Windows phone 8 WP8，LoopingSelector数据源中的空值

我正在使用Windows Phone Toolkit nuget软件包（）。我想在我的WP8应用程序中创建一个LoopingSelector，在这里我可以选择日期，但我不希望LoopingSelector在它的选择中显示今天之前的几天

我的控件如下所示：

直到我选择了源代码的第一个元素（今天是2004年12月），我的应用程序才会有响应（我可以按按钮，显示效果，但事件处理程序不会执行）。我猜这是一个无限循环或死锁，但它不在我自己的代码中（我调试了它）

LoopingSelector数据源的定义：

public class NextDatesDataSource : LoopingDataSource<NextDateModel>
{
    public NextDatesDataSource() : base(NextDateModel.Create(DateTime.Today)) { }
    protected override NextDateModel GetNext(NextDateModel relativeTo)
    {
        if (relativeTo.Date == DateTime.Today + TimeSpan.FromDays(10))
            return null;
        return NextDateModel.Create(relativeTo.Date + TimeSpan.FromDays(1));
    }
    protected override NextDateModel GetPrevious(NextDateModel relativeTo)
    {
        //if i comment the next two lines, then everything works perfect
        if (relativeTo.Date == DateTime.Today)
            return null;
        return NextDateModel.Create(relativeTo.Date - TimeSpan.FromDays(1));
    }
}

多谢各位

public abstract class LoopingDataSource<T> : ILoopingSelectorDataSource
{
    protected LoopingDataSource() { selectedItem = default(T); }
    protected LoopingDataSource(T initialSelection) { selectedItem = initialSelection; }

    protected abstract T GetNext(T relativeTo);
    protected abstract T GetPrevious(T relativeTo);

    public object GetNext(object relativeTo)
    {
        if (relativeTo == null) return null;
        return GetNext((T)relativeTo);
    }

    public object GetPrevious(object relativeTo)
    {
        if (relativeTo == null) return null;
        return GetPrevious((T)relativeTo);
    }

    private T selectedItem;
    public object SelectedItem
    {
        get
        {
            return selectedItem;
        }
        set
        {
            object oldVal = selectedItem;
            selectedItem = (T)value;
            if (SelectionChanged != null)
                SelectionChanged(this, new SelectionChangedEventArgs(new object[] { oldVal }, new object[] { value }));
        }
    }
    public T Selected
    {
        get { return (T)SelectedItem; }
        set { SelectedItem = value; }
    }

    public event EventHandler<SelectionChangedEventArgs> SelectionChanged;
}

   <p:LoopingSelector
            x:Name="DaySelector"
            Grid.Column="0" Grid.Row="1"
            ItemSize="230,110"
            ItemMargin="6" Margin="10,5,5,5">
        <p:LoopingSelector.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <TextBlock Text="{Binding Header}" FontSize="28" VerticalAlignment="Top"/>
                    <TextBlock Text="{Binding Body}" FontSize="42" FontWeight="Bold" VerticalAlignment="Bottom"/>
                </Grid>
            </DataTemplate>
        </p:LoopingSelector.ItemTemplate>
    </p:LoopingSelector>

        DaySelector.DataSource = new NextDatesDataSource();