使用 AsyncCallback 委派和狀態物件
當您使用 AsyncCallback 委派以處理不同執行緒中非同步作業的結果，可以使用狀態物件傳遞回呼之間的資訊以擷取最終結果。 本主題透過展開使用 AsyncCallback 委派結束非同步作業中的範例來示範該做法。
範例
下列範例示範在 Dns 類別中使用非同步方法，以擷取使用者指定電腦的網域名稱系統 (DNS) 資訊。 此範例定義和使用 HostRequest 類別來儲存狀態資訊。 為使用者所輸入的每個電腦名稱建立 HostRequest 物件。 此物件會傳遞給 BeginGetHostByName 方法。 每次要求完成時都會呼叫 ProcessDnsInformation 方法。 使用 AsyncState 屬性擷取 HostRequest 物件。 ProcessDnsInformation 方法會使用 HostRequest 物件儲存要求傳回的 IPHostEntry 或要求所擲回的 SocketException。 當所有要求都完成時，應用程式會逐一查看 HostRequest 物件並顯示 DNS 資訊或 SocketException 錯誤訊息。
/*
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
