Why AsyncLocal is different from CallContext

Question

Running the bellow code you can see that there is a different between CallContext and AsyncLocal.

using System;
using System.Runtime.Remoting.Messaging;
using System.Threading;

namespace AsyncLocalIsDifferentThanCallContext
{
    class Program
    {
        public static AsyncLocal<int> AsyncLocal = new AsyncLocal<int>();

        public static int CallContextValue
        {
            get
            {
                var data = CallContext.GetData("CallContextValue");
                if (data == null)
                    return 0;
                return (int) data;
            }
            set { CallContext.SetData("CallContextValue", value); }
        }

        static void Main(string[] args)
        {
            AsyncLocal.Value = 1;
            CallContextValue = 1;
            new Thread(() =>
            {
                Console.WriteLine("From thread AsyncLocal: " + AsyncLocal.Value); // Should be 0 but is 1
                Console.WriteLine("From thread CallContext: " + CallContextValue); // Value is 0, as it should be
            }).Start();
            Console.WriteLine("Main AsyncLocal: " + AsyncLocal.Value);
            Console.WriteLine("Main CallContext: " + CallContextValue);
        }
    }
}

Can you explain why?

I expected the value of AsyncLocal to be unique per thread as the documentation says it should behave and as CallContext behaves.

Why would you expect different values? The object you are using is static so just 1 instance can exist of this object. You set the value of this object to a specific value. And then get and print this value, the only thing is you do the very same thing in different threads. I think your expectance is disturbed by using static variables. Read up on static variables here: http://stackoverflow.com/questions/10795502/what-is-the-use-of-static-variable-in-c-when-to-use-it-why-cant-i-declare-th — Totumus Maximus, Jan 24 '17 at 10:25
My understanding that AsyncLocal should replace CallContext on .net core (as there is no CallContext there) and it should hold a value unique to the thread. Also see the link to the documentation. — Fitzchak Yitzchaki, Jan 24 '17 at 11:03
There's no task in your example, which is thread based. `AsyncLocal` isn't relevant to anything you are doing here. — Jon Hanna, Jan 24 '17 at 13:53
Use LogicalGet/SetData() instead. Although it isn't exactly clear what the point might be. — Hans Passant, Jan 24 '17 at 14:01

score 1 · Accepted Answer · answered Jan 24 '17 at 13:35

1

You thinking of ThreadLocal? AsyncLocal can flow across threads as it says

Because the task-based asynchronous programming model tends to abstract the use of threads, AsyncLocal instances can be used to persist data across threads.

answered Jan 24 '17 at 13:35

Ben Adams

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Why AsyncLocal is different from CallContext

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