Docs: Not clear of SemaphoreSlim usage with ConcurrentQueue

[IndikaUdagedara]

ConcurrentQueue handles all synchronization internally, so it's not clear why SemaphoreSlim is used here. Can you please add a sentence describing why it is required?

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• ID: c3460b0d-9c60-3a06-ac63-18cf990e53c3
• Version Independent ID: b2dece1f-15b8-03af-6d52-7cc070432b4b
• Content: Background tasks with hosted services in ASP.NET Core
• Content Source: aspnetcore/fundamentals/host/hosted-services.md
• Product: aspnet-core
• GitHub Login: @guardrex
• Microsoft Alias: riande

[guardrex]

I can't say outside of their goal to base it on QueueBackgroundWorkItem. SemaphoreSlim is used in their BackgroundTaskQueue example. They plan to port QueueBackgroundWorkItem to Core for 3.0. The topic sample was based on a prototype, and there's an issue for that, as well. I recommend that you ask over on the aspnet/Hosting issue (it's still open for discussion), and one of the engineers will probably answer.

https://github.com/aspnet/Hosting/issues/1280 You should probably ask here.
#3352 You can see the prototype for the topic sample here.

[pr3sto]

TryDequeue in ConcurrentQueue returns false if there are no items in the queue. So SemaphoreSlim is used here to block executing of ExecuteAsync method until some workItem is pushed in QueueBackgroundWorkItem method.
Probably you can use BlockingCollection here instead.
Anyway it is just an example, as @guardrex mentioned - it is taken from aspnet/Hosting#1280

[Rick-Anderson]

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