Async when continuing on another thread

I know that a Task may continue the execution on another thread, proven by this code.

public async Task Test()
{
var id1 = System.Environment.CurrentManagedThreadId;
await Task.Delay(1000);
var id2 = System.Environment.CurrentManagedThreadId;
Console.Write($"First {id1} then {id2}");
}


I expect the framework to handle memory barrieres, so that id1 is visible when accessed in the last statement.

But what if you are using some kind of framework e.g. NHibernate where the ISession is not thread safe. Frameworks may even check that the thread ID is still the same. How does this mix?

When building website, I'll to use a IOC container with a nested container per request, but when then thread could change within the same request, wouldn't this lead to all kind of problems? ThreadStatic would not work as expected

1 Answer
1

Not being thread safe usually means don't use it from multiple threads at the same time, rather than don't use it from one thread and then from another thread later.

I don't know about NHibernate specifically, but if it's a problem, consider using EF Core.

In general with async, don't use variables attached to specific threads, like thread local or thread static.

However, local variables, class members, logical call contexts, HttpContext if you're on asp.net etc continue to work. If you do have something that will be lost after an await, you can usually just save it to a local variable first.

HttpContext

await

The default ConfigureAwait(true) also restores some context on the continuation, but it can be hard to know what is restored and what isn't. Later versions of .net do a better job of this, like restoring the culture so that resources keep working after an await.

ConfigureAwait(true)

await

There's a good article by Stephen Toub here:
https://blogs.msdn.microsoft.com/pfxteam/2012/06/15/executioncontext-vs-synchronizationcontext/

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