为什么TPL Dataflow阻塞.LinkTo不提供任何输出？

Question

我对TPL Dataflow这个话题很陌生.在C#中的Concurrency一书中,我测试了以下示例.我无法弄清楚为什么没有输出应该是2*2-2=2;

static void Main(string[] args)
{
    //Task tt = test();
    Task tt = test1(); 
    Console.ReadLine();
}
static async Task test1()
{
    try
    {
        var multiplyBlock = new TransformBlock<int, int>(item =>
        {
            if (item == 1)
                throw new InvalidOperationException("Blech.");
            return item * 2;
        });
        var subtractBlock = new TransformBlock<int, int>(item => item - 2);
        multiplyBlock.LinkTo(subtractBlock,
        new DataflowLinkOptions { PropagateCompletion = true });
        multiplyBlock.Post(2);
        await subtractBlock.Completion;
        int temp = subtractBlock.Receive();
        Console.WriteLine(temp);
    }
    catch (AggregateException e)
    {
        // The exception is caught here.
        foreach (var v in e.InnerExceptions)
        {
            Console.WriteLine(v.Message);
        }

    }

}

Update1:​​我尝试了另一个例子.我仍然没有使用,Block.Complete()但我想当第一个块完成时,结果会自动传递到第二个块.

private static async Task test3()
{
    TransformManyBlock<int, int> tmb = new TransformManyBlock<int, int>((i) => { return new int[] {i, i + 1}; });
    ActionBlock<int> ab = new ActionBlock<int>((i) => Console.WriteLine(i));

    tmb.LinkTo(ab);

    for (int i = 0; i < 4; i++)
    {
        tmb.Post(i);
    }
    //tmb.Complete();
    await ab.Completion;
    Console.WriteLine("Finished post");
}

Answer 1

这部分代码:

await subtractBlock.Completion;
int temp = subtractBlock.Receive();

首先(异步)等待减法块完成,然后尝试从块中检索输出.

有两个问题:源块永远不会完成,代码正在尝试从已完成的块中检索输出.一旦块完成,它将不再产生任何数据.

(我假设您指的是配方4.2中的示例,它将发布1,导致异常,从而使块处于故障状态).

因此,您可以通过完成源块来修复此测试(并且完成将沿着链接subtractBlock自动传播),并通过在(异步)等待subtractBlock完成之前读取输出:

multiplyBlock.Complete();
int temp = subtractBlock.Receive();
await subtractBlock.Completion;