Utilizzo di un oggetto di stato e di un delegato AsyncCallback

Quando si usa un delegato AsyncCallback per elaborare i risultati dell'operazione asincrona in un thread separato, è possibile usare un oggetto di stato per passare le informazioni tra i metodi di callback e per recuperare un risultato finale. In questo argomento viene illustrata tale pratica analizzando ulteriormente l'esempio descritto in Uso di un delegato AsyncCallback per terminare un'operazione asincrona.

Esempio

Nell'esempio di codice seguente viene illustrato come usare metodi asincroni nella classe Dns per recuperare le informazioni sul DNS (Domain Name System) per i computer specificati dall'utente. Questo esempio definisce e usa la classe HostRequest per archiviare le informazioni sullo stato. Viene creato l'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 tramite la proprietà AsyncState. Il metodo ProcessDnsInformation usa l'oggetto HostRequest per archiviare l'oggetto IPHostEntry restituito dalla richiesta o un'eccezione SocketException generata dalla richiesta. Al completamento 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.
*/

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);
            }
        }
    }
}

' The following example demonstrates using asynchronous methods to
' get Domain Name System information for the specified host computer.

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

Vedi anche

• Event-based Asynchronous Pattern (EAP) (Modello asincrono basato su eventi, EAP)
• Panoramica sul modello asincrono basato su eventi
• Uso di un delegato AsyncCallback per terminare un'operazione asincrona