CODEWITHSUNDEEP
c#multithreadingasync-awaitsystem.threading.channels
skitoy4321
skitoy4321

Reputation: 45

How to avoid InvalidOperationException was thrown when ChannelReader.WaitToReadAsync?

I wrote the asynchronous queue using System.Threading.Channels. but when I ran the program for testing, following exception was thrown at random time and worker thread was stopped.

System.InvalidOperationException: The asynchronous operation has not completed.
   at System.Threading.Channels.AsyncOperation.ThrowIncompleteOperationException()
   at System.Threading.Channels.AsyncOperation`1.GetResult(Int16 token)
   at AsyncChannels.Worker() in g:\src\gitrepos\dotnet-sandbox\channelstest\AsyncChannelsTest.cs:line 26

If Exception was caught and ignore, the code is working. But I want to get rid of the error whose cause is not clear.

here is my environment and the least code.

using System.Threading.Channels;
using System.Threading;
using System.Threading.Tasks;
using System;
using System.Linq;

class AsyncChannels : IDisposable
{
    Channel<TaskCompletionSource<bool>> _Channel;
    Thread _Thread;
    CancellationTokenSource _Cancellation;
    public AsyncChannels()
    {
        _Channel = Channel.CreateUnbounded<TaskCompletionSource<bool>>();
        _Thread = new Thread(Worker);
        _Thread.Start();
        _Cancellation = new CancellationTokenSource();
    }
    private void Worker()
    {
        while (!_Cancellation.IsCancellationRequested)
        {
            // System.InvalidOperationException is thrown
            if (!_Channel.Reader.WaitToReadAsync(_Cancellation.Token).Result)
            {
                break;
            }
            while (_Channel.Reader.TryRead(out var item))
            {
                item.TrySetResult(true);
            }
        }
    }
    public void Dispose()
    {
        _Cancellation.Cancel();
        _Channel.Writer.TryComplete();
        _Thread.Join();
    }
    public Task<bool> Enqueue()
    {
        var tcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
        _Channel.Writer.TryWrite(tcs);
        return tcs.Task;
    }
    public static async Task Test()
    {
        using (var queue = new AsyncChannels())
        {
            for (int i = 0; i < 100000; i++)
            {
                await queue.Enqueue().ConfigureAwait(false);
            }
        }
    }
}

Upvotes: 3

Views: 1833

Answers (1)

Stephen Cleary
Stephen Cleary

Reputation: 457057

You can't directly block on a ValueTask<T> like this:

_Channel.Reader.WaitToReadAsync(_Cancellation.Token).Result

You can only do two things with ValueTask<T>: await it (only once), or convert it to a Task<T> by calling AsTask(). If you need to do anything complex, like awaiting it more than once or blocking on it, then you need to use AsTask().

Or, in this case, just use await in the standard Channels consumption pattern:

class AsyncChannels : IDisposable
{
  Channel<TaskCompletionSource<bool>> _Channel;
  Task _Thread;
  CancellationTokenSource _Cancellation;
  public AsyncChannels()
  {
    _Channel = Channel.CreateUnbounded<TaskCompletionSource<bool>>();
    _Thread = Task.Run(() => WorkerAsync());
    _Cancellation = new CancellationTokenSource();
  }
  private async Task WorkerAsync()
  {
    try
    {
      while (await _Channel.Reader.WaitToReadAsync(_Cancellation.Token))
        while (_Channel.Reader.TryRead(out var item))
          item.TrySetResult(true);
    }
    catch (OperationCanceledException)
    {
    }
  }
  public void Dispose()
  {
    _Cancellation.Cancel();
    _Channel.Writer.TryComplete();
  }
  ...
}

Upvotes: 9

Related Questions