How Can I Throttle Asynchronous Tasks in C#?

Limit asynchronous tasks

In asynchronous programming, it is often necessary to limit the number of concurrent tasks that can be executed at any given time to achieve throttling. This is especially useful when making multiple API calls or processing large data sets.

ThrottleTasksAsync method

The provided ThrottleTasksAsync extension method solves this situation. It accepts an enumeration of projects, a maximum number of concurrent tasks, and a task creation function. It utilizes BlockingCollection and SemaphoreSlim to enforce throttling.

<code class="language-c#">public static async Task<result_t> ThrottleTasksAsync<enumerable_t, result_t>(
    this IEnumerable<enumerable_t> enumerable,
    int maxConcurrentTasks,
    int maxDegreeOfParallelism,
    Func<enumerable_t, Task> taskToRun)
{
    // ...
}</code>

This method runs the throttle on a separate thread, using SemaphoreSlim to control the number of concurrent tasks. It also utilizes Parallel.ForEach to achieve parallelization, within a specified degree of parallelism.

TPL data flow as an alternative

While the ThrottleTasksAsync method provides a solution, TPL data flow provides a more elegant approach. Specifically, the TransformBlock classes can be used to define maximum parallelism limits for task execution.

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

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

By linking a TransformBlock to a BufferBlock, block completion is not blocked by consumer availability. This allows the buffer chunk to collect HttpResponse objects when generated, and the program can wait for the downloader to complete before processing the response.

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