TPL Dataflow, guarantee completion only when ALL source data blocks completed

Question

How can I re-write the code that the code completes when BOTH transformblocks completed? I thought completion means that it is marked complete AND the \" out queue\" is empt

Answer 1

Here is a method that is functionally equivalent to pkt's CompleteWhenAll method, but with slightly less code:

public static void PropagateCompletionOfAll(IDataflowBlock[] sources,
    IDataflowBlock target)
{
    _ = Task.WhenAll(sources.Select(b => b.Completion)).ContinueWith(t =>
    {
        if (t.IsFaulted)
        {
            target.Fault(t.Exception);
        }
        else
        {
            target.Complete();
        }
    }, TaskScheduler.Default);
}

Usage example:

PropagateCompletionOfAll(new[] { transformBlock1, transformBlock2 }, processorBlock);

You could make the case about adding the condition: else if (t.IsCanceled) { target.Fault(new OperationCanceledException()); }, but this would be inconsistent with how the built-in completion propagation works.