How to to make UdpClient.ReceiveAsync() cancelable?

Question

I have an interface INetwork with a method:

Task<bool> SendAsync(string messageToSend, CancellationToken ct)

One implementation of the interface has code like this:

public async Task<bool> SendAsync(string messageToSend, CancellationToken ct)
{
  var udpClient = new UdpClient();
  var data = Encoding.UTF8.GetBytes (messageToSend);
  var sentBytes = await udpClient.SendAsync(data);
  return sentBytes == data.Length; 
}

Unfortunately, SendAsync() of the UdpClient class does not accept a CancellationToken.

So I started changing it to:

public Task<bool> SendAsync(string messageToSend, CancellationToken ct)
{
  var udpClient = new UdpClient();
  var data = Encoding.UTF8.GetBytes (messageToSend);
  var sendTask = udpClient.SendAsync(data);
  sendTask.Wait(ct);

  if(sendTask.Status == RanToCompletion)
  {
    return sendTask.Result == data.Length;
  }
}

Obviously this won't work because there is no Task being returned. However if I return the Task, the signatures don't match anymore. SendAsync() returns a Task<int>, but I need a Task<bool>.

And now I'm confused. :-) How to resolve this?

Answer 1

Fisrt: You can't "cancel" the UdpClient.ReceiveAsync() directly, but you can simply ignore it after waiting for some time or the time you want cancel, but if you have anxiety about this "infinity wait task thread", you can:

Let's analyze the usage scenario:

1. Won't reuse the UdpClient instance:
   Just using it, the ReceiveAsync will be shutdown after disposed.

using (var udpClient = new UdpClient())
{
    await udpClient.SendAsync(sendData, sendData.Length, remoteEndPoint);
    var receiveAsyncTask = udpClient.ReceiveAsync();
    // wait time or use CancellationToken
    if (receiveAsyncTask.Wait(1000))
    {
        var data = receiveAsyncTask.Result;
        ProcessResult(data);
    }
}

2. Will reuse the UdpClient instance:
   Don't worry. When you "Send data" will shutdown the previously ReceiveAsync task.

await udpClient.SendAsync(sendData, sendData.Length, remoteEndPoint);
var receiveAsyncTask = udpClient.ReceiveAsync();
// wait time or use CancellationToken
if (receiveAsyncTask.Wait(1000))
{
    var data = receiveAsyncTask.Result;
    ProcessResult(data);
}

3. "I want it shutdown immediately!"
   The (2.) tells you "Send data" will shutdown the previously ReceiveAsync task. So just send a new Empty message.

await udpClient.SendAsync(sendData, sendData.Length, remoteEndPoint);
var receiveAsyncTask = udpClient.ReceiveAsync();
// wait time or use CancellationToken
if (receiveAsyncTask.Wait(1000))
{
    var data = receiveAsyncTask.Result;
    ProcessResult(data);
}
else
{
    udpClient.Send(new byte[0], 0, "127.0.0.1", 1);
    await receiveAsyncTask.ContinueWith(task => task.Dispose());
}

Answer 2

I know this is a little late, but I just recently had to make a UdpClient ReceiveAsync/SendAsync cancellable.

Your first code block is sending without a cancel (your title says receive by the way...).

Your second code block is defintely not the way to do it. You are calling *Async, and then Task.Wait, which blocks until the call is complete. This makes the call effectively synchronous and there's no point in calling the *Async version. The best solution is to use Async as follows:

...
var sendTask = udpClient.SendAsync(data);
var tcs = new TaskCompletionSource<bool>();
using( ct.Register( s => tcs.TrySetResult(true), null) )
{
    if( sendTask != await Task.WhenAny( task, tcs.Task) )
        // ct.Cancel() called
    else
        // sendTask completed first, so .Result will not block
}
...

There's no built-in way to cancel on UdpClient (none of the functions accept a CancellationToken), but you can take advantage of the ability to await multiple tasks with Task.WhenAny. This will return with the first task that completes (this is also an easy way to use Task.Delay() to implement timeouts). We then just need to create a Task that will complete when the CancellationToken is canceled, which we can do by creating a TaskCompletionSource and setting it with the CancellationToken's callback.

Once canceled, we can close the socket to actually "cancel" the underlying read/write.

The original idea for this came from another SO answer dealing with file handles, but it works with sockets too. I generally wrap it up in an extension method like so:

public static class AsyncExtensions
{
    public static async Task<T> WithCancellation<T>( this Task<T> task, CancellationToken cancellationToken )
    {
        var tcs = new TaskCompletionSource<bool>();
        using( cancellationToken.Register( s => ( (TaskCompletionSource<bool>)s ).TrySetResult( true ), tcs ) )
        {
            if( task != await Task.WhenAny( task, tcs.Task ) )
            {
                throw new OperationCanceledException( cancellationToken );
            }
        }

        return task.Result;
    }
}

Then use it like so:

try
{
    var data = await client.ReceiveAsync().WithCancellation(cts.Token);
    await client.SendAsync(data.Buffer, data.Buffer.Length, toep).WithCancellation(cts.Token);
}
catch(OperationCanceledException)
{
    client.Close();
}

Answer 3

First of all, if you want to return Task<bool>, you can simply do that by using Task.FromResult(). But you probably shouldn't do that, it doesn't make much sense to have an async method that's actually synchronous.

Apart from that, I also think you shouldn't pretend that the method was canceled, even if it wasn't. What you can do is to check the token before you start the real SendAsync(), but that's it.

If you really want to pretend that the method was cancelled as soon as possible, you could use ContinueWith() with cancellation:

var sentBytes = await sendTask.ContinueWith(t => t.Result, ct);