如何设置ViewModela视图的数据上下文?

我试图将视图的数据上下文设置为包含在它的ViewModel中的列表。但是当我测试当前设置时，似乎ViewModel和View之间的data context设置不正确。

为了调试这个问题，我在View的构造函数中设置了一个消息框，我得到了以下错误消息，它提示数据上下文没有正确设置:"对象引用没有设置为对象的实例"

列表也在另一个ViewModel中使用，显示列表不是空的，这进一步暗示了数据上下文问题。

有人知道在视图和ViewModel之间设置数据上下文的缺陷是什么吗?

这是包含列表的ViewModel:

namespace LC_Points.ViewModel
{
    public class ViewSubjectGradeViewModel 
    {

        public ViewSubjectGradeViewModel()
        {
            AddedSubjectGradePairs = new ObservableCollection<ScoreModel>();
        }

        public ObservableCollection<ScoreModel> AddedSubjectGradePairs { get; set; }       
    }
}

后面是View和View代码:

<Page x:Class="LC_Points.View.ViewSubjectGradePage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:local="using:LC_Points.View"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:vms="using:LC_Points.ViewModel"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
      DataContext="{Binding ViewSubjectGradeViewModelProperty1}"
      mc:Ignorable="d">

    <Grid x:Name="LayoutRoot">
        <Grid.ChildrenTransitions>
            <TransitionCollection>
                <EntranceThemeTransition />
            </TransitionCollection>
        </Grid.ChildrenTransitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="40*" />
            <RowDefinition Height="20*" />
            <RowDefinition Height="30*" />
            <RowDefinition Height="30*" />
            <RowDefinition Height="20*" />
            <RowDefinition Height="20*" />
        </Grid.RowDefinitions>
        <!--  Title Panel  -->
        <StackPanel Grid.Row="0" Margin="19,0,0,0">
            <TextBlock Margin="0,12,0,0"
                       Style="{ThemeResource TitleTextBlockStyle}"
                       Text="LC POINTS" />
            <TextBlock Margin="0,-6.5,0,26.5"
                       CharacterSpacing="{ThemeResource PivotHeaderItemCharacterSpacing}"
                       Foreground="DarkGreen"
                       Style="{ThemeResource HeaderTextBlockStyle}"
                       Text="View Grades" />
        </StackPanel>
        <!--  TODO: Content should be placed within the following grid  -->
        <Grid x:Name="ContentRoot"
              Grid.Row="1"
              Margin="19,9.5,19,0">
            <ListBox Height="400"
                     Margin="0,0,0,-329"
                     VerticalAlignment="Top"
                     ItemsSource="{Binding AddedSubjectGradePairs}">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <TextBlock>
                            <Run Text="{Binding Subject}" /><Run Text=" - " /><Run Text="{Binding Points}" />
                        </TextBlock>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>
    </Grid>
</Page>

查看背后代码:

namespace LC_Points.View
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class ViewSubjectGradePage : Page
    {
        private NavigationHelper navigationHelper;
        private ObservableDictionary defaultViewModel = new ObservableDictionary();

        public ViewSubjectGradePage()
        {
            this.InitializeComponent();
            this.navigationHelper = new NavigationHelper(this);
            this.navigationHelper.LoadState += this.NavigationHelper_LoadState;
            this.navigationHelper.SaveState += this.NavigationHelper_SaveState;
            var messageDialog = new MessageDialog(DataContext.GetType().ToString());
            messageDialog.ShowAsync();
        }
        /// <summary>
        /// Gets the <see cref="NavigationHelper"/> associated with this <see cref="Page"/>.
        /// </summary>
        public NavigationHelper NavigationHelper
        {
            get { return this.navigationHelper; }
        }

        /// <summary>
        /// Gets the view model for this <see cref="Page"/>.
        /// This can be changed to a strongly typed view model.
        /// </summary>
        public ObservableDictionary DefaultViewModel
        {
            get { return this.defaultViewModel; }
        }

        private void NavigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
        }

        private void NavigationHelper_SaveState(object sender, SaveStateEventArgs e)
        {
        }
        #region NavigationHelper registration

        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            this.navigationHelper.OnNavigatedTo(e);
        }
        protected override void OnNavigatedFrom(NavigationEventArgs e)
        {
            this.navigationHelper.OnNavigatedFrom(e);
        }
        #endregion
    }
}

    public ViewSubjectGradePage()
    {
        this.InitializeComponent();
        this.navigationHelper = new NavigationHelper(this);
        this.navigationHelper.LoadState += this.NavigationHelper_LoadState;
        this.navigationHelper.SaveState += this.NavigationHelper_SaveState;
        this.DataContext = this.DefaultViewModel;
    }