在一个 JoinBlock 中，当另一个 Target 被填充时接收一个 Target

Question

I'm connecting a JoinBlock to a WriteOnceBlock and a BufferBlock to fill Targets 1 and 2. My goal is that every time the JoinBlock receives a message from the BufferBlock , it also requests the value that the WriteOnceBlock is holding.

My first guess was that I could add a ContinueWith delegate to the Target2 Completion event, but that's not quite right - I need to attach to something like a Filled event that doesn't seem to exist.

I also tried using the join block in non-greedy mode as a last-ditch effort, but that didn't change the output.

Am I missing something obvious here?

Example:

static void Main(string[] args)
    {
        var writeOnceBlockTest = new WriteOnceBlock<int>(i => i);
        var queueBlockTest = new BufferBlock<string>();
        var joinBlockTest = new JoinBlock<int, string>();
        var actionBlockTest = new ActionBlock<System.Tuple<int, string>>(tuple => Console.WriteLine($"I received int {tuple.Item1} and string {tuple.Item2}."));

        writeOnceBlockTest.LinkTo(joinBlockTest.Target1);
        queueBlockTest.LinkTo(joinBlockTest.Target2, new DataflowLinkOptions{PropagateCompletion = true});
        joinBlockTest.LinkTo(actionBlockTest, new DataflowLinkOptions { PropagateCompletion = true });

        writeOnceBlockTest.Post(3);
        queueBlockTest.Post("String1");
        queueBlockTest.Post("String2");
        writeOnceBlockTest.Post(4);
        writeOnceBlockTest.Post(5);
        queueBlockTest.Post("String3");
        queueBlockTest.Post("String4");
        queueBlockTest.Complete();
        Console.ReadLine();
    }

Desired output:

I received int 3 and string String1.

I received int 3 and string String2.

I received int 3 and string String3.

I received int 3 and string String4.

Actual output:

I received int 3 and string String1.

Answer 1

The JoinBlock isn't the right choice here although it does seem like the perfect fit. The WriteOnceBlock , as you've found, only offers it's value once. However, you can read that value many times. With this you can use a TransformBlock to get the behavior your looking for.

public class JoinFlow
{
    [Test]
    public async Task TestWriteOnceBlock()
    {
        var writeOnceBlockTest = new WriteOnceBlock<int>(i => i);
        var queueBlockTest = new BufferBlock<string>();
        var transformBlockTest = new TransformBlock<string, Tuple<int, string>>(async str => Tuple.Create(await writeOnceBlockTest.ReceiveAsync(), str));
        var actionBlockTest = new ActionBlock<Tuple<int, string>>(tuple => Console.WriteLine($"I received int {tuple.Item1} and string {tuple.Item2}."));

        queueBlockTest.LinkTo(transformBlockTest, new DataflowLinkOptions { PropagateCompletion = true });
        transformBlockTest.LinkTo(actionBlockTest, new DataflowLinkOptions { PropagateCompletion = true });

        writeOnceBlockTest.Post(3);
        queueBlockTest.Post("String1");
        queueBlockTest.Post("String2");
        writeOnceBlockTest.Post(4);
        writeOnceBlockTest.Post(5);
        queueBlockTest.Post("String3");
        queueBlockTest.Post("String4");
        queueBlockTest.Complete();
        await actionBlockTest.Completion;
    }
}

Outputs:

I received int 3 and string String1.
I received int 3 and string String2.
I received int 3 and string String3.
I received int 3 and string String4.