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IAsyncResult Interface


Represents the status of an asynchronous operation.

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

public interface IAsyncResult


Gets a user-defined object that qualifies or contains information about an asynchronous operation.


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


Gets a value that indicates whether the asynchronous operation completed synchronously.


Gets a value that indicates whether the asynchronous operation has completed.

The IAsyncResult interface is implemented by classes containing methods that can operate asynchronously. It is the return type of methods that initiate an asynchronous operation, such as FileStream.BeginRead, and it is passed to methods that conclude an asynchronous operation, such as FileStream.EndRead. IAsyncResult objects are also passed to methods invoked by AsyncCallback delegates when an asynchronous operation completes.

An object that supports the IAsyncResult interface stores state information for an asynchronous operation and provides a synchronization object to allow threads to be signaled when the operation completes.


The AsyncResult class is the implementation of IAsyncResult that is returned by the BeginInvoke method when you use a delegate to call a method asynchronously.

For a detailed description of how the IAsyncResult interface is used, see the Calling Synchronous Methods Asynchronously topic.

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.");
            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);

            Console.WriteLine("Main thread {0} does some work.",

            // Wait for the WaitHandle to become signaled.

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

            // Close the wait handle.

            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.".

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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