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 BeginOperationName 方法返回的 IAsyncResult 的 IsCompleted 属性来确定此操作是否已完成。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。
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 BeginGetHostByNameAsyncCallback 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.
*/
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);
}
}
}
下面的例子是同时执行多个异步操作，类似于多线程，要处理好并好，使用了Interlocked.Increment(requestCounter)/Interlocked.Decrement(requestCounter)
原子唯一性操作，(即不可中断的一个或一列系操作)
AsyncCallback 委托在一个单独的线程中处理异步操作的结果。AsyncCallback 委托表示在异步操作完成时调用的回调方法。回调方法采用 IAsyncResult 参数，该参数随后可用来获取异步操作的结果。
有关异步编程的更多信息，请参见使用 AsyncCallback 委托和状态对象。
下面的代码示例演示在 AsyncCallback 委托。对于 DNS 信息的每个异步请求，都调用一次此方法。
The following example demonstrates using asynchronous methods to
get Domain Name System information for the specified host computers.
This example uses a delegate to obtain the results of each asynchronous
operation.
*/
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Collections.Specialized;
using System.Collections;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
public class UseDelegateForAsyncCallback
{
static int requestCounter;
static ArrayList hostData = new ArrayList();
static StringCollection hostNames = new StringCollection();
static void UpdateUserInterface()
{
// Print a message to indicate that the application
// is still working on the remaining requests.
Console.WriteLine("{0} requests remaining.", requestCounter);
}
public static void Main()
{
// Create the delegate that will process the results of the
// asynchronous request.
AsyncCallback callBack = new AsyncCallback(ProcessDnsInformation);
string host;
do
{
Console.Write(" Enter the name of a host computer or <enter> to finish: ");
host = Console.ReadLine();
if (host.Length > 0)
{
// Increment the request counter in a thread safe manner.
Interlocked.Increment(ref requestCounter);
// Start the asynchronous request for DNS information.
Dns.BeginGetHostEntry(host, callBack, host);
}
} while (host.Length > 0);
// The user has entered all of the host names for lookup.
// Now wait until the threads complete.
while (requestCounter > 0)
{
UpdateUserInterface();
}
// Display the results.
for (int i = 0; i< hostNames.Count; i++)
{
object data = hostData [i];
string message = data as string;
// A SocketException was thrown.
if (message != null)
{
Console.WriteLine("Request for {0} returned message: {1}",
hostNames[i], message);
continue;
}
// Get the results.
IPHostEntry h = (IPHostEntry) data;
string[] aliases = h.Aliases;
IPAddress[] addresses = h.AddressList;
if (aliases.Length > 0)
{
Console.WriteLine("Aliases for {0}", hostNames[i]);
for (int j = 0; j < aliases.Length; j++)
{
Console.WriteLine("{0}", aliases[j]);
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses for {0}", hostNames[i]);
for (int k = 0; k < addresses.Length; k++)
{
Console.WriteLine("{0}",addresses[k].ToString());
}
}
}
}
// The following method is called when each asynchronous operation completes.
static void ProcessDnsInformation(IAsyncResult result)
{
string hostName = (string) result.AsyncState;
hostNames.Add(hostName);
try
{
// Get the results.
IPHostEntry host = Dns.EndGetHostEntry(result);
hostData.Add(host);
}
// Store the exception message.
catch (SocketException e)
{
hostData.Add(e.Message);
}
finally
{
// Decrement the request counter in a thread-safe manner.
Interlocked.Decrement(ref requestCounter);
}
}
}
}