通过结束异步操作来阻止应用程序执行
更新：2007 年 11 月
如果应用程序在等待异步操作结果时不能继续执行其他工作，则在操作完成之前，必须阻止执行其他工作。可以使用下列方法之一来在等待异步操作完成时阻止应用程序的主线程。
调用异步操作的 End操作名称 方法。本主题中演示了此方法。
使用异步操作的 Begin操作名称 方法返回的 IAsyncResult 的 AsyncWaitHandle 属性。有关演示此方法的示例，请参见 使用 AsyncWaitHandle 阻止应用程序的执行。
在异步操作完成之前使用 End操作名称 方法进行阻止的应用程序通常会调用 Begin操作名称 方法，执行任何不需要等待操作结果也可以执行的工作，然后调用 End操作名称。
示例
下面的代码示例演示如何使用 Dns 类中的异步方法来检索用户指定的计算机的域名系统信息。请注意，示例中为 BeginGetHostByNamerequestCallback 和 stateObject 参数传递了 null（在 Visual Basic 中为 Nothing），因为在使用此方法时不需要这两个参数。
' 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
Namespace Examples.AdvancedProgramming.AsynchronousOperations
Public Class BlockUntilOperationCompletes
Public Shared Sub Main(args() as String)
' Make sure the caller supplied a host name.
If(args.Length = 0)
' Print a message and exit.
Console.WriteLine("You must specify the name of a host computer.")
End
End If
' Start the asynchronous request for DNS information.
' This example does not use a delegate or user-supplied object
' so the last two arguments are Nothing.
Dim result as IAsyncResult = Dns.BeginGetHostEntry(args(0), Nothing, Nothing)
Console.WriteLine("Processing your request for information...")
' Do any additional work that can be done here.
Try
' EndGetHostByName blocks until the process completes.
Dim host as IPHostEntry = Dns.EndGetHostEntry(result)
Dim aliases() as String = host.Aliases
Dim addresses() as IPAddress= host.AddressList
Dim i as Integer
If aliases.Length > 0
Console.WriteLine("Aliases")
For i = 0 To aliases.Length -1
Console.WriteLine("{0}", aliases(i))
Next i
End If
If addresses.Length > 0
Console.WriteLine("Addresses")
For i = 0 To addresses.Length -1
Console.WriteLine("{0}", addresses(i).ToString())
Next i
End If
Catch e as SocketException
Console.WriteLine("An exception occurred while processing the request: {0}", e.Message)
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;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
public class BlockUntilOperationCompletes
{
public static void Main(string[] args)
{
// Make sure the caller supplied a host name.
if (args.Length == 0 || args[0].Length == 0)
{
// Print a message and exit.
Console.WriteLine("You must specify the name of a host computer.");
return;
}
// Start the asynchronous request for DNS information.
// This example does not use a delegate or user-supplied object
// so the last two arguments are null.
IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);
Console.WriteLine("Processing your request for information...");
// Do any additional work that can be done here.
try
{
// EndGetHostByName blocks until the process completes.
IPHostEntry host = Dns.EndGetHostEntry(result);
string[] aliases = host.Aliases;
IPAddress[] addresses = host.AddressList;
if (aliases.Length > 0)
{
Console.WriteLine("Aliases");
for (int i = 0; i < aliases.Length; i++)
{
Console.WriteLine("{0}", aliases[i]);
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses");
for (int i = 0; i < addresses.Length; i++)
{
Console.WriteLine("{0}",addresses[i].ToString());
}
}
}
catch (SocketException e)
{
Console.WriteLine("An exception occurred while processing the request: {0}", e.Message);
}
}
}
}