How do you use SemaphoreSlim for Concurrency Throttling and protecting sensitive resources in async code?

Short answer

Use it as a non-blocking lock that allows a fixed number of threads into a critical section through await semaphore.WaitAsync(), and always release it in finally with semaphore.Release().

Explanation

C#'s lock is fully synchronous and cannot contain await. SemaphoreSlim solves this with the non-blocking WaitAsync(). With a capacity of (1, 1), it acts as an exclusive Async Lock for one request. With a capacity of (10, 10), it limits the critical section to ten concurrent calls to protect external servers from overload.

Code example

private static readonly SemaphoreSlim _gate = new(initialCount: 5, maxCount: 5);

public async Task<Data> FetchThrottledAsync() {
    await _gate.WaitAsync();
    try {
        return await CallExternalServiceAsync();
    } finally {
        _gate.Release(); // Always required in finally
    }
}

Common mistakes

• Forgetting _gate.Release() in a finally block, gradually exhausting Semaphore permits and hanging the system indefinitely.

Follow-up questions

• How does System.Threading.Channels work, and what does it offer for the Producer-Consumer pattern?
• How do you process asynchronous work in parallel with Parallel.ForEachAsync in .NET 6+?

Sources

• Microsoft Learn — Semaphore and SemaphoreSlim