM) with functionality to upload a file to a FTP-server.
The user can view the uploading progress: percentage completed is shown to the user by vm.Busycontent, which is a property in my viewmodel bound to a UI element in my view.
Here is the code for reading the file and uploading via FTP (which is part of the Task vm.FtpUploadTask)
using (FileStream inputStream = File.OpenRead(file))
{
using (outputStream = request.GetRequestStream())
{
var buffer = new byte[1024 * 1024];
int totalReadBytesCount = 0;
int readBytesCount;
while ((readBytesCount = inputStream.Read(buffer, 0, buffer.Length)) > 0 && (!vm.Token.IsCancellationRequested))
{
vm.Token.ThrowIfCancellationRequested();
outputStream.Write(buffer, 0, readBytesCount);
totalReadBytesCount += readBytesCount;
var progress = totalReadBytesCount * 100.0 / inputStream.Length;
vm.BusyContent = ((int)progress).ToString();
}
}
}
MainWindow.xaml
I am using WPF extended toolkit BusyIndicator
<xctk:BusyIndicator IsBusy='{Binding IsBusy}'>
<xctk:BusyIndicator.BusyContentTemplate>
<DataTemplate>
<StackPanel>
<TextBlock Text='{Binding Path=DataContext.BusyContent,
RelativeSource={RelativeSource AncestorType={x:Type Window}}}' />
</StackPanel>
</DataTemplate>
</xctk:BusyIndicator.BusyContentTemplate>
</xctk:BusyIndicator>
UploadToFTPCommand.cs
try
{
vm.FtpUploadTask = new Task(() => FTPUpload(file), vm.Token);
vm.FtpUploadTask.Start();
vm.FtpUploadTask.Wait(vm.Token);
vm.BusyContent = "upload done!";
}
catch (OperationCanceledException)
{
vm.BusyContent = "Canceled";
}
CancelCommand.cs
public class CancelCommand : ICommand
{
public void Execute(object parameter)
{
vm.TokenSource.Cancel();
}
}
The cancel function works but sometimes vm.Busycontent equals to
((int)progress).ToString()
canceled in uploadftpcommand
When pressing the Cancel button, the while loop should be exited and the user should only see the message in the catch (OperationCanceledException).
Any ideas how to solve this?
Notes
I am using .NET 4.0
This program is a part of a larger project, which includes multipe Tasks that should be executed in a synchronous manner. That's why I am using Task.Start() and Task.Wait() methods.
Edit
Problem still remains
using (FileStream inputStream = File.OpenRead(file))
{
using (outputStream = request.GetRequestStream())
{
var buffer = new byte[1024 * 1024];
int totalReadBytesCount = 0;
int readBytesCount;
while ((readBytesCount = inputStream.Read(buffer, 0, buffer.Length)) > 0)
{
if (vm.Token.IsCancellationRequested)
{
break;
}
outputStream.Write(buffer, 0, readBytesCount);
totalReadBytesCount += readBytesCount;
var progress = totalReadBytesCount * 100.0 / inputStream.Length;
vm.BusyContent = ((int)progress).ToString();
}
if (vm.Token.IsCancellationRequested)
{
inputStream.Close();
outputStream.Close();
vm.Token.ThrowIfCancellationRequested();
}
}
}

Answer 1

You may have an issue relating to exactly how you've set up your code. Without going through every line of it, it's difficult to tell. But more to the point:
When pressing the Cancel button, the while loop should be exited and the user should only see the message in the catch (OperationCanceledException).
This shows exactly how to exit your while loop and throw an exception you can catch. Note the test asserts TaskCanceledException but you are right in just catching OperationCanceledException.
using System.Threading.Tasks;
using System.IO;
using System.Threading;
using NUnit.Framework;
namespace CancellationTests {
[TestFixture]
public class WhileCancellation {
[Test]
public void While_Loop_Is_Canceled_When_Cancel_Is_Requested() {
var inputFile = new FileInfo("somebigfile.txt");
var outputFile = new FileInfo("outputFile.txt");
var cts = new CancellationTokenSource();
cts.Cancel();
Assert.ThrowsAsync<TaskCanceledException>(() => TheWhileLoop(inputFile, outputFile, cts.Token));
}
private async Task TheWhileLoop(FileInfo inputFile, FileInfo outputFile, CancellationToken token) {
using (var inputStream = inputFile.OpenRead())
using (var outputStream = outputFile.OpenWrite()) {
var buffer = new byte[1024 * 1024];
var totalReadBytesCount = 0;
var readBytesCount = 0;
while ((readBytesCount = await inputStream.ReadAsync(buffer, 0, buffer.Length, token)) > 0) {
await outputStream.WriteAsync(buffer, 0, readBytesCount, token);
token.ThrowIfCancellationRequested();
totalReadBytesCount += readBytesCount;
var progress = totalReadBytesCount * 100.0 / inputStream.Length;
vm.BusyContent = ((int)progress).ToString();
}
}
}
private static class vm {
public static string BusyContent { get; set; }
}
}
}
Hopefully this gets you on the right track.