Question

我在项目中使用Async CTP库时遇到问题。这是代码：

BookPage

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
    {
        base.OnNavigatedTo(e);

        if (this.DataContext is BookViewModel)
        {
            var bookViewModel = this.DataContext as BookViewModel;
            bookViewModel.UpdateReviews();
        }
    }

BookViewModel

public async void UpdateReviews()
    {
        Reviews.Clear();

        IEnumerable<Review> newReviews = null;
        try
        {
            newReviews = await BooksManager.GetBookReviews(this.Book.Sysno, 10, 0);
        }
        catch (Exception ex)
        {

        }

        if (newReviews != null)
        {
            foreach (var review in newReviews)
            {
                Reviews.Add(review);
            }
        }
    }

BooksManager

public static async Task<IEnumerable<Review>> GetBookReviews(string sysno, uint limit, uint offset)
    {
        if (sysno == null)
            throw new ArgumentNullException("sysno");
        if (string.IsNullOrWhiteSpace(sysno))
            throw new ArgumentException("sysno");

        string url = CreateBookReviewsURL(sysno, limit, offset);

        var reviews = await DownloadDataAsync<IEnumerable<Review>>(url);
        return reviews;
    }

public static async Task<T> DownloadDataAsync<T>(string url)
    {
        if (url == null)
            throw new ArgumentNullException("url");

        var newUrl = url.Contains("?") ? 
            string.Format("{0}d={1}", url, DateTime.Now) :
             string.Format("{0}?d={1}", url, DateTime.Now); //to avoid caching

        string data = null;

        WebRequest webRequest = WebRequest.CreateHttp(newUrl);

        using (WebResponse response = await webRequest.GetResponseAsync())
        {

            if (response.Headers["StatusCode"] == "200")
            {
                using (var stm = response.GetResponseStream())
                {
                    using (var reader = new StreamReader(stm))
                    {
                        data = await reader.ReadToEndAsync();
                    }
                }
            }
        }

        var books = await ParseDataAsync<T>(data);
        return books;
    }

抛出NullReferenceException

在   SmartLib.ViewModels.BooksViewModel.d_ 8.MoveNext（）
  在   System.Runtime.CompilerServices.TaskAwaiter＆LT;。＆以及c _DisplayClass5.b__1（对象   国家）   System.Reflection.RuntimeMethodInfo.InternalInvoke（RuntimeMethodInfo   rtmi，Object obj，BindingFlags invokeAttr，Binder binder，Object   参数，CultureInfo文化，布尔isBinderDefault，汇编   调用者，布尔验证访问，StackCrawlMark＆amp; stackMark）at   System.Reflection.RuntimeMethodInfo.InternalInvoke（Object obj，   BindingFlags invokeAttr，Binder binder，Object []参数，   CultureInfo culture，StackCrawlMark＆amp; stackMark）at   System.Reflection.MethodBase.Invoke（Object obj，Object []参数）
  在System.Delegate.DynamicInvokeOne（Object [] args）at   System.MulticastDelegate.DynamicInvokeImpl（Object [] args）at   System.Delegate.DynamicInvoke（Object [] args）at   System.Windows.Threading.DispatcherOperation.Invoke（）at   System.Windows.Threading.Dispatcher.Dispatch（的DispatcherPriority   System.Windows.Threading.Dispatcher.OnInvoke（Object。的优先级）   System.Windows.Hosting.CallbackCookie.Invoke（Object []上下文）   args）at   System.Windows.Hosting.DelegateWrapper.InternalInvoke（Object [] args）
  在System.Windows.RuntimeHost.ManagedHost.InvokeDelegate（IntPtr   pHandle，Int32 nParamCount，ScriptParam [] pParams，ScriptParam＆amp;   pResult）

在App.xaml.cs中调用BooksManager.GetBookReviews（）可以正常工作。

Async CTP NullReference或ArgumentNullException（Windows Phone）

0 个答案: