通过

使用 AsyncWaitHandle 阻止应用程序执行

在等待异步操作结果时无法继续执行其他工作的应用程序必须阻止，直到操作完成。 使用以下选项之一在等待异步操作完成时阻止应用程序的主线程：

使用一个或多个 WaitHandle 对象阻止的应用程序在异步操作完成之前通常会调用 BeginOperationName 方法，执行无需操作结果即可完成的任何工作，然后在异步操作完成之前阻止。 应用程序可以通过使用AsyncWaitHandle调用WaitOne中的一种方法来阻止单个操作。 若要在等待一组异步操作完成时阻止，请将关联的 AsyncWaitHandle 对象存储在数组中并调用其中 WaitAll 一种方法。 若要在等待一组异步操作完成时阻止，请将关联的 AsyncWaitHandle 对象存储在数组中并调用其中 WaitAny 一种方法。

示例

下面的代码示例演示如何使用 DNS 类中的异步方法检索用户指定的计算机的域名系统信息。 该示例演示了如何使用与异步操作关联的 WaitHandle 进行阻塞。 请注意，对于 BeginGetHostByNamerequestCallback 和 stateObject 参数，传递 null（在 Visual Basic 中为 Nothing），因为使用此方法时不需要这些参数。

/*
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;

namespace Examples.AdvancedProgramming.AsynchronousOperations
{
    public class WaitUntilOperationCompletes
    {
        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.
            IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);
            Console.WriteLine("Processing request for information...");
            // Wait until the operation completes.
            result.AsyncWaitHandle.WaitOne();
            // The operation completed. Process the results.
            try
            {
                // Get the results.
                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($"{aliases[i]}");
                    }
                }
                if (addresses.Length > 0)
                {
                    Console.WriteLine("Addresses");
                    for (int i = 0; i < addresses.Length; i++)
                    {
                        Console.WriteLine($"{addresses[i].ToString()}");
                    }
                }
            }
            catch (SocketException e)
            {
                Console.WriteLine($"Exception occurred while processing the request: {e.Message}");
            }
        }
    }
}

' 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

namespace Examples.AdvancedProgramming.AsynchronousOperations
    Public Class WaitUntilOperationCompletes

        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.
            Dim result as IAsyncResult = Dns.BeginGetHostEntry(args(0), Nothing, Nothing)
            Console.WriteLine("Processing request for information...")
            ' Wait until the operation completes.
            result.AsyncWaitHandle.WaitOne()
            ' The operation completed. Process the results.
            Try
                ' Get the results.
                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

另见

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