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    <p>I think the best way to do something like this using C# 5/.Net 4.5 is to use <a href="http://msdn.microsoft.com/en-us/devlabs/gg585582">TPL Dataflow</a>. There even is <a href="http://blogs.microsoft.co.il/blogs/bnaya/archive/2012/01/28/tpl-dataflow-walkthrough-part-5.aspx">a walkthrough on how to implement web crawler using it</a>.</p> <p>Basically, you create one "block" that takes care of downloading one URL and getting the link from it:</p> <pre><code>var cts = new CancellationTokenSource(); Func&lt;LinkItem, Task&lt;IEnumerable&lt;LinkItem&gt;&gt;&gt; downloadFromLink = async link =&gt; { // WebClient is not guaranteed to be thread-safe, // so we shouldn't use one shared instance var client = new WebClient(); string html = await client.DownloadStringTaskAsync(link.Href); return LinkFinder.Find(html, link.BaseURL); }; var linkFinderBlock = new TransformManyBlock&lt;LinkItem, LinkItem&gt;( downloadFromLink, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4, CancellationToken = cts.Token }); </code></pre> <p>You can set <code>MaxDegreeOfParallelism</code> to any value you want. It says at most how many URLs can be downloaded concurrently. If you don't want to limit it at all, you can set it to <code>DataflowBlockOptions.Unbounded</code>.</p> <p>Then you create one block that processes all the downloaded links somehow, like storing them all in a list. It can also decide when to cancel downloading:</p> <pre><code>var links = new List&lt;LinkItem&gt;(); var storeBlock = new ActionBlock&lt;LinkItem&gt;( linkItem =&gt; { links.Add(linkItem); if (links.Count == maxSize) cts.Cancel(); }); </code></pre> <p>Since we didn't set <code>MaxDegreeOfParallelism</code>, it defaults to 1. That means using collection that is not thread-safe should be okay here.</p> <p>We create one more block: it will take a link from <code>linkFinderBlock</code>, and pass it both to <code>storeBlock</code> and back to <code>linkFinderBlock</code>.</p> <pre><code>var broadcastBlock = new BroadcastBlock&lt;LinkItem&gt;(li =&gt; li); </code></pre> <p>The lambda in its constructor is a "cloning function". You can use it to create a clone of the item if you want to, but it shouldn't be necessary here, since we don't modify the <code>LinkItem</code> after creation.</p> <p>Now we can connect the blocks together:</p> <pre><code>linkFinderBlock.LinkTo(broadcastBlock); broadcastBlock.LinkTo(storeBlock); broadcastBlock.LinkTo(linkFinderBlock); </code></pre> <p>Then we can start processing by giving the first item to <code>linkFinderBlock</code> (or <code>broadcastBlock</code>, if you want to also send it to <code>storeBlock</code>):</p> <pre><code>linkFinderBlock.Post(firstItem); </code></pre> <p>And finally wait until the processing is complete:</p> <pre><code>try { linkFinderBlock.Completion.Wait(); } catch (AggregateException ex) { if (!(ex.InnerException is TaskCanceledException)) throw; } </code></pre>
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    1. COWow! Thank you for the brilliant explanation. Could you just confirm one thing? If we set MaxDegreeOfParallelism to a number > 1 does this mean I need change the collection type to something like the ConcurrentStack for it to be thread safe?
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      1. USMaxim Gershkovich
    2. CODo you mean the collection in `storeBlock`? And where do you set `MaxDegreeOfParallelism`? If you set `MDOP` of `storeBlock` to > 1, then, yes, you would need to use some thread-safe collection there (or use locks). But if you set `MDOP` of some other block to > 1, it doesn't affect the parallelism of `storeBlock`, so you don't need to consider thread-safety.
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    3. COBoy this is going to make me upgrade to 2012! +1
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