我想制作一个Windows Phone应用程序，它简要介绍一些新闻。然后，如果我点击某个新闻，它将导航到详细描述。问题是它有不同的json源。
以下是详细说明的来源：http://apibiru.herokuapp.com/v0.1/snack/1?auth_token=64d362d2e483e8023c46595f83ca8d9555ff6d7cc700a2474fbdbd341c43c1fb（将零食后的数字改为不同的数字以查看其他对象。
这是我的MainPage.xaml：
<phone:Pivot Title="Plumber">
<!--Pivot item one-->
<phone:PivotItem Header="readnow">
<!--Double line list with text wrapping-->
<phone:LongListSelector x:Name="FeedsLongListSelector" Margin="0,0,-12,0" ItemsSource="{Binding Feeds.RootObject}" SelectionChanged="FeedsLongListSelector_SelectionChanged">
<phone:LongListSelector.ItemTemplate>
<DataTemplate>
<StackPanel Margin="0,0,0,17">
<TextBlock Text="{Binding main_text}" TextWrapping="Wrap" Margin="12,0,0,0" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
<Image Source="{Binding main_image_url}" VerticalAlignment="Top"></Image>
<TextBlock Text="{Binding type}" TextWrapping="Wrap" Margin="12,-6,0,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
</StackPanel>
</DataTemplate>
</phone:LongListSelector.ItemTemplate>
</phone:LongListSelector>
</phone:PivotItem>
这是我的MainPage.xaml.cs：
private void LoadDataReadnow()
{
WebClient readnowClient = new WebClient();
readnowClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(readnow_DownloadStringCompleted);
readnowClient.DownloadStringAsync(new Uri("http://apibiru.herokuapp.com/v0.1/feeds/1?auth_token=64d362d2e483e8023c46595f83ca8d9555ff6d7cc700a2474fbdbd341c43c1fb"));
}
private void readnow_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
try
{
string jsonStr = e.Result;
if (!string.IsNullOrEmpty(jsonStr))
{
JArray array = JArray.Parse(jsonStr);
Feeds.RootObject[] feeds = new Feeds.RootObject[3];
List<Feeds.RootObject> listFeeds = array.ToObject<List<Feeds.RootObject>>();
FeedsLongListSelector.ItemsSource = listFeeds;
}
}
catch (Exception ex)
{
MessageBox.Show("Please check your internet connection");
}
}
这里是Feeds.cs的模型：
public class Feeds
{
public class Topic
{
public string name { get; set; }
public string screen_name { get; set; }
public int id { get; set; }
}
public class RootObject
{
public int updated_at { get; set; }
public string type { get; set; }
public List<Topic> topics { get; set; }
public string main_text { get; set; }
public string uuid { get; set; }
public Uri main_image_url { get; set; }
public int id { get; set; }
public bool is_rumour { get; set; }
public int created_at { get; set; }
}
最后一个是Snacks的模型：
public class Snack
{
public class Topic
{
public string name { get; set; }
public string screen_name { get; set; }
public int id { get; set; }
}
public class RootObject
{
public int updated_at { get; set; }
public string type { get; set; }
public List<Topic> topics { get; set; }
public string main_text { get; set; }
public string uuid { get; set; }
public string main_image_url { get; set; }
public int id { get; set; }
public bool is_rumour { get; set; }
public int created_at { get; set; }
}
}
这是我尝试导航它的代码（它不起作用）：
private void MainLongListSelector_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (FeedsLongListSelector.SelectedItem == null)
return;
NavigationService.Navigate(new Uri("/View/ReadnowDetailPage.xaml?selectedItem=" + (FeedsLongListSelector.SelectedItem as ReadnowViewModel).ID, UriKind.Relative));
}
感谢您的帮助：）
这是错误：
System.NullReferenceException was unhandled by user code
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=Plumber
StackTrace:
at Plumber.MainPage.FeedsLongListSelector_SelectionChanged(Object sender, SelectionChangedEventArgs e)
at Microsoft.Phone.Controls.LongListSelector.set_SelectedItem(Object value)
at Microsoft.Phone.Controls.LongListSelector.OnItemTap(Object sender, GestureEventArgs e)
at MS.Internal.CoreInvokeHandler.InvokeEventHandler(Int32 typeIndex, Delegate handlerDelegate, Object sender, Object args)
at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)
InnerException:
这是ReadnowViewModel：
public class ReadnowViewModel : INotifyPropertyChanged
{
private int _updated_at;
private string _type;
private List<Feeds.Topic> _topics;
private string _main_text;
private string _uuid;
private Uri _main_image_url;
private int _id;
private bool _is_rumour;
private int _created_at;
public int Updated_At
{
get
{
return this._updated_at;
}
set
{
if (value != this._updated_at)
{
this._updated_at = value;
NotifyPropertyChanged("Updated_At");
}
}
}
public string Type
{
get
{
return this._type;
}
set
{
if (value != this._type)
{
this._type = value;
NotifyPropertyChanged("Type");
}
}
}
public List<Feeds.Topic> Topics
{
get
{
return this._topics;
}
set
{
if (value != this._topics)
{
this._topics = value;
NotifyPropertyChanged("Topic");
}
}
}
public string Main_Text
{
get
{
return this._main_text;
}
set
{
if (value != this._main_text)
{
this._main_text = value;
NotifyPropertyChanged("Main_Text");
}
}
}
public string UUID
{
get
{
return this._uuid;
}
set
{
if (value != this._uuid)
{
this._uuid = value;
NotifyPropertyChanged("UUID");
}
}
}
public Uri Main_Image_URL
{
get
{
return this._main_image_url;
}
set
{
if (value != this._main_image_url)
{
this._main_image_url = value;
NotifyPropertyChanged("Main_Image_URL");
}
}
}
public int ID
{
get
{
return this._id;
}
set
{
if (value != this._id)
{
this._id = value;
NotifyPropertyChanged("ID");
}
}
}
public bool Is_Rumour
{
get
{
return this._is_rumour;
}
set
{
if (value != this._is_rumour)
{
this._is_rumour = value;
NotifyPropertyChanged("Is_Rumour");
}
}
}
public int Created_At
{
get
{
return this._created_at;
}
set
{
if (value != this._created_at)
{
this._created_at = value;
NotifyPropertyChanged("Created_At");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (null != handler)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
答案 0 :(得分：0)
查看代码的这一部分：
List<Feeds.RootObject> listFeeds = array.ToObject<List<Feeds.RootObject>>();
FeedsLongListSelector.ItemsSource = listFeeds;
似乎FeedsLongListSelector包含Feeds.RootObject而不是ReadnowViewModel的列表。因此，我建议尝试更改此行：
NavigationService.Navigate(
new Uri("/View/ReadnowDetailPage.xaml?selectedItem=" +
(FeedsLongListSelector.SelectedItem as ReadnowViewModel).ID
, UriKind.Relative));
到此（请注意SelectedItem已投放到RootObject）：
NavigationService.Navigate(
new Uri("/View/ReadnowDetailPage.xaml?selectedItem=" +
(FeedsLongListSelector.SelectedItem as Feeds.RootObject).id
, UriKind.Relative));