Utilizzo di un oggetto di stato e di un delegato AsyncCallback
Aggiornamento: novembre 2007
Si consiglia di non bloccare le applicazioni che consentono di eseguire altre attività in attesa dei risultati di un'operazione asincrona fino al completamento di quest'ultima. Utilizzare una delle opzioni riportate di seguito per continuare l'esecuzione delle istruzioni durante l'attesa del completamento di un'operazione asincrona.
Utilizzare un delegato AsyncCallback per elaborare i risultati dell'operazione asincrona in un thread diverso. Tale approccio verrà descritto nel presente argomento.
Utilizzare la proprietà IsCompleted dell'oggetto IAsyncResult restituito dal metodo BeginNomeOperazione dell'operazione asincrona per determinare se l'operazione è stata completata. Per un esempio in cui venga illustrato questo approccio, vedere Esecuzione del polling dello stato di un'operazione asincrona.
Esempio
Nell'esempio di codice riportato di seguito viene illustrato come utilizzare i metodi asincroni nella classe Dns per recuperare informazioni DNS (Domain Name System) relative ai computer specificati dall'utente. Viene definita e utilizzata la classe HostRequest per memorizzare informazioni sullo stato. Viene creato un oggetto HostRequest per ogni nome computer immesso dall'utente. Questo oggetto viene passato al metodo BeginGetHostByName. Il metodo ProcessDnsInformation viene chiamato ogni volta che viene completata una richiesta. L'oggetto HostRequest viene recuperato mediante la proprietà AsyncState. Il metodo ProcessDnsInformation utilizza l'oggetto HostRequest per memorizzare l'oggetto IPHostEntry restituito dalla richiesta o una SocketException generata dalla richiesta. Al termine dell'esecuzione di tutte le richieste, l'applicazione scorre gli oggetti HostRequest e visualizza le informazioni DNS o il messaggio di errore 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);
}
}
}
}
Vedere anche
Concetti
Cenni preliminari sul modello asincrono basato su eventi
Utilizzo di un delegato AsyncCallback per terminare un'operazione asincrona