使用异常等待HttpClient.GetAsync便携式类库 [英] Exception using await HttpClient.GetAsync in Portable Class Library

问题描述

我有便携式类库（net40 + SL4 + win8的+ wp71）与code：结果

I have Portable Class Library(net40+sl4+win8+wp71) with code:

    public static async Task<Dictionary<string, FeedItem>> GetFeeds()  
    {  
        try
        {
            string data;
            using (HttpMessageHandler handler = new HttpClientHandler())
            {
                using (HttpClient httpClient = new HttpClient(handler))
                {
                    data = await httpClient.GetStringAsync(FeedsUrl + Guid.NewGuid()).ConfigureAwait(false);
                }
            }

            if (data != null)
            {
                //...
            }
            return null;
        }
        catch
        {
            return null;
        }
    }

Microsoft.Bcl，Microsoft.Bcl.Async和Microsoft.Net.Http参考。

Microsoft.Bcl, Microsoft.Bcl.Async and Microsoft.Net.Http are referenced.

我使用Windows 8地铁应用这个库。然后，我通过VS2012 debugg的应用程序，它是确定。但是，当我创建应用程序包并安装它，第一次启动每次应用程序崩溃与错误：

I use this library in Windows 8 Metro app. Then I debugg app through VS2012, it is ok. But when I create App Package and install it, the app crashes on first launch everytime with error:

Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.Runtime.InteropServices.COMException
Stack:
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__1(System.Object)
   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

编辑：如果我评论在线数据=等待httpClient.GetStringAsync这将无一例外工作。

If I comment line "data = await httpClient.GetStringAsync" it will work without exception.

在第二次发射它总是有效。

On second launch it always works.

任何建议？

推荐答案

我已经想通了。我使用MVVM轻，所以我必须用ViewModelLocator code：结果

I've figured it out. I use MVVM Light, so I have ViewModelLocator with code:

public ViewModelLocator()
{
    ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);

    SimpleIoc.Default.Register<MainViewModel>(true);
}

所以，当我删除立即创建它停止崩溃的模式：结果

So when I removed immediate creation of the model it stopped crashing:

SimpleIoc.Default.Register<MainViewModel>(/*true*/);

我不知道为什么，但我认为这是因为MVVM光有一些问题，Microsoft.Bcl，Microsoft.Bcl.Async或Microsoft.Net.Http。

I have no idea why but I think it's because MVVM Light has some problems with Microsoft.Bcl, Microsoft.Bcl.Async or Microsoft.Net.Http.

希望这会帮助别人，例如<一href=\"http://stackoverflow.com/questions/14960268/await-with-httpclient-getasyncurl-and-getting-error-system-nullreferenceexcept\">this问题

Hope this will help someone, e.g. this question

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