Question

当你使用 AsyncCallback 委托的一个单独的线程中的异步操作对结果进行处理，则可以使用的状态对象，两个回调之间传递信息以及如何检索最终结果。 本主题通过扩展中的示例演示这种做法 使用 AsyncCallback 委托结束异步操作 。

示例

下面的代码示例演示如何使用中的异步方法 Dns 类检索指定用户的计算机的域名系统 (DNS) 信息。 此示例定义并使用 HostRequest 类来存储状态信息。 A HostRequest 获取用户输入的每个计算机名创建对象。 此对象传递给 BeginGetHostByName 方法。 ProcessDnsInformation 每次请求完成后调用方法。 HostRequest 对象使用检索 AsyncState 属性。 ProcessDnsInformation 方法使用 HostRequest 要存储对象 IPHostEntry 该请求返回的或 SocketException 引发的请求。 应用程序完成后的所有请求，循环访问 HostRequest 对象，并显示的 DNS 信息或 SocketException 错误消息。

/*
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
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

另请参阅

基于事件的异步模式 (EAP)
基于事件的异步模式概述
使用 AsyncCallback 委托停止异步操作