BufferBlock getting blocked on OutputAvailableAsync

Question

I am new to using TPL. I've been able to implement a simple post / receive transaction on a BufferBlock but when I try to go asynchronous, I get hung.

Here is a simplified version of what I am trying.

Declare the message buffer (made it global)

Dim msgBuffer As New BufferBlock(Of String)

A simple function to post the messages which is just a list of file names in a directory

Private Sub PostMessagesToBuffer()

    filesList = Directory.GetFiles(txtBoxSrcFilesDir.Text, fileFilter).ToList
    For Each file In filesList
        msgBuffer.Post(file)
    Next
    msgBuffer.Complete()
End Sub

I created this function for processing the messages:

Private Async Function ProcessMessagesAsync() As Task(Of Integer)

    Dim msgsProcessed = 0

    While Await msgBuffer.OutputAvailableAsync
        Dim msg = msgBuffer.Receive
        Console.WriteLine(msg)
        msgsProcessed += 1
    End While

    Return msgsProcessed

End Function

Then just this set of calls to run it all.

Dim msgProcessor = ProcessMessagesAsync()
PostMessagesToBuffer()
msgProcessor.Wait()
Console.writeLine("Processed " & msgProcessor.Result & " Messages.")

I can debug and see the messages getting added to the buffer but the "While Await" never gets the signal that a message is available on the queue. It just sits there, never getting into the loop to do the work on the message. Am I missing something rather simple here?

Answer 1

You are mixing synchronous and asynchronous code. Deadlocks are pretty common when blocking on async code. Instead of Wait:

msgProcessor.Wait()

...it is safer to Await:

Await msgProcessor

Also the DataflowBlock.Receive is a blocking method. The proper method to use after awaiting the DataflowBlock.OutputAvailableAsync is the method DataflowBlock.TryReceive.

I should also note that retrieving manually the elements of a DataflowBlock is feasible but uncommon. Usually the last block in a dataflow pipeline is an ActionBlock, that doesn't generate output.