AsyncCallback Delegate

References a method to be called when a corresponding asynchronous operation completes.

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

public delegate void AsyncCallback(
	IAsyncResult ar


Type: System.IAsyncResult

The result of the asynchronous operation.

Use an AsyncCallback delegate to process the results of an asynchronous operation in a separate thread. The AsyncCallback delegate represents a callback method that is called when the asynchronous operation completes. The callback method takes an IAsyncResult parameter, which is subsequently used to obtain the results of the asynchronous operation.

For more information about asynchronous programming, see Using an AsyncCallback Delegate to End an Asynchronous Operation and Using an AsyncCallback Delegate and State Object in Asynchronous Programming Design Patterns.

The following code example demonstrates using asynchronous methods in the Dns class to retrieve Domain Name System (DNS) information for user-specified computers. This example creates an AsyncCallback delegate that references the ProcessDnsInformation method. This method is called once for each asynchronous request for DNS information.

The following example demonstrates using asynchronous methods to
get Domain Name System information for the specified host computers.
This example uses a delegate to obtain the results of each asynchronous 

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Collections.Specialized;
using System.Collections;

namespace Examples.AdvancedProgramming.AsynchronousOperations
    public class UseDelegateForAsyncCallback
        static int requestCounter;
        static ArrayList hostData = new ArrayList();
        static StringCollection hostNames = new StringCollection();
        static void UpdateUserInterface()
            // Print a message to indicate that the application 
            // is still working on the remaining requests.
            Console.WriteLine("{0} requests remaining.", requestCounter);
        public static void Main()
            // Create the delegate that will process the results of the  
            // asynchronous request.
            AsyncCallback callBack = new AsyncCallback(ProcessDnsInformation);
            string host;
                Console.Write(" Enter the name of a host computer or <enter> to finish: ");
                host = Console.ReadLine();
                if (host.Length > 0)
                    // Increment the request counter in a thread safe manner.
                    Interlocked.Increment(ref requestCounter);
                    // Start the asynchronous request for DNS information.
                    Dns.BeginGetHostEntry(host, callBack, host);
            } while (host.Length > 0);
            // The user has entered all of the host names for lookup. 
            // Now wait until the threads complete. 
            while (requestCounter > 0)
            // Display the results. 
            for (int i = 0; i< hostNames.Count; i++)
                object data = hostData [i];
                string message = data as string;
                // A SocketException was thrown. 
                if (message != null)
                    Console.WriteLine("Request for {0} returned message: {1}", 
                        hostNames[i], message);
                // Get the results.
                IPHostEntry h = (IPHostEntry) data;
                string[] aliases = h.Aliases;
                IPAddress[] addresses = h.AddressList;
                if (aliases.Length > 0)
                    Console.WriteLine("Aliases for {0}", hostNames[i]);
                    for (int j = 0; j < aliases.Length; j++)
                        Console.WriteLine("{0}", aliases[j]);
                if (addresses.Length > 0)
                    Console.WriteLine("Addresses for {0}", hostNames[i]);
                    for (int k = 0; k < addresses.Length; k++)

        // The following method is called when each asynchronous operation completes. 
        static void ProcessDnsInformation(IAsyncResult result)
            string hostName = (string) result.AsyncState;
                // Get the results.
                IPHostEntry host = Dns.EndGetHostEntry(result);
            // Store the exception message. 
            catch (SocketException e)
                // Decrement the request counter in a thread-safe manner.
                Interlocked.Decrement(ref requestCounter);

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0