AsyncCallback Delegate

Definition

Namespace:

System

Assembly:

System.Runtime.dll

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

public delegate void AsyncCallback(IAsyncResult ^ ar);

public delegate void AsyncCallback(IAsyncResult ar);

type AsyncCallback = delegate of IAsyncResult -> unit

Public Delegate Sub AsyncCallback(ar As IAsyncResult)

Parameters

ar

IAsyncResult

The result of the asynchronous operation.

Examples

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);
            }
        }
    }
}

// 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.

open System
open System.Net
open System.Net.Sockets
open System.Threading
open System.Collections.Specialized

let mutable requestCounter = 0
let hostData = ResizeArray<Result<IPHostEntry, string>>()
let hostNames = StringCollection()

let updateUserInterface () =
    // Print a message to indicate that the application
    // is still working on the remaining requests.
    printfn $"{requestCounter} requests remaining."

// The following function is called when each asynchronous operation completes.
let processDnsInformation (result: IAsyncResult) =
    string result.AsyncState
    |> hostNames.Add 
    |> ignore
    try
        try
            // Get the results.
            Dns.EndGetHostEntry result
            |> Ok
            |> hostData.Add
        // Store the exception message.
        with :? SocketException as e ->
            Error e.Message
            |> hostData.Add 
    finally
        // Decrement the request counter in a thread-safe manner.
        Interlocked.Decrement &requestCounter 
        |> ignore

// Create the delegate that will process the results of the asynchronous request.
let callBack = AsyncCallback processDnsInformation

let mutable host = " "
while host.Length > 0 do
    printf " Enter the name of a host computer or <enter> to finish: "
    host <- stdin.ReadLine()
    if host.Length > 0 then
        // Increment the request counter in a thread safe manner.
        Interlocked.Increment &requestCounter |> ignore
        // Start the asynchronous request for DNS information.
        Dns.BeginGetHostEntry(host, callBack, host) |> ignore

// The user has entered all of the host names for lookup.
// Now wait until the threads complete.
while requestCounter > 0 do
    updateUserInterface ()

// Display the results.
for i = 0 to hostNames.Count - 1 do
    match hostData[i] with 
    | Error message ->
        // A SocketException was thrown.
        printfn $"Request for {hostNames[i]} returned message: {message}"
    | Ok entry ->    
        // Get the results.
        let aliases = entry.Aliases
        let addresses = entry.AddressList
        if aliases.Length > 0 then
            printfn $"Aliases for {hostNames[i]}"
            for alias in aliases do
                printfn $"{alias}"

        if addresses.Length > 0 then
            printfn $"Addresses for {hostNames[i]}"
            for address in addresses do
                printfn $"{address}"

'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.

Imports System.Net
Imports System.Net.Sockets
Imports System.Threading
Imports System.Collections.Specialized
Imports System.Collections

Namespace Examples.AdvancedProgramming.AsynchronousOperations

    Public Class UseDelegateForAsyncCallback
    
        Dim Shared requestCounter as Integer
        Dim Shared hostData as ArrayList = new ArrayList()
        Dim Shared hostNames as StringCollection = new StringCollection()
        Shared Sub UpdateUserInterface()
        
            ' Print a message to indicate that the application
            ' is still working on the remaining requests.
            Console.WriteLine("{0} requests remaining.", requestCounter)
        End Sub
        Public Shared Sub Main()
            ' Create the delegate that will process the results of the 
            ' asynchronous request.
            Dim callBack as AsyncCallback
            Dim host as string
            Dim i, j, k as Integer
            callback = AddressOf ProcessDnsInformation
            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(requestCounter)
                    ' Start the asynchronous request for DNS information.
                    Dns.BeginGetHostEntry(host, callBack, host)
                End If
            Loop While (host.Length > 0)
            
            ' The user has entered all of the host names for lookup.
            ' Now wait until the threads complete.
            Do While requestCounter > 0
            
                UpdateUserInterface()
            Loop
            
            ' Display the results.
            For i = 0 To hostNames.Count -1
                Dim dataObject as Object = hostData (i)
                Dim message as String 
                
                ' Was a SocketException was thrown?
                If TypeOf dataObject is String
                    message = CType(dataObject, String)
                    Console.WriteLine("Request for {0} returned message: {1}", _ 
                        hostNames(i), message)
                Else
                    ' Get the results.
                    Dim h as IPHostEntry = CType(dataObject, IPHostEntry) 
                    Dim aliases() as String = h.Aliases
                    Dim addresses() as IPAddress = h.AddressList
                    If aliases.Length > 0
                        Console.WriteLine("Aliases for 0}", hostNames(i))
                        For j = 0 To aliases.Length -1
                            Console.WriteLine("{0}", aliases(j))
                        Next j
                    End If
                    If addresses.Length > 0
                        Console.WriteLine("Addresses for {0}", hostNames(i))
                        For k = 0 To addresses.Length -1
                            Console.WriteLine("{0}",addresses(k).ToString())
                        Next k
                    End If
                End If
            Next i
       End Sub

        ' The following method is called when each asynchronous operation completes.
        Shared Sub ProcessDnsInformation(result as IAsyncResult)
        
            Dim hostName as String = CType(result.AsyncState, String)
            hostNames.Add(hostName)
            Try 
                ' Get the results.
                Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
                hostData.Add(host)
            ' Store the exception message.
            Catch e as SocketException
                hostData.Add(e.Message)
            Finally 
                ' Decrement the request counter in a thread-safe manner.
                Interlocked.Decrement(requestCounter)
            End Try
        End Sub
    End Class
End Namespace

Remarks

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).

Extension Methods

GetMethodInfo(Delegate)

Gets an object that represents the method represented by the specified delegate.

See also

• IAsyncResult
• Using an AsyncCallback Delegate to End an Asynchronous Operation
• Using an AsyncCallback Delegate and State Object
• Calling Synchronous Methods Asynchronously
• Event-based Asynchronous Pattern (EAP)
• Handling and Raising Events