使用 AsyncWaitHandle 阻止应用程序的执行

  • 项目

如果应用无法在等待异步操作结果期间继续执行其他工作，必须阻止应用一直到操作完成。 请使用下列方法之一，在应用等待异步操作完成期间阻止应用的主线程：

在异步操作完成前使用一个或多个 WaitHandle 对象阻止的应用，通常会调用 BeginOperationName 方法，执行任何不需要等待操作结果也可以执行的工作，并在一个或多个异步操作完成前一直处于阻止状态。 可以使用 AsyncWaitHandle 调用 WaitOne 方法之一，对单个操作阻止应用。 若要在等待一组异步操作完成期间阻止应用，请将关联的 AsyncWaitHandle 对象存储到数组中，并调用 WaitAll 方法之一。 若要在等待一组异步操作中任一操作完成期间阻止应用，请将关联的 AsyncWaitHandle 对象存储到数组中，并调用 WaitAny 方法之一。

示例

下面的代码示例展示了如何使用 DNS 类中的异步方法，检索用户指定计算机的域名系统信息。 此示例展示了如何使用与异步操作关联的 WaitHandle 阻止应用。 请注意，对 BeginGetHostByNamerequestCallback 和 stateObject 参数传递的是 null（Visual Basic 中的 Nothing），因为使用这种方法时这些是可选参数。

/*
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("{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("Exception occurred while processing the request: {0}",
                    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

另请参阅