How to wait Tasks when unable to await?

[seppo498573908457]

Eventually we run into a scenario, where we cannot go async/await for the full call stack. It's unclear what to do in such case, please help by extending this documentation.

What we run into:

• We need to call a method that returns a Task or Task<T>, but we cannot use await keyword. The code cannot be changed to accommodate async/await.
• At worst, the request starts with an async/await method, but calls synchronized functions and they call async/await methods again, all in the same stack.
• There are number of options, all of which are prone to deadlocks and their use is discouraged.
  • Wait() blocks and deadlocks. Exceptions are ugly.
  • GetAwaiter().GetResult() is better for exceptions, but still blocks.
  • RunSynchronously() is for special uses only.
  • ConfigureAwait(false) requires await and also all decendant awaits need to call ConfigureAwait(false), not acceptable.
• Nonetheless, we still need to somehow wait for the task and get a result.
  • Do we need to use something like var result = Task.Run(async () => await task).GetAwaiter().GetResult();

The internet is not helpful in giving a silver bullet. We need working code, not fancy theories.

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Document Details

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• ID: d885f1eb-2b37-693b-c91f-74b748c9424a
• Version Independent ID: 77bdee53-c2a0-e16f-c93a-3ca0afae0b93
• Content: Asynchronous programming - C#
• Content Source: docs/csharp/asynchronous-programming/async-scenarios.md
• Product: dotnet-csharp
• Technology: csharp-async
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

Adding @stephentoub

GetAwaiter().GetResult() is the best of the options I know of.

[stephentoub]

The internet is not helpful in giving a silver bullet

There is no silver bullet. The definition of the problem you've outlined is you need to invoke an asynchronous operation and you need that operation to complete before you allow the current thread to make forward progress. That necessarily means blocking the current thread.

https://devblogs.microsoft.com/pfxteam/should-i-expose-synchronous-wrappers-for-asynchronous-methods/

[seppo498573908457]

Thanks for the quick replies. I've read the linked blog post and many others, they do well advertising the parallel programming.

Just to be clear, the asyncness has to eventually be waited somewhere, so blocking may be unavoided and that's fine. But the real issue here is minimizing "downtime" (like when UI responsiveness is affected) and avoiding deadlocks without getting unnecessary performance hits. On the internet there are many sources that contradict each other, some are outdated. It would be easier for everyone, if there was a clear documentation about how to properly wait and sync with tasks when awaiting is not an option. Better yet, that would rank first on search results, like most on MSDN.

[BillWagner]

I'll leave this in the backlog as an enhancement.

I think the best fix is to link to the various blog posts that point out the pitfalls with this scenario, and how to minimize the risk.

I'd prefer keeping those actions a couple clicks away because there isn't a general solution that doesn't have risks.