What does the Asp.net Identity task extension WithCurrentCulture() do and why?

Question

I am trying to understand the inner workings of the Asp.net Identity system and have seen that all of the async calls it seems to make use of the WithCurrentCulture() task extension. I have looked at the source code for that extension and I cannot figure out what it is doing or why.

From documentation

Configures an awaiter used to await this Task to avoid marshalling the continuation back to the original context, but preserve the current culture and UI culture.

Can someone please shed some light on it?

Answer 1

It executes the task continuation with Thread.CurrentCulture and Thread.CurrentUICulture set to the same values as they are on the thread where it was called from.

By default, the culture of new threads is set to the Windows system culture. For example, say you are awaiting a task, and you have explicitly set Thread.CurrentCulture to a different culture. Now when you await a task with ConfigureAwait(false), the code after the await might execute on a different thread than the code before the await. Since it is a different thread, it may have CurrentCulture set back to the system culture.

Thread.CurrentThread.CurrentCulture = someCulture;
await SomeTask().ConfigureAwait(false);

bool equal = Thread.CurrentThread.CurrentCulture == someCulture; // Might be false!

If you want to have the culture stay the same after the await, use WithCurrentCulture(). It does the same thing as ConfigureAwait(false), but also keeps the culture from the original thread.

Thread.CurrentThread.CurrentCulture = someCulture;
await SomeTask().WithCurrentCulture();

bool equal = Thread.CurrentThread.CurrentCulture == someCulture; // Will be true

Note that if you OMIT ConfigureAwait(false) and WithCurrentCulture(), the code after the await will run on the same thread as before, so you don't have to worry about any of this. WithCurrentCulture() only applies in the case where you want to optimize your code by avoiding an extra context switch.

Side note: you can also control the default culture for new threads by setting CultureInfo.DefaultThreadCurrentCulture and CultureInfo.DefaultThreadCurrentUICulture.