AsyncLocal<T> Class

.NET Framework (current version)

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Represents ambient data that is local to a given asynchronous control flow, such as an asynchronous method.

Namespace:   System.Threading
Assembly:  mscorlib (in mscorlib.dll)


public sealed class AsyncLocal<T>

Type Parameters


The type of the ambient data.


Instantiates an AsyncLocal<T> instance that does not receive change notifications.


Instantiates an AsyncLocal<T> local instance that receives change notifications.


Gets or sets the value of the ambient data.


Determines whether the specified object is equal to the current object.(Inherited from Object.)


Serves as the default hash function. (Inherited from Object.)


Gets the Type of the current instance.(Inherited from Object.)


Returns a string that represents the current object.(Inherited from Object.)

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

The AsyncLocal<T> class also provides optional notifications when the value associated with the current thread changes, either because it was explicitly changed by setting the Value property, or implicitly changed when the thread encountered an await or other context transition.

The following example uses the AsyncLocal<T> class to persist a string value across an asynchronous flow. It also contrasts the use of AsyncLocal<T> with ThreadLocal<T>.

using System;
using System.Threading;
using System.Threading.Tasks;

class Example
    static AsyncLocal<string> _asyncLocalString = new AsyncLocal<string>();

    static ThreadLocal<string> _threadLocalString = new ThreadLocal<string>();

    static async Task AsyncMethodA()
        // Start multiple async method calls, with different AsyncLocal values.
        // We also set ThreadLocal values, to demonstrate how the two mechanisms differ.
        _asyncLocalString.Value = "Value 1";
        _threadLocalString.Value = "Value 1";
        var t1 = AsyncMethodB("Value 1");

        _asyncLocalString.Value = "Value 2";
        _threadLocalString.Value = "Value 2";
        var t2 = AsyncMethodB("Value 2");

        // Await both calls
        await t1;
        await t2;

    static async Task AsyncMethodB(string expectedValue)
        Console.WriteLine("Entering AsyncMethodB.");
        Console.WriteLine("   Expected '{0}', AsyncLocal value is '{1}', ThreadLocal value is '{2}'", 
                          expectedValue, _asyncLocalString.Value, _threadLocalString.Value);
        await Task.Delay(100);
        Console.WriteLine("Exiting AsyncMethodB.");
        Console.WriteLine("   Expected '{0}', got '{1}', ThreadLocal value is '{2}'", 
                          expectedValue, _asyncLocalString.Value, _threadLocalString.Value);

    static void Main(string[] args)
// The example displays the following output:
//   Entering AsyncMethodB.
//      Expected 'Value 1', AsyncLocal value is 'Value 1', ThreadLocal value is 'Value 1'
//   Entering AsyncMethodB.
//      Expected 'Value 2', AsyncLocal value is 'Value 2', ThreadLocal value is 'Value 2'
//   Exiting AsyncMethodB.
//      Expected 'Value 2', got 'Value 2', ThreadLocal value is ''
//   Exiting AsyncMethodB.
//      Expected 'Value 1', got 'Value 1', ThreadLocal value is ''

Universal Windows Platform
Available since 10
.NET Framework
Available since 4.6

Any public static ( Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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