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AsyncCallback Class

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

Namespace:  System
Assemblies:   System.Runtime (in System.Runtime.dll)
  mscorlib (in mscorlib.dll)

public sealed class AsyncCallback : MulticastDelegate

The AsyncCallback type exposes the following members.

  NameDescription
Public methodSupported in .NET for Windows Store appsAsyncCallback
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  NameDescription
Public propertySupported in .NET for Windows Store appsTargetGets the class instance on which the current delegate invokes the instance method. (Inherited from Delegate.)
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  NameDescription
Public methodSupported in .NET for Windows Store appsBeginInvoke
Public methodSupported in .NET for Windows Store appsDynamicInvokeDynamically invokes (late-bound) the method represented by the current delegate. (Inherited from Delegate.)
Public methodSupported in .NET for Windows Store appsEndInvoke
Public methodSupported in .NET for Windows Store appsEqualsDetermines whether this multicast delegate and the specified object are equal. (Inherited from MulticastDelegate.)
Public methodSupported in .NET for Windows Store appsGetHashCodeReturns the hash code for this instance. (Inherited from MulticastDelegate.)
Public methodSupported in .NET for Windows Store appsGetInvocationListReturns the invocation list of this multicast delegate, in invocation order. (Inherited from MulticastDelegate.)
Public methodSupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported in .NET for Windows Store appsInvoke
Public methodSupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Public Extension MethodSupported in .NET for Windows Store appsGetMethodInfoGets an object that represents the method represented by the specified delegate. (Defined by RuntimeReflectionExtensions.)
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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 Event-based Asynchronous Pattern (EAP).

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 
operation.
*/ 

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;
            do
            {
                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)
            {
                UpdateUserInterface();
            }
            // 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);
                    continue;
                }
                // 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++)
                    {
                        Console.WriteLine("{0}",addresses[k].ToString());
                    }
                }
            }
       }

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

.NET Framework

Supported in: 4.5.3, 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

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

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.

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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