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IAsyncResult.AsyncWaitHandle Property

Gets a WaitHandle that is used to wait for an asynchronous operation to complete.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
WaitHandle AsyncWaitHandle { get; }

Property Value

Type: System.Threading.WaitHandle
A WaitHandle that is used to wait for an asynchronous operation to complete.

The return value allows the client to wait for an asynchronous operation to complete instead of polling IsCompleted until the operation concludes. The return value can be used to perform a WaitOne, WaitAny, or WaitAll operation.

The common language runtime supplies a number of waitable objects, such as ManualResetEvent, AutoResetEvent, and Mutex, all of which mirror Win32 synchronization primitives.

Notes to Implementers

The object that implements IAsyncResult does not need to create the WaitHandle until the AsyncWaitHandle property is read. It is the choice of the IAsyncResult implementer. However, if the implementer creates AsyncWaitHandle, it is the responsibility of the implementer to signal the WaitHandle that will terminate the wait at the appropriate time. For example, System.Runtime.Remoting.Messaging.AsyncResult terminates the wait on behalf of the caller when an asynchronously invoked method returns. Once created, AsyncWaitHandle should be kept alive until the user calls the method that concludes the asynchronous operation. At that time the object behind AsyncWaitHandle can be discarded.

Notes to Callers

Clients that wait for the operation to complete (as opposed to polling) use this property to obtain a synchronization object to wait on.

NoteNote

When you use the BeginInvoke method of a delegate to call a method asynchronously and obtain a wait handle from the resulting IAsyncResult, we recommend that you close the wait handle as soon as you are finished using it, by calling the WaitHandle.Close method. If you simply release all references to the wait handle, system resources are freed when garbage collection reclaims the wait handle, but garbage collection works more efficiently when disposable objects are explicitly closed or disposed. For more information, see the AsyncResult.AsyncWaitHandle property.

The following example demonstrates how to use the AsyncWaitHandle property to get a WaitHandle, and how to wait for an asynchronous call on a delegate. The WaitHandle is signaled when the asynchronous call completes, and you can wait for it by calling the WaitOne method.

The example consists of two classes: the class that contains the method that is called asynchronously, and the class that contains the Main method that makes the call.

For more information and more examples of calling methods asynchronously by using delegates, see Calling Synchronous Methods Asynchronously.

using System;
using System.Threading; 

namespace Examples.AdvancedProgramming.AsynchronousOperations
{
    public class AsyncDemo 
    {
        // The method to be executed asynchronously. 
        public string TestMethod(int callDuration, out int threadId) 
        {
            Console.WriteLine("Test method begins.");
            Thread.Sleep(callDuration);
            threadId = Thread.CurrentThread.ManagedThreadId;
            return String.Format("My call time was {0}.", callDuration.ToString());
        }
    }
    // The delegate must have the same signature as the method 
    // it will call asynchronously. 
    public delegate string AsyncMethodCaller(int callDuration, out int threadId);
}


...


using System;
using System.Threading;

namespace Examples.AdvancedProgramming.AsynchronousOperations
{
    public class AsyncMain 
    {
        static void Main() 
        {
            // The asynchronous method puts the thread id here. 
            int threadId;

            // Create an instance of the test class.
            AsyncDemo ad = new AsyncDemo();

            // Create the delegate.
            AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod);

            // Initiate the asychronous call.
            IAsyncResult result = caller.BeginInvoke(3000, 
                out threadId, null, null);

            Thread.Sleep(0);
            Console.WriteLine("Main thread {0} does some work.",
                Thread.CurrentThread.ManagedThreadId);

            // Wait for the WaitHandle to become signaled.
            result.AsyncWaitHandle.WaitOne();

            // Perform additional processing here. 
            // Call EndInvoke to retrieve the results. 
            string returnValue = caller.EndInvoke(out threadId, result);

            // Close the wait handle.
            result.AsyncWaitHandle.Close();

            Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".",
                threadId, returnValue);
        }
    }
}

/* This example produces output similar to the following:

Main thread 1 does some work.
Test method begins.
The call executed on thread 3, with return value "My call time was 3000.".
 */

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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