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What Happens in the Default Implementation of Stream.ReadAsync Will Shock You

tl;dr The base implementation of Stream.ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken) doesn’t do what you might reasonably expect with respect to cancellation. Make sure you check whether your Stream subclass provides a proper implementation before relying on it.

A common pattern when cancelling a Task is to then wait for it to complete. This ensures that any unmanaged resources that the Task might be using are then safe to clean up:

cancellationTokenSource.Cancel();

try
{
  task.GetAwaiter().GetResult();
}
catch (OperationCanceledException)
{
}

// dispose of other stuff...

However, this pattern can lead to application hangs if the Task in question doesn’t do a good job of supporting cancellation, and the .NET Framework makes it surprisingly easy to fall into Pits of Failure. One such pit of failure is Stream.ReadAsync.

Background

I recently needed to build a system to communicate between a couple of processes running on the same machine. Eventually, I settled on using Windows’ named pipes, which are exposed in the .NET Framework as NamedPipeServerStream and NamedPipeClientStream.

For both ends of the pipe, I created a simple manager class that creates a Task that handles connecting/reconnecting to the pipe and listening for messages from the other end until canceled. In the Dispose method of the manager, I followed the pattern described above. In the Task, the CancellationToken was checked at each iteration, and it was also passed to all async calls. To a casual reader, the implementation appeared flawless.

The processes also both hung on Dispose if they ever succeeded in connecting to each other. Pausing the process in the debugger indicated that the hanging thread was waiting for the Task to finish, and no threads were currently associated with the Task.

Huh?

In terms of the Win32 API, a named pipe is mostly just a file: named pipe clients can even create them by calling CreateFile directly. This means that under the hood, they support nearly all of the same operations as files, including overlapped I/O and CancelIoEx.

If you take a look at .NET Core’s implementation, you’ll even find that the Windows-specific implementation of ReadAsync creates an instance of a subclass of TaskCompletionSource that uses these features. You could easily be forgiven for assuming that the .NET Framework would be implemented in exactly the same way.

But you’d be wrong.

Instead, the .NET Framework, as of .NET 4.7, provides no override of ReadAsync, which means that it delegates to Stream’s default implementation. This implementation checks the value of the CancellationToken at the beginning of the method; if it has not been triggered, it discards the token and delegates to a BeginRead/EndRead-based implementation of async I/O.

This is what caused my hang.

Workarounds

I’m not aware of a general-purpose workaround that will work for every subclass of Stream.

In the case of NamedPipeClientStream and NamedPipeServerStream, it is possible to Dispose the stream during an asynchronous read, which will have the effect of terminating the read. If you do this, you may also need to be prepared to catch an ObjectDisposedException when waiting for your Task to complete.

You could also use P/Invoke to drop down directly to the Win32 API to create a named pipe as a SafeFileHandle and pass it to the FileStream constructor: FileStream does override ReadAsync in terms of overlapped I/O and CancelIoEx, so you could call it without worrying about your Task becoming orphaned. On the other hand, you’d also have to continue dropping to the Win32 API to handle all of the connection-oriented logic.

In any case, if you’re writing code that deals with Streams and needs to handle cancellation, make sure your subclass properly supports it!

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