轮询异步操作的状态
在等待异步操作结果的同时可以进行其他工作的应用程序不应在操作完成之前阻止等待。 可以使用下列方法之一来在等待异步操作完成的同时继续执行指令。
可使用异步操作的 Begin操作名称 方法返回的 IAsyncResult 的 IsCompleted 属性来确定该操作是否已完成。 此方法叫做轮询；本主题中将演示轮询。
可使用 AsyncCallback 委托来处理另一个线程中的异步操作的结果。 有关演示此方法的示例，请参见 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 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。 此示例开始异步操作，然后在控制台输出句点（“.”），直到操作完成。 请注意，示例中为 BeginGetHostByName AsyncCallback 和 Object 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.
'This example polls to detect the end of the asynchronous operation.
Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Threading
Namespace Examples.AdvancedProgramming.AsynchronousOperations
Public Class PollUntilOperationCompletes
Shared Sub UpdateUserInterface()
' Print a period to indicate that the application
' is still working on the request.
Console.Write(".")
End Sub
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...")
' Poll for completion information.
' Print periods (".") until the operation completes.
Do while result.IsCompleted <> True
UpdateUserInterface()
Loop
' The operation is complete. Process the results.
' Print a new line.
Console.WriteLine()
Try
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.
This example polls to detect the end of the asynchronous operation.
*/
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
public class PollUntilOperationCompletes
{
static void UpdateUserInterface()
{
// Print a period to indicate that the application
// is still working on the request.
Console.Write(".");
}
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 asychronous request for DNS information.
IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);
Console.WriteLine("Processing request for information...");
// Poll for completion information.
// Print periods (".") until the operation completes.
while (result.IsCompleted != true)
{
UpdateUserInterface();
}
// The operation is complete. Process the results.
// Print a new line.
Console.WriteLine();
try
{
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);
}
}
}
}