Question

If you have a pending operation, eg

stream.BeginRead(_buffer, 0, _buffer.Length, _asyncCallbackRead, this); 

and you close the stream provider, eg

serialPort.Close(); 

you unsurprisingly cause an exception.

Is there an preferred method by which one might cancel a pending APM operation, prior to closing the port?

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The world has changed

Use IStream.ReadAsync(Memory<Byte>, CancellationToken)

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Colby’s reply is not the answer I hoped for, but he does at least close off a fruitless avenue in inquiry.

Happily I have found a solution.

As noted by a commenter, this will probably leak but as I do this because the app is trying to exit, it hardly matters. You should use a cancellation token anyway.

For each stream I maintain various state information in a class DeviceSession. This class has a method ReadStream providing the implementation for the AsyncCallback that handles incoming data.

Note that _asyncCallbackRead and every other variable beginning with an underscore is a class private member assigned in the constructor of DeviceSession.

The constructor also provides the initial call to _stream.BeginRead.

void ReadStream(IAsyncResult ar) {   if (IsOpen)      try     {       DevicePacket packet;       int cbRead = _stream.EndRead(ar);       _endOfValidData += cbRead;       while ((packet = GetPacket()) != null)         CommandStrategy.Process(this, packet);       _stream.BeginRead(_buffer, _endOfValidData,          _buffer.Length - _endOfValidData,          _asyncCallbackRead, null);     }     catch (Exception ex)     {       Trace.TraceError("{0}\r\n{1}", ex.Message, ex.StackTrace);       _restart(_streamProvider, _deviceId);     } } 

Notice that I haven’t bothered to set ar.AsyncState. Because the callback delegate refers to the method of a specific instance of DeviceSession, detailed and strongly typed context information (contained in the members of this instance of DeviceSession) is automatically in scope. This is the point of having a session object.

Back on the subject of aborting a listener, closing the stream provider triggers the callback but attempting to invoke EndRead results in an IOException.

Generally such an exception indicates a fault requiring a listener restart and one will want to respond by restarting the stream provider and recreating the session. This is complicated by the absence of a reliable stream-provider-independent way to determine whether the provider has faulted or the user is trying to restart the connection (eg plugged a new device into the port).

The trick is to add more context (IsOpen) to the DeviceSession to indicate whether the session is open or has been closed, and use it to smoothly complete the final abortive execution of ReadStream.

If IsOpen is true then an IOException represents a failure in need of recovery. If IsOpen is false the failure was deliberately induced and no action is required.

Answer 1

This is not directly supported in the framework. Your best bet is to write a wrapper that spawns a thread and use synchronization primitives like events to signal a cancel request.

HTH

Colby Africa