如何等待WebClient的OpenReadAsync完成 [英] how to wait for webclient OpenReadAsync to complete

问题描述

我使用Web客户端下载从互联网中的Windows Phone 8.1应用中的一些东西。
下面是示例code我用我的应用程序 - 我在哪里调用下面的方法，但我的Web客户端不等待完成读操作和OpenReadAsync电话后立即返回

我怎样才能确保我的方法的返回操作必须等到完成OpenReadCompleted事件？
我已经看到多个类似的问题，但无法找到一个解决方案。

  MyCustomObject externalObj; //我的自定义对象
私有静态无效CheckNetworkFile（）
    {
        尝试
        {
            Web客户端Web客户端=新的WebClient（）;
            webClient.OpenReadCompleted + =（S，E）=＆GT;
            {
              externalObj = myReadWebclientResponse（e.Result）; //我的自定义的方法来读取响应
            };            webClient.OpenReadAsync（新的URI（http://externalURl.com/sample.xml，UriKind.Absolute））;
        }
        赶上（例外）
        {
          externalObj = NULL;
        }
    }
 

解决方案

我会建议你使用的 WebClient.OpenReadTaskAsync 用的 异步/的await 关键字引入在.NET 4.5来代替。您需要将异步关键字添加到您的方法，使其返回一个工作，这是明智结束你的方法与异步后缀：

  MyCustomObject externalObj;私有静态异步任务CheckNetworkFileAsync（）
{
    尝试
    {
        Web客户端Web客户端=新的WebClient（）;        流流=等待webClient.OpenReadTaskAsync（新的URI（http://externalURl.com/sample.xml，UriKind.Absolute））;
        externalObj = myReadWebclientResponse（流）;
    }
    赶上（例外）
    {
      externalObj = NULL;
    }
}
 

编辑：

正如你所说， WebClient.OpenReadTaskAsync 不适用于WP8.1，所以让我们创建一个扩展方法所以这将是：

 公共静态类WebClientExtensions
{
    公共静态任务＆LT;流＆gt; OpenReadTaskAsync（这WebClient的客户，开放的URI）
    {
       VAR TCS =新TaskCompletionSource＆LT;流＆gt;（）;       OpenReadCompletedEventHandler openReadEventHandler = NULL;
       openReadEventHandler =（发件人，参数）=＆GT;
       {
          尝试
          {
             tcs.SetResult（args.Result）;
          }
          赶上（例外五）
          {
             tcs.SetException（E）;
          }
       };       client.OpenReadCompleted + = openReadEventHandler;
       client.OpenReadAsync（URI）;       返回tcs.Task;
    }
}
 

现在你可以用它在你的 Web客户端。

您可以在的异步等待维基的和伟大的阅读材料只需在搜索栏标签过滤。

I am using WebClient to download some stuff from internet in Windows Phone 8.1 app. Below is the sample code i am using in my app - where i am calling below method, but my webclient is not waiting to complete the read operation and returning immediately after OpenReadAsync call.

how can i make sure that my method return operation must wait till OpenReadCompleted event is completed? I have seen multiple similar questions, but couldn't find a solution.

MyCustomObject externalObj;  // my custom object


private static void CheckNetworkFile()
    {
        try
        {
            WebClient webClient = new WebClient();
            webClient.OpenReadCompleted += (s, e) =>
            {
              externalObj=myReadWebclientResponse(e.Result); // my custom method to read the response
            };

            webClient.OpenReadAsync(new Uri("http://externalURl.com/sample.xml", UriKind.Absolute));                
        }
        catch (Exception)
        {
          externalObj=null;
        }
    }

解决方案

I would advise you to use WebClient.OpenReadTaskAsync with a combination of the async/await keywords introduced in .NET 4.5 instead. You need to add the async keyword to your method, make it return a Task and it is advisable to end your method with the Async postfix:

MyCustomObject externalObj;

private static async Task CheckNetworkFileAsync()
{
    try
    {
        WebClient webClient = new WebClient();

        Stream stream = await webClient.OpenReadTaskAsync(new Uri("http://externalURl.com/sample.xml", UriKind.Absolute));                
        externalObj = myReadWebclientResponse(stream);
    }
    catch (Exception)
    {
      externalObj = null;
    }
}

Edit:

As you said, WebClient.OpenReadTaskAsync isn't available for WP8.1, So lets create an Extension Method so it will be:

public static class WebClientExtensions 
{
    public static Task<Stream> OpenReadTaskAsync(this WebClient client, Uri uri)
    {
       var tcs = new TaskCompletionSource<Stream>();

       OpenReadCompletedEventHandler openReadEventHandler = null;
       openReadEventHandler = (sender, args) => 
       {
          try 
          {
             tcs.SetResult(args.Result);
          } 
          catch (Exception e)
          {
             tcs.SetException(e);
          }
       };

       client.OpenReadCompleted += openReadEventHandler;
       client.OpenReadAsync(uri);

       return tcs.Task;
    }
}

Now you can use it on your WebClient.

You can find great reading material in the async-await wiki and by simply filtering by that tag in the search bar.

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