How to Limit Parallelism in Parallel.ForEach() for Throttled Operations?

Managing Concurrent Tasks in Parallel.ForEach() with Resource Constraints

When working with parallel loops where the individual operations are subject to resource limitations (like network bandwidth or API rate limits), controlling the level of concurrency is crucial. The Parallel.ForEach() method, in conjunction with the ParallelOptions class, offers a straightforward solution.

The ParallelOptions class provides the MaxDegreeOfParallelism property, which sets the maximum number of threads allowed to execute concurrently. This effectively limits the number of simultaneous operations, preventing resource overload.

Consider a web scraping scenario where network bandwidth restricts concurrent downloads to, say, four at a time. The following code snippet demonstrates how to enforce this limitation:

<code class="language-csharp">Parallel.ForEach(
    listOfWebpages,
    new ParallelOptions { MaxDegreeOfParallelism = 4 },
    webpage => { DownloadWebpage(webpage); }
);</code>

Setting MaxDegreeOfParallelism to 4 ensures that a maximum of four threads download webpages concurrently, thus respecting the bandwidth constraint.

Further Reading:

• MSDN: Parallel.ForEach
• MSDN: ParallelOptions.MaxDegreeOfParallelism

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