異步操作狀態輪詢

能在等待非同步操作結果時執行其他工作的應用程式，不應在過程完成前阻塞等待。 使用下列其中一個選項，在等候異步作完成時繼續執行指令：

• 使用異步作 IsCompletedIAsyncResult 方法傳回之 的 屬性，判斷作業是否已完成。 此方法稱為輪詢，本主題中對此進行了示範。

• 使用 AsyncCallback 委派來在單獨的執行緒中處理異步操作的結果。 如需示範此方法的範例，請參閱使用 AsyncCallback 委派結束異步作業。

範例

下列程式代碼範例示範如何使用 Dns 類別中的異步方法來擷取使用者指定計算機的域名系統資訊。 這個範例會啟動異步操作，然後在控制台印出句號（“.”），直到操作完成。 請注意， Null （Visual Basic 中的 Nothing ） 會針對 BeginGetHostByNameAsyncCallback 和 Object 參數傳遞，因為這些自變數在使用此方法時並非必要。

/*
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

另請參閱

• 事件驅動非同步模式 （EAP）
• 事件為基礎的非同步模式概述