Observable ForEachAsync等待所有操作完成

Question

我有以下代码：

static async Task TestTwo()
{
    int x = 0;

    // The table writer is going to be faster 
    var ab = new ActionBlock<int>(async item =>
    {
        await Task.Delay(30);
        Interlocked.Increment(ref x);

    },
    new ExecutionDataflowBlockOptions() { MaxDegreeOfParallelism = 1, BoundedCapacity = 10 });

    await MyObservable().ForEachAsync(async i =>
    {
        await ab.SendAsync(i); // if the ActionBlock is full, this will complete when it has some room
    });

    await Task.Delay(10); // just to let it get ahead a little
    ab.Complete();
    await ab.Completion; // all items proccess by action block
    Console.WriteLine("Test two {0}", x);
}

static IObservable<int> MyObservable()
{
    return Observable.Create<int>(async o =>
    {
        for (int i = 0; i < 500; i++)
        {
            await Task.Delay(10);
            o.OnNext(i);
        }
        o.OnCompleted();
    });
}

这将输出 241 ，但是如果我将await ab.SendAsync(i)更改为ab.SendAsync(i).Wait()，则它将输出 500 。我猜这是由于它只是继续而已，它将在await ab.SendAsync(i)之后运行任何代码。我希望从ForEachAsync返回的等待也将等待它调用的所有内容。

我想念什么吗？