在等待异步操作的结果时可以执行其他工作的应用程序不应在操作完成之前阻止等待。 使用以下选项之一在等待异步操作完成时继续执行指令：
使用异步操作的IsCompletedIAsyncResult方法返回的属性来确定操作是否已完成。 此方法称为轮询，本主题对此进行了演示。
使用AsyncCallback委托在单独的线程中处理异步操作的结果。 有关展示这种方法的示例，请参阅使用 AsyncCallback 委托结束异步操作。
示例：
下面的代码示例演示如何在类中使用 Dns 异步方法检索用户指定的计算机的域名系统信息。 此示例启动异步操作，然后在控制台打印句点 (".")，直到操作完成。 请注意， null（在 Visual Basic 中为Nothing）被传递给 BeginGetHostByNameAsyncCallback 和 Object 参数，因为在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.
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 asynchronous 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($"{aliases[i]}");
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses");
for (int i = 0; i < addresses.Length; i++)
{
Console.WriteLine($"{addresses[i].ToString()}");
}
}
}
catch (SocketException e)
{
Console.WriteLine($"An 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.
'This example polls to detect the end of the asynchronous operation.
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
