【发布时间】：2015-06-07 01:08:48
【问题描述】：
我正在尝试将 View 的数据上下文设置为其 ViewModel 中包含的列表。但是当我测试了当前的设置时，似乎 ViewModel 和 View 之间的data context 设置不正确。
为了调试这个问题，我在视图的构造函数中设置了一个消息框，我收到以下错误消息，提示数据上下文设置不正确：“对象引用未设置为实例对象的"
该列表还在另一个 ViewModel 中使用，显示该列表不为空，这进一步暗示了数据上下文问题。
有谁知道在 View 和 ViewModel 之间设置数据上下文的缺陷是什么？
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; }
}
}
这是 View 和 View 背后的代码：
<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>
查看后面的代码：
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
}
}
【问题讨论】：
-
尝试在
OnNavigatedTo或等效事件中显示对话框。不在构造函数上。 -
这里是一个 -1 表示异常的快照。永远不要在问题中这样做。看看那个对话框——在底部它说“将异常详细信息复制到剪贴板”。您单击该链接，然后将其粘贴到edit。一张异常的照片比无用更糟糕。
标签： c# listview design-patterns mvvm win-universal-app