C# LongListSelector和ObservableCollection抛出InvalidCastException

我试图将我的收藏绑定到LLS，但我得到了一个无效的例外。如果我使用ListBox而不是LLS，那么所有绑定都可以很好地工作。这是我的收藏：

    public class Friend : INotifyPropertyChanged
{
    private string name;
    public string Name { get { return name; } set { name = value; OnPropertyChanged("Name"); } }
    private string image;
    public string Image { get { return image; } set { image = value; OnPropertyChanged("Image"); } }
    private string sourseId;
    public string SourseId { get { return sourseId; } set { sourseId = value; OnPropertyChanged("SourseId"); } }
    private string online_image;
    public string Online_Image { get { return online_image; } set { online_image = value; if (online_image == "1") online_image = @"/icons/appbar.power.png"; OnPropertyChanged("Online_Image"); } }
    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string info)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(info));
        }
    }
    public Friend() { }


}
public class FriendList : ObservableCollection<Friend>
{
    public FriendList()
        : base()
    {

    }
}

和

public-FriendList-FriendList=new-FriendList（）就在页面下方。我做错了什么？异常在字符串this.AllFriendsList.ItemsSource=FriendsList上激发

这是我的xaml：

                    <toolkit:LongListSelector Name="AllFriendsList" ItemsSource="{Binding FriendList}">
                    <toolkit:LongListSelector.ItemTemplate>
                        <DataTemplate>
                            <Grid Height="75" Width="460" Margin="0,10,10,0">
                                <Line Style="{StaticResource Line1}" ></Line>
                                <Line Style="{StaticResource Line2}" ></Line>
                                <TextBlock Margin="75,15,40,0" FontSize="30" Name="Name" Text="{Binding Name}" Tap="GetUserInfo" />
                                <Image HorizontalAlignment="Left" Width="75" Name="Photo" >
                                    <Image.Source>
                                        <BitmapImage UriSource="{Binding Image}" CreateOptions="BackgroundCreation" />
                                    </Image.Source>
                                </Image>
                                <Image HorizontalAlignment="Right" Name="IsOnline" Margin="0,0,0,0" Width="60" Height="60" Source="{Binding Online_Image}" />
                            </Grid>
                        </DataTemplate>
                    </toolkit:LongListSelector.ItemTemplate>
                </toolkit:LongListSelector>



UPD：如果我使用示例中的group，并将我的列表分组为

            var FriendsGroup = from fr in FriendList
                            group fr by fr.Online_Image into c
                            orderby c.Key
                            select new Group<Friend>(c.Key, c);

        AllFriendsList.ItemsSource = FriendsGroup;

var FriendsGroup=来自FriendsList中的fr
按fr.Online\u图像将fr分组到c中
orderby c.Key
选择新组（c键，c）；
AllFriendsList.ItemsSource=FriendsGroup；

它很好用。如果我理解正确-LLS正在等待分组的源。
您将绑定两次到您的AllFiendsList。第一次通过这里的xaml

<toolkit:LongListSelector Name="AllFriendsList" ItemsSource="{Binding FriendList}">



第二次通过代码。这可能会导致异常。
我通过将IsFlat更改为True解决了相同的问题

<toolkit:LongListSelector Name="AllFriendsList" ItemsSource="{Binding FriendList}" IsFlat="True">

我尝试绑定代码、xaml、code和xaml-相同的异常。
<toolkit:LongListSelector Name="AllFriendsList" ItemsSource="{Binding FriendList}" IsFlat="True">