C# 如何“检查”、“取消”和等待异步 WebRequest？

Question

假设我开始使用 BeginGetResponse 有 50 个请求。

如何检查每个请求的状态？
以及如何取消它（有时它们会挂起）？
当所有请求都完成或取消时，我该如何执行操作？

Answer 1

对 BeginGetResponse 的调用返回一个 IAsyncResult。保留对它的引用。您可以使用 IAsyncResult.AsyncState 检查请求的状态。

要取消请求，请调用原始 WebRequest 实例的 WebRequest.Abort。

要在所有请求完成或取消时执行某些操作，请从每个请求的 IAsyncResult.AsyncWaitHandle 获取 WaitHandle，然后等待所有请求。示例代码此处。

Answer 2

您最好实施基于事件的异步模式，以便您的开发能够捕获各个不同阶段的事件。

基本上，我将如何首先创建 IAsyncCommand 接口，如下所示。

    public interface IAsyncCommand<TRequest, TResponse, TCorrelation> : ICommand<TRequest, TResponse>
{
    event CommandProgressChangedEventHandler<string, TCorrelation> ProgressChanged;
    event CommandCompletedEventHandler<TResponse, TCorrelation> Completed;

    bool CancellationPending { get; }
    bool IsBusy { get; }

    TCorrelation CorrelationId(TRequest request);
    void ExecuteAsync(TRequest request);
    void ExecuteAsync(TRequest request, CommandCompletedEventHandler<TResponse, TCorrelation> completedCallback, CommandProgressChangedEventHandler<string, TCorrelation> progressCallback);
    void CancelAsync();
}

根据您的场景实现这两个 CommandProgressChangedEventHander 和 CommandCompletedEventHandler 并相应地填充 Argument。

如果我们假设我们的线程应该检查所讨论的特定 URL 是否是有效的 URL，则代码如下......

    public class UrlCheckCommand : AsyncCommandBase<string, bool, string>, IUrlCheckCommand
{
    public override string CorrelationId(string request)
    {
        return request;   //Guid.NewGuid().ToString();
    }

    public override bool Execute(string request)
    {
        return CommandHelper.CheckUrlValidity(request);
    }
}

AsyncCommandBase 类是一个实现 IAsyncCommand 接口的抽象类。
该类的框架定义如下。

    public abstract class AsyncCommandBase<TRequest, TResponse, TCorrelation> : IAsyncCommand<TRequest, TResponse, TCorrelation>
{
    protected AsyncOperation operation = null;
    protected BackgroundWorker worker = null;

    #region IAsyncCommand<TRequest,TResponse,TCorrelation> Members

        //Implement all the interface members as per your use....

    #endregion

    protected void worker_DoWork(object sender, DoWorkEventArgs e)
    {
        e.Result = Execute((TRequest)e.Argument);
    }
    protected void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        OnProgressChanged(e);
    }
    protected void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        OnCompleted(e);
    }

    protected void ReportProgress(int percentageProgress, string message)
    {
        if (worker != null && worker.WorkerReportsProgress)
            worker.ReportProgress(percentageProgress, message);
        else
            OnProgressChanged(new ProgressChangedEventArgs(percentageProgress, message));
    }
    protected void OnProgressChanged(ProgressChangedEventArgs e)
    {
        if (ProgressChanged != null)
        {
            SendOrPostCallback callback = new SendOrPostCallback(delegate { ProgressChanged(this, new CommandProgressChangedEventArgs<string, TCorrelation>(e.ProgressPercentage, e.UserState as string, (TCorrelation)operation.UserSuppliedState)); });
            operation.Post(callback, null);
        }
    }
    protected void OnCompleted(RunWorkerCompletedEventArgs e)
    {
        if (Completed != null)
        {
            TResponse response = default(TResponse);
            if (e.Error == null)
                response = (TResponse)e.Result;

            SendOrPostCallback callback = new SendOrPostCallback(delegate { Completed(this, new CommandCompletedEventArgs<TResponse, TCorrelation>(response, e.Error, (TCorrelation)operation.UserSuppliedState, e.Cancelled)); });
            operation.PostOperationCompleted(callback, null);
        }
    }
}

您可以填充进度和已完成事件处理程序，它们的关键是参数填充。你甚至可以用它来填充进度百分比、用户状态等......