AsyncCallback Delegate

Definition

public delegate void AsyncCallback(IAsyncResult ^ ar);

public delegate void AsyncCallback(IAsyncResult ar);

[System.Serializable]
public delegate void AsyncCallback(IAsyncResult ar);

[System.Serializable]
[System.Runtime.InteropServices.ComVisible(true)]
public delegate void AsyncCallback(IAsyncResult ar);

type AsyncCallback = delegate of IAsyncResult -> unit

[<System.Serializable>]
type AsyncCallback = delegate of IAsyncResult -> unit

[<System.Serializable>]
[<System.Runtime.InteropServices.ComVisible(true)>]
type AsyncCallback = delegate of IAsyncResult -> unit

Public Delegate Sub AsyncCallback(ar As IAsyncResult)

Parameters

Attributes

SerializableAttribute ComVisibleAttribute

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).

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