How Can TPL Dataflow Limit Concurrent Asynchronous Tasks?

Use TPL Dataflow to limit the number of concurrent asynchronous tasks

This article explores how to use TPL Dataflow to efficiently limit the number of concurrent asynchronous tasks while taking full advantage of multi-threading. Compared to the BlockingCollection approach using SemaphoreSlim and ThrottleTasksAsync, TPL Dataflow provides a cleaner solution.

By configuring the TransformBlock attribute of MaxDegreeOfParallelism, we can directly specify the maximum number of concurrently executed tasks. Add URLs to TransformBlock and it will handle them asynchronously. Once the processing is complete, the results are available.

<code class="language-csharp">var downloader = new TransformBlock<string, HttpResponse>(
    url => Download(url),
    new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 50 }
);

var buffer = new BufferBlock<HttpResponse>();
downloader.LinkTo(buffer);

foreach (var url in urls)
    downloader.Post(url);

downloader.Complete();
await downloader.Completion;

IList<HttpResponse> responses;
if (buffer.TryReceiveAll(out responses))
{
    // 处理结果
}</code>

It should be noted that TransformBlock buffers input and output. Therefore, we connect it with BufferBlock to prevent TransformBlock from blocking until all output items are consumed. The BufferBlock.TryReceiveAll method is used to retrieve all results after TransformBlock has completed.

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