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System.Threading.Tasks.Dataflow: Why are the standard block types implemented with List rather than Queue?

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The debug session shown is from the sample program provided on How to: Implement a Producer-Consumer Dataflow Pattern.

In System.Threading.Tasks.Dataflow, why are the standard block types such as BufferBlock implemented with a List rather than a Queue? Most of the dataflow blocks (especially BufferBlock<T>) are intended to be used as FIFO, but popping the head of a list is O(n) rather than O(1) for Queue. Mainly I'm curious if anyone has any insight into a possible rationale for this design.

Solution

  • Never make any conclusions based on debugging session, examine the source first.

    Consider the class information for BufferBlock<T>

    public IEnumerable<T> Queue { get { return _sourceDebuggingInformation.OutputQueue; } }

    As you can see, the Queue property here coming from a debugging information field.

    Let's take a look for that class:

    internal IEnumerable<TOutput> OutputQueue { get { return _source._messages.ToList(); } }

    Again, as you can see, this is not list-based queue, it's the queue being transformed to list for debugging.

    Finally, let's see what type has the _messages field. It's SingleProducerSingleConsumerQueue<T>, which works exactly how it should.