Utilisation d'un délégué AsyncCallback et objet d'état
Mise à jour : novembre 2007
Les applications qui peuvent effectuer un autre travail en attendant les résultats d'une opération asynchrone ne doivent pas bloquer l'attente jusqu'à la fin de l'opération. Utilisez l'une des options suivantes pour continuer à exécuter les instructions en attendant la fin d'une opération asynchrone :
Utilisez un délégué AsyncCallback pour traiter les résultats de l'opération asynchrone dans un thread séparé. Cette approche est présentée dans cette rubrique.
Utilisez la propriété IsCompleted du IAsyncResult retourné par la méthode BeginNomOpération de l'opération asynchrone pour déterminer si l'opération est terminée. Pour obtenir un exemple illustrant cette approche, consultez Interrogation de l'état d'une opération asynchrone.
Exemple
L'exemple de code suivant illustre l'utilisation de méthodes asynchrones dans la classe Dns pour récupérer les informations du système de noms de domaines (DNS, Domain Name System) pour des ordinateurs spécifiés par l'utilisateur. Cet exemple définit et utilise la classe HostRequest pour stocker des informations d'état. Un objet HostRequest est créé pour chaque nom d'ordinateur entré par l'utilisateur. Cet objet est passé à la méthode BeginGetHostByName. La méthode ProcessDnsInformation est appelée chaque fois qu'une demande est terminée. L'objet HostRequest est récupéré à l'aide de la propriété AsyncState. La méthode ProcessDnsInformation utilise l'objet HostRequest pour stocker le IPHostEntry retourné par la demande ou un SocketException levé par la demande. Lorsque toutes les demandes sont terminées, l'application parcourt les objets HostRequest et affiche les informations DNS ou le message d'erreur SocketException.
' The following example demonstrates using asynchronous methods to
' get Domain Name System information for the specified host computer.
Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Threading
Imports System.Collections
Namespace Examples.AdvancedProgramming.AsynchronousOperations
' Create a state object that holds each requested host name,
' an associated IPHostEntry object or a SocketException.
Public Class HostRequest
' Stores the requested host name.
Dim hostNameValue as string
' Stores any SocketException returned by the Dns EndGetHostByName method.
Dim e as SocketException
' Stores an IPHostEntry returned by the Dns EndGetHostByName method.
Dim entry as IPHostEntry
Public Sub New(name as String)
hostNameValue = name
End Sub
ReadOnly Public Property HostName as String
Get
return hostNameValue
End Get
End Property
Public Property ExceptionObject as SocketException
Get
return e
End Get
Set
e = value
End Set
End Property
Public Property HostEntry as IPHostEntry
Get
return entry
End Get
Set
entry = value
End Set
End Property
End Class
Public Class UseDelegateAndStateForAsyncCallback
' The number of pending requests.
Dim Shared requestCounter as Integer
Dim Shared hostData as ArrayList = new ArrayList()
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 = AddressOf ProcessDnsInformation
Dim host as string
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)
' Create and store the state object for this request.
Dim request as HostRequest = new HostRequest(host)
hostData.Add(request)
' Start the asynchronous request for DNS information.
Dns.BeginGetHostEntry(host, callBack, request)
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 Each r as HostRequest In hostData
If IsNothing(r.ExceptionObject) = False
Console.WriteLine( _
"Request for host {0} returned the following error: {1}.", _
r.HostName, r.ExceptionObject.Message)
Else
' Get the results.
Dim h as IPHostEntry = r.HostEntry
Dim aliases() as String = h.Aliases
Dim addresses() as IPAddress = h.AddressList
Dim j, k as Integer
If aliases.Length > 0
Console.WriteLine("Aliases for {0}", r.HostName)
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}", r.HostName)
For k = 0 To addresses.Length -1
Console.WriteLine("{0}",addresses(k).ToString())
Next k
End If
End If
Next r
End Sub
' The following method is invoked when each asynchronous operation completes.
Shared Sub ProcessDnsInformation(result as IAsyncResult)
' Get the state object associated with this request.
Dim request as HostRequest= CType(result.AsyncState,HostRequest)
Try
' Get the results and store them in the state object.
Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
request.HostEntry = host
Catch e as SocketException
' Store any SocketExceptions.
request.ExceptionObject = e
Finally
' Decrement the request counter in a thread-safe manner.
Interlocked.Decrement(requestCounter)
End Try
End Sub
End Class
End Namespace
/*
The following example demonstrates using asynchronous methods to
get Domain Name System information for the specified host computer.
*/
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Collections;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
// Create a state object that holds each requested host name,
// an associated IPHostEntry object or a SocketException.
public class HostRequest
{
// Stores the requested host name.
private string hostName;
// Stores any SocketException returned by the Dns EndGetHostByName method.
private SocketException e;
// Stores an IPHostEntry returned by the Dns EndGetHostByName method.
private IPHostEntry entry;
public HostRequest(string name)
{
hostName = name;
}
public string HostName
{
get
{
return hostName;
}
}
public SocketException ExceptionObject
{
get
{
return e;
}
set
{
e = value;
}
}
public IPHostEntry HostEntry
{
get
{
return entry;
}
set
{
entry = value;
}
}
}
public class UseDelegateAndStateForAsyncCallback
{
// The number of pending requests.
static int requestCounter;
static ArrayList hostData = new ArrayList();
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);
// Create and store the state object for this request.
HostRequest request = new HostRequest(host);
hostData.Add(request);
// Start the asynchronous request for DNS information.
Dns.BeginGetHostEntry(host, callBack, request);
}
} 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.
foreach(HostRequest r in hostData)
{
if (r.ExceptionObject != null)
{
Console.WriteLine("Request for host {0} returned the following error: {1}.",
r.HostName, r.ExceptionObject.Message);
}
else
{
// Get the results.
IPHostEntry h = r.HostEntry;
string[] aliases = h.Aliases;
IPAddress[] addresses = h.AddressList;
if (aliases.Length > 0)
{
Console.WriteLine("Aliases for {0}", r.HostName);
for (int j = 0; j < aliases.Length; j++)
{
Console.WriteLine("{0}", aliases[j]);
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses for {0}", r.HostName);
for (int k = 0; k < addresses.Length; k++)
{
Console.WriteLine("{0}",addresses[k].ToString());
}
}
}
}
}
// The following method is invoked when each asynchronous operation completes.
static void ProcessDnsInformation(IAsyncResult result)
{
// Get the state object associated with this request.
HostRequest request = (HostRequest) result.AsyncState;
try
{
// Get the results and store them in the state object.
IPHostEntry host = Dns.EndGetHostEntry(result);
request.HostEntry = host;
}
catch (SocketException e)
{
// Store any SocketExceptions.
request.ExceptionObject = e;
}
finally
{
// Decrement the request counter in a thread-safe manner.
Interlocked.Decrement(ref requestCounter);
}
}
}
}
Voir aussi
Concepts
Vue d'ensemble du modèle asynchrone basé sur des événements
Utilisation d'un délégué AsyncCallback pour terminer une opération asynchrone