多重绑定中的DependencyProperty.unsetValue [英] DependencyProperty.unsetValue in a multibinding

问题描述

我有一个listView项是 Book实例，当我单击一本书时，组合框应显示其关键字；实际上，这有点棘手：组合框包含所有书籍（已删除重复项）的所有关键字的列表（comboboxItems为复选框），并且选中了所选书籍的所有关键词。
是多重绑定：

I have a listView of items which are 'Book' instances, and when I click on a book the combobox should display its keywords; in fact it's a little trickier : the combobox contains the list of all the keywords of all books (duplicates removed)(the comboboxItems are checkboxes), and those of the selected book are checked. here is the multibinding:

<ComboBox
                        x:Name="cbb_