转换器中的 WPF 多重绑定失败 ==> DependencyProperty.UnsetValue

Question

我的代码在启动时失败，因为Multibinding调用的Converter中的值数组未填充适当的值，但具有 DependencyProperty.UnsetValue的值。

看一下转换器，看看我在哪里得到错误

public class ButtonColorConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            string val1 = string.Format("  {0}  ", values[0]);
            string val2 = (string)values[1];  **//Here i am getting ==> {DependencyProperty.UnsetValue}** 
            return val1.Equals(val2)
                ? Brushes.NavajoWhite
                : Brushes.White;  
        }
        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

你可以下载完整的代码或看到我的代码片段如下。

MainWindow.xaml

<Window x:Class="DataPager.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        xmlns:Local="clr-namespace:DataPager.Convertor_For_BackGround">
    <Grid>
        <Grid.Resources>
            <Local:ButtonColorConverter x:Key="currentPageSetter"/>
        </Grid.Resources>
        <Grid.RowDefinitions>
            <RowDefinition Height="36*" />
            <RowDefinition Height="275*" />
        </Grid.RowDefinitions>
        <ItemsControl Name="pageControl" ItemsSource="{Binding Path=PageCollection}" Grid.Row="0">
            <ItemsControl.Template>
                <ControlTemplate TargetType="ItemsControl">
                    <Border >
                        <StackPanel>
                            <ItemsPresenter></ItemsPresenter>
                        </StackPanel>
                    </Border>
                </ControlTemplate>
            </ItemsControl.Template>
            <ItemsControl.ItemsPanel x:Uid="pageItemTemplate">
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Button x:Name="pageNumberButton" Margin="3,4" Content="{Binding Path=Page_Number}">
                        <Button.Background>
                            <MultiBinding Converter="{StaticResource currentPageSetter}">
                                <Binding Path="Page_Number" />
                                <Binding Path="CurrentPage.Page_Number" /> **//This Binding not resolves properly**
                            </MultiBinding>
                        </Button.Background>
                    </Button>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
        <TextBox Text="{Binding Path=CurrentPage.Page_Number,Mode=TwoWay, FallbackValue=asdf}" Grid.Row="1" Height="23"  Margin="79,62,257,0" Name="textBox1" VerticalAlignment="Top" Width="167" />
    </Grid>
</Window>

MainWindow.xaml.cs

 public partial class MainWindow : Window
    {
        public MainWindow()
        {

            MyPageViewModel = new PageViewModel();
            MyPageViewModel.PageCollection.Add(new PageNumberViewModel(string.Format("  {0}  ",0)));
            MyPageViewModel.PageCollection.Add(new PageNumberViewModel(string.Format("  {0}  ",1)));
            MyPageViewModel.PageCollection.Add(new PageNumberViewModel(string.Format("  {0}  ",2)));
            MyPageViewModel.PageCollection.Add(new PageNumberViewModel(string.Format("  {0}  ",3)));
            InitializeComponent();
        }
        public PageViewModel MyPageViewModel
        {
            get
            {
                return this.DataContext as PageViewModel;
            }
            set
            {
                this.DataContext = value;
            }
        }
    }

这是ViewModel Classes

PageViewModel.cs

public class PageViewModel:ViewModelBase
    {
        private ObservableCollection<PageNumberViewModel> m_pageCollection = new ObservableCollection<PageNumberViewModel>();
        private PageNumberViewModel m_currentPage = new PageNumberViewModel(string.Format("  {0}  ",0));
        public PageViewModel()
        {
            m_currentPage = new PageNumberViewModel(string.Format("  {0}  ", 1000));
        }
        public PageNumberViewModel CurrentPage 
        {
            get
            {
                return this.m_currentPage;
            }
            set
            {
                if (m_currentPage == value)
                    return;
                this.m_currentPage = value;
                base.OnPropertyChanged("CurrentPage");
            }
        }
        public ObservableCollection<PageNumberViewModel> PageCollection
        {
            get
            {
                return this.m_pageCollection;
            }
            set
            {
                if (m_pageCollection == value)
                    return;
                this.m_pageCollection = value;
                base.OnPropertyChanged("PageCollection");
            }
        }
    }

