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WHY DOES ASYNCLOCAL<T> RETURN DIFFERENT RESULTS WHEN CODE IS REFACTORED
SLIGHTLY?

Follow this link AsyncLocal Class on MSDN

> AsyncLocal<T> represents ambient data that is local to a given asynchronous
> control flow, such as an asynchronous method

It means that your code uses different values when it's accesses from another
async method such as WrapperAsync and your main thread contains another value

00:10 / 00:20

T-Mobile G2 Unboxing


[UPDATE]
Not obvious thing to understand, but here is explanation. Control Flow in Async
Programs. This is how your thread is changed when you do not expect this.


This is how Control Flow working with async

public class Program
{
    private static readonly AsyncLocal<string> AsyncLocalContext = new AsyncLocal<string>();

    public static void Main(string[] args)
    {
        AsyncLocalContext.Value = "No surprise";
        WrapperAsync("surprise!");
        Console.WriteLine("Main: " + AsyncLocalContext.Value);
    }

    private static async void WrapperAsync(string text)
    {
        Console.WriteLine("WrapperAsync before: " + AsyncLocalContext.Value);
        AsyncLocalContext.Value = text;
        Console.WriteLine("WrapperAsync after: " + AsyncLocalContext.Value);
    }
}


Output is:

WrapperAsync before: No surprise
WrapperAsync after: surprise!
Main: No surprise


[/UPDATE]


AsyncLocal<T> is ambient data stored on the ExecutionContext of the current
thread. ExecutionContext is flowed across threads automagically in async/await
call chains (see Stephen Toub's blog for details). When the app starts, the
default ExecutionContext is used, but once data is stored via
AsyncLocal<T>.Value, a new ExecutionContext is created for the current async
call chain (see here) and the ambient data is added to it. This new context is
propagated to downstream calls.

Stephen Cleary discusses this behavior here (scroll down to the AsyncLocal
section) and makes the point:

> [AsyncLocal] provides a way for contextual information to flow “down”
> asynchronous calls. Note that the value does not flow “up”.

This is why AsyncLocal<T> updates down the call chain are not reflected in
upstream methods.


TAGS:

C#

.NET

ASYNC AWAIT


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