Utilisation d'un délégué AsyncCallback pour terminer une opération asynchrone
Les applications qui peuvent continuer à effectuer d’autres tâches en attendant les résultats d’une opération asynchrone ne doivent pas se bloquer en attendant que cette opération se termine. Pour poursuivre l’exécution des instructions tout en attendant la fin d’une opération asynchrone, utilisez l’une des options suivantes :
Utilisez un délégué AsyncCallback pour traiter les résultats de l’opération asynchrone dans un thread distinct. Cette approche est illustrée dans cette rubrique.
Utilisez la propriété IsCompleted du IAsyncResult retourné par la méthode BeginOperationName de l’opération asynchrone pour déterminer si cette dernière est terminée. Vous trouverez un exemple illustrant cette approche sur la page Demander l’état d’une opération asynchrone.
Exemple
L’exemple de code suivant montre comment utiliser des méthodes asynchrones dans la classe Dns afin de récupérer les informations DNS (Domain Name System) pour des ordinateurs spécifiés par l’utilisateur. Cet exemple crée un délégué AsyncCallback qui référence la méthode ProcessDnsInformation, appelée une fois par demande asynchrone d’informations DNS.
Vous remarquerez que l’hôte spécifié par l’utilisateur est transmis au paramètre BeginGetHostByNameObject. Vous trouverez un exemple illustrant la définition et l’utilisation d’un objet d’état plus complexe sur la page Utiliser un délégué AsyncCallback et un objet d’état.
/*
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.
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
Voir aussi
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