PageNumberViewModel.cs

public class PageNumberViewModel : ViewModelBase
    {
        private string m_pageNumber;
        public PageNumberViewModel()              
        {
        }
        public PageNumberViewModel(string pageNumgerArg)
        {
            this.m_pageNumber = pageNumgerArg;
        }
        public string Page_Number
        {
            get { return m_pageNumber; }
            set
            {
                if (m_pageNumber == value)
                    return;
                m_pageNumber = value;
                OnPropertyChanged("PageNumber");
            }
        }
    }

Answer 1

当您在任何ItemsSource中设置列表时，单个项目的DataTemplate的DataContext将为列表中的每个项目设置。

你的TextBlock绑定工作正确，因为DataContext被设置为你的主要对象:PageViewModel

但是在您的DataTemplate中，DataContext将被设置为PageNumberViewModel，因为这些是您的集合中的项目。

因此，与Path=CurrentPage.Page_Number的绑定将导致UnsetValue，因为CurrentPage不是PageNumberViewModel的属性

希望这能澄清一些事情!

如果你真的想绑定Window的DataContext的CurrentPage属性，考虑使用ElementName绑定:

给window一个名字，绑定到

<Binding ElementName="name" Path="DataContext.CurrentPage.Page_Number" />

或使用RelativeSource绑定:

<Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}" Path="DataContext.CurrentPage.Page_Number" />

Answer 2

看起来您可能没有正确的DataContext来解析路径CurrentPage.Page_Number。调试这类事情的一个好方法是删除路径，以便您可以检查值转换器中的DataContext:

<Binding Path="." />

然后在你的ButtonColorConverter中设置一个断点，看看你到底想转换什么

Answer 3

DependencyProperty.UnsetValue仅仅是DependencyProperty类中的一个常量。

你可以这样做:

 if (values[1] == DependencyProperty.UnsetValue)
 {
     return null;  // or default value
 }

Answer 4

我也遇到了这个问题，并在另一个帖子(https://stackoverflow.com/a/3139397/500099)中找到了解决方案。关键是像这样使用FallbackValue="属性:

<MultiBinding Converter="{StaticResource StringFormatConverter}">
    <Binding Path="ResultValueControl.Min" FallbackValue=""/>
    <Binding Path="Format" />
</MultiBinding>

Answer 5

我刚刚更改了主窗口。xaml按照Mr。Arcturus的建议和它的工作良好。

非常感谢Mr.Arcturus

可以看到ManiWindow。xaml after change

<Window x:Class="DataPager.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        xmlns:Local="clr-namespace:DataPager.Convertor_For_BackGround">
    <Grid>
        <Grid.Resources>
            <Local:ButtonColorConverter x:Key="currentPageSetter"/>
        </Grid.Resources>
        <Grid.RowDefinitions>
            <RowDefinition Height="36*" />
            <RowDefinition Height="275*" />
        </Grid.RowDefinitions>
        <ItemsControl Name="pageControl" ItemsSource="{Binding Path=PageCollection}" Grid.Row="0">
            <ItemsControl.Template>
                <ControlTemplate TargetType="ItemsControl">
                    <Border >
                        <StackPanel>
                            <ItemsPresenter></ItemsPresenter>
                        </StackPanel>
                    </Border>
                </ControlTemplate>
            </ItemsControl.Template>
            <ItemsControl.ItemsPanel x:Uid="pageItemTemplate">
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Button x:Name="pageNumberButton" Margin="3,4" Content="{Binding Path=Page_Number}">
                        <Button.Background>
                            <MultiBinding Converter="{StaticResource currentPageSetter}">
                                <Binding Path="Page_Number" />
                                **<Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}" Path="DataContext.CurrentPage.Page_Number" />** 
                            </MultiBinding>
                        </Button.Background>
                    </Button>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
        <TextBox Text="{Binding Path=CurrentPage.Page_Number,Mode=TwoWay, FallbackValue=asdf}" Grid.Row="1" Height="23"  Margin="79,62,257,0" Name="textBox1" VerticalAlignment="Top" Width="167" />
        <Button Content="Button" Grid.Row="1" Height="23" HorizontalAlignment="Left" Margin="121,110,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
    </Grid>
</Window>