Gabe's Code
Stuff I've learned along the way
.NET: Don’t use ConfigureAwait(false)
Except for library code. I’ll get to that, but hear me out.
I’ve seen this advice all over:
As a general rule,
ConfigureAwait(false)should be used for every await unless the method needs its context.
It’s even what Stephen Cleary (a Microsoft MVP) says in his Async and Await article:
A good rule of thumb is to use ConfigureAwait(false) unless you know you do need the context.
Stephen definitely knows his stuff, and I agree that the advice is technically accurate, but I’ve always thought that this is bad advice for two reasons:
- Beginners, and
- Maintenance risk
Beginnners
First, it’s bad advice for beginners because synchronization context is a complex subject. If you start learning async/await by being told that “ConfigureAwait(false) should be used for every await unless the method needs its context”, but you don’t even know what “context” is and what it means to “need it”, then you don’t know when you shouldn’t use it, so you end up always using it. That means you can run into bugs that will be very difficult to figure out unless you happen to learn that, yes, you did actually need that “context” thing and this magical “ConfigureAwait” thing made you lose it. You can lose hours trying to figure that out.
For applications of any kind, I believe the advice really should be the opposite: Don’t use ConfigureAwait(false) at all, unless you know what it does and you have determined that you absolutely don’t need the context after that line. However…
Maintenance Risk
Determining you don’t need the context can be either simple, or quite complex depending on what methods are called after. But even then - and this is the second reason I disagree with that advice - just because you don’t need the context after that line right now, doesn’t mean some code won’t be added later that will use the context. You’ll have to hope that whoever makes that change knows what ConfigureAwait(false) does, sees it, and removes it. Using ConfigureAwait(false) everywhere creates a maintenance risk.
A Note on Synchronization Context
A couple simple examples of needing the synchronization context are:
- Changing a UI control in a Windows Forms or WPF project
- Accessing
HttpContext.Currentin an ASP.NET (not Core) project
Doing one of those things after the use of await with ConfigureAwait(false) might throw an exception. In the case of a Windows Forms or WPF app, a UI control can only be changed by the thread that created it. The synchronization context ensures that the continuation runs of the same thread. However, if you use ConfigureAwait(false), you tell it that you don’t care where it resumes, so it might resume on the same thread, or might not. Using ConfigureAwait(false) leaves it up to chance whether an exception is thrown, making your debugging more difficult.
If you’d like to read more about synchronization context, you can read the article What is Synchronization Context? by Hamid Mosalla. If you’d like to dig even deeper and see some of the code that goes behind a synchronization context, you can read the article Exploring the async/await State Machine – Synchronization Context by Vasil Kosturski.
Conclusion
This is what another Stephen, Stephen Toub (a Microsoft employee), recommends in the ConfigureAwait FAQ under the subheading “When should I use ConfigureAwait(false)?”:
When writing applications, you generally want the default behavior (which is why it is the default behavior). … This leads to the general guidance of: if you’re writing app-level code, do not use
ConfigureAwait(false)
In my own application code, I don’t bother trying to figure out where I can and can’t use it. I just ignore that ConfigureAwait exists. Sure, there can be a performance improvement by using it where you can, but I really doubt that it will be a noticeable difference to any human, even if it is measurable by a timer. I don’t believe the return on investment is positive.
Libraries
The only exception to this is when you’re writing libraries (code compiled into a DLL that will be used in other applications), as Stephen Toub points out in his article:
if you’re writing general-purpose library code, use
ConfigureAwait(false)
That’s for two reasons:
- A library has no idea about the context of the application it’s being used in, so it can’t use the context anyway, and
- If the person using the library decides to wait synchronously on your asynchronous library code, it could cause a deadlock that they cannot change because they can’t change your code. (ideally, they shouldn’t do that, but it can happen)
And keep in mind that it’s not always enough to use ConfigureAwait(false) on the first await and not the rest. Use it on every await in your library code. Stephen Toub’s article under the heading “Is it ok to use ConfigureAwait(false) only on the first await in my method and not on the rest?” says, in part:
If the
await task.ConfigureAwait(false)involves a task that’s already completed by the time it’s awaited (which is actually incredibly common), then theConfigureAwait(false)will be meaningless, as the thread continues to execute code in the method after this and still in the same context that was there previously.
It may seem arrogant of me to disagree with Stephen Cleary on this subject: he’s well respected. However, I first wrote this as an answer on Stack Overflow. Stephen Cleary commented on that post saying:
I believe I will update that
asyncblog post. For the last several years, I have also recommended only usingConfigureAwait(false)for library code.
So it’s good to know we agree on that now.
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