今天我花了一些时间搜索我们面临的一个具体案例，该案例要求使用以下要求调用（外部）ASP.NET Web服务：
在互联网和StackOverflow上出现了许多关于此主题的问题，但这些问题要么过时，要么建议使用WebRequest.TimeOut属性，而该属性仅适用于同步调用。
其中一个替代方案是使用一个System.Threading.Timer。在开始调用之前启动计时器，并在到达TimerCallback时取消它。
然而，我认为应该有更常见的方法来处理这种情况。可惜到目前为止还没有找到。有人知道如何设置异步Web服务调用的客户端超时吗？
提前致谢。
事实上，您不能总是在异步操作中使用WebRequest.TimeOut；至少不是对于所有抽象WebRequest类的实现者。例如，在msdn文档中记录了当调用HttpWebRequest.BeginGetResponse开始异步操作时，该属性被忽略。明确指出TimeOut属性被忽略，并且如果需要，由用户负责实现超时行为。
在附带HttpWebRequest.BeginGetResponsemsdn文档的示例代码中，使用ManualResestEvent allDone与WaitOrTimerCallback结合使用，如下所示:
IAsyncResult result = (IAsyncResult) myHttpWebRequest.BeginGetResponse(
new AsyncCallback(RespCallback), myRequestState);
// TimeoutCallback aborts the request if the timer fires.
ThreadPool.RegisterWaitForSingleObject (result.AsyncWaitHandle,
new WaitOrTimerCallback(TimeoutCallback),
myHttpWebRequest,
DefaultTimeout,
true);
// The response came in the allowed time. The work processing will happen in the
// callback function RespCallback.
allDone.WaitOne();
<system.serviceModel>
<bindings>
<basicHttpBinding>
<binding name="HeaderedServiceSoap"
closeTimeout="00:01:00"
openTimeout="00:01:00"
receiveTimeout="00:10:00"
sendTimeout="00:01:00"
allowCookies="false"
bypassProxyOnLocal="false"
hostNameComparisonMode="StrongWildcard"
maxBufferSize="65536"
maxBufferPoolSize="524288"
maxReceivedMessageSize="65536"
messageEncoding="Text"
textEncoding="utf-8"
transferMode="Buffered"
useDefaultWebProxy="true">
<readerQuotas maxDepth="32"
maxStringContentLength="8192"
maxArrayLength="16384"
maxBytesPerRead="4096"
maxNameTableCharCount="16384" />
<security mode="None">
<transport clientCredentialType="None"
proxyCredentialType="None"
realm="" />
<message clientCredentialType="UserName"
algorithmSuite="Default" />
</security>
</binding>
</basicHttpBinding>
</bindings>
<client>
<endpoint
address="http://localhost/MyService.asmx"
binding="basicHttpBinding"
bindingConfiguration="HeaderedServiceSoap"
contract="WSTest.HeaderedServiceSoap"
name="HeaderedServiceSoap" />
</client>
</system.serviceModel>
尝试先移除再重新添加引用，确保您的应用程序目标框架为4.0，并且您正在添加服务引用（而不是Web引用）。
现在继续！;)
用于执行回调的helper： AsyncCallHelper.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
// contains base classes for webservice calls
using System.ServiceModel;
// contains the DispatcherTimer class for callback timers
using System.Windows.Threading;
namespace ASPSandcastleWPFClient
{
/// <summary>
/// DispatcherTimer usage info thanks to:
///
/// Wildermuth, Shawn, "Build More Responsive Apps With The Dispatcher", MSDN Magazine, October 2007
/// Original URL: http://msdn.microsoft.com/en-us/magazine/cc163328.aspx
/// Archived at http://www.webcitation.org/605qBiUEC on July 11, 2011.
///
/// this class is not set up to handle multiple outstanding calls on the same async call;
/// if you wish to do that, there would need to be some sort of handling for multiple
/// outstanding calls designed into the helper.
/// </summary>
public class AsyncCallHelper
{
#region Static Defaults
private static TimeSpan myDefaultTimeout;
/// <summary>
/// default timeout for all instances of the helper; should different timeouts
/// be required, a member should be created that can override this setting.
///
/// if this is set to null or a value less than zero, the timout will be set
/// to TimeSpan.Zero, and the helper will not provide timeout services to the
/// async call.
/// </summary>
public static TimeSpan DefaultTimeout
{
get
{
return myDefaultTimeout;
}
set
{
if ((value == null) || (value < TimeSpan.Zero))
myDefaultTimeout = TimeSpan.Zero;
else
myDefaultTimeout = value;
}
}
#endregion
/// <summary>
/// creates an instance of the helper to assist in timing out on an async call
/// </summary>
/// <param name="AsyncCall">the call which is represented by this instance. may not be null.</param>
/// <param name="FailureAction">an action to take, if any, upon the failure of the call. may be null.</param>
public AsyncCallHelper(Action AsyncCall, Action FailureAction)
{
myAsyncCall = AsyncCall;
myFailureAction = FailureAction;
myTimer = new DispatcherTimer();
myTimer.Interval = DefaultTimeout;
myTimer.Tick += new EventHandler(myTimer_Tick);
}
/// <summary>
/// Make the call
/// </summary>
public void BeginAsyncCall()
{
myAsyncCall();
if (DefaultTimeout > TimeSpan.Zero)
{
myTimer.Interval = DefaultTimeout;
myTimer.Start();
}
}
/// <summary>
/// The client should call this upon receiving a response from the
/// async call. According to the reference given above, it seems that
/// the WPF will only be calling this on the same thread as the UI,
/// so there should be no real synchronization issues here.
///
/// In a true multi-threading situation, it would be necessary to use
/// some sort of thread synchronization, such as lock() statements
/// or a Mutex in order to prevent the condition where the call completes
/// successfully, but the timer fires prior to calling "CallComplete"
/// thus firing the FailureAction after the success of the call.
/// </summary>
public void CallComplete()
{
if ((DefaultTimeout != TimeSpan.Zero) && myTimer.IsEnabled)
myTimer.Stop();
}
private void myTimer_Tick(object sender, EventArgs e)
{
CallComplete();
if (myFailureAction != null)
myFailureAction();
}
/// <summary>
/// WPF-friendly timer for use in aborting "Async" Webservice calls
/// </summary>
private DispatcherTimer myTimer;
/// <summary>
/// The call to be made
/// </summary>
private Action myAsyncCall;
/// <summary>
/// What action the helper should take upon a failure
/// </summary>
private Action myFailureAction;
}
}
具有相关代码的MainWindow.xaml.cs文件：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using ASPSandcastleWPFClient.ASPSandcastleWebserviceClient;
namespace ASPSandcastleWPFClient
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private ASPSandcastleWebservice myClient = null;
private AsyncCallHelper myHelloWorldHelper = null;
public MainWindow()
{
InitializeComponent();
}
private void InitializeClient()
{
myClient = new ASPSandcastleWebservice();
myHelloWorldHelper =
new AsyncCallHelper
(
myClient.HelloWorldAsync,
HelloWorldTimeout
);
}
private void Window_Initialized(object sender, EventArgs e)
{
InitializeClient();
}
/// <summary>
/// this is called prior to making a call so that we do not end up with multiple
/// outstanding async calls
/// </summary>
private void DisableButtons()
{
btnStartAsyncCall.IsEnabled = false;
btnStartAsyncCallWithTimeout.IsEnabled = false;
}
/// <summary>
/// this is called after a result is received or the call is cancelled due to timeout
/// so that we know it's safe to make another call.
/// </summary>
private void EnableButtons()
{
btnStartAsyncCall.IsEnabled = true;
btnStartAsyncCallWithTimeout.IsEnabled = true;
}
private void btnStartAsyncCall_Click(object sender, RoutedEventArgs e)
{
DisableButtons();
// disable the timeout handling
AsyncCallHelper.DefaultTimeout = TimeSpan.Zero;
myClient.HelloWorldCompleted += new HelloWorldCompletedEventHandler(myClient_HelloWorldCompleted);
myHelloWorldHelper.BeginAsyncCall();
lblResponse.Content = "waiting...";
}
private void btnStartAsyncCallWithTimeout_Click(object sender, RoutedEventArgs e)
{
DisableButtons();
// enable the timeout handling
AsyncCallHelper.DefaultTimeout = TimeSpan.FromSeconds(10);
lblResponse.Content = "waiting for 10 seconds...";
myHelloWorldHelper.BeginAsyncCall();
}
/// <summary>
/// see note RE: possible multi-thread issues when not using WPF in AsyncCallHelper.cs
/// </summary>
private void HelloWorldTimeout()
{
myClient.CancelAsync(null);
lblResponse.Content = "call timed out...";
EnableButtons();
}
void myClient_HelloWorldCompleted(object sender, HelloWorldCompletedEventArgs e)
{
myHelloWorldHelper.CallComplete();
if (!e.Cancelled)
lblResponse.Content = e.Result;
EnableButtons();
}
}
}
我不知道这是否习惯用语，但当我通过 WebClient.DownloadStringAsync(...) 发布异步请求时，我也使用来自 Silverlight 的计时器（DispatchTimer）。