使用 AsyncWaitHandle 阻止应用程序的执行
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:
使用异步操作的 BeginOperationName 方法返回的 IAsyncResult 的 AsyncWaitHandle 属性。 本主题介绍的就是这种方法。
调用异步操作的 EndOperationName 方法。 有关展示这种方法的示例,请参阅通过结束异步操作阻止应用执行。
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($"{aliases[i]}");
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses");
for (int i = 0; i < addresses.Length; i++)
{
Console.WriteLine($"{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