Can't stop circular dataflow blocks

[ZhaoXiangXML]

I'm trying to implement a crawler with System.Threading.Tasks.Dataflow, the core feature is to have a TransformBlock, which accepts an url as input, request the url, parse the response, and find more urls for the same block.

Then I noticed a problem that I cannot complete/stop this pipeline. The following code simulates my problem:

using System;
using System.Threading;
using System.Threading.Tasks.Dataflow;

namespace ConsoleApp1
{
 class Program
 {
 static void Main()
 {
 int count = 0;

 var block = new TransformBlock<string, string>(uri =>
 {
 //await Task.Delay(10);
 Console.WriteLine($"Downloading {count}");

 if (Interlocked.Increment(ref count) > 10)
 {
 return null;
 }

 return uri;
 });

 var linkOptions = new DataflowLinkOptions { PropagateCompletion = false };

 block.LinkTo(block, linkOptions, (x) => !string.IsNullOrEmpty(x));

 block.Post("");
 block.Post("");

 while (true)
 {
 Console.WriteLine($"Check Count: {count}");

 if (count >= 2)
 {
 Console.WriteLine("Complete");
 block.Complete();
 break;
 }

 Thread.Sleep(20);
 }

 Console.WriteLine("After Check");

 block.Completion.Wait();

 Console.WriteLine("Done");
 }
 }
}

The output is:

Check Count: 0
Downloading 0
Downloading 1
Check Count: 2
Complete
After Check

And program stuck here, block.Completion.Wait() never return.

I've also tried to have two blocks link to each other, yet the result is same.

So what can I do about my code? How can I stop this dataflow?

[ZhaoXiangXML]

Versions of things: Windows 10.0.17763.864 64bit, .NET Core 3.0, System.Threading.Tasks.Dataflow 4.11.0

[ZhaoXiangXML]

And I've also tried PropagateCompletion = true, the result is all same

[ZhaoXiangXML]

Well problem solved by adding block.LinkTo(DataflowBlock.NullTarget<string>(), linkOptions);, thanks to the help of the sample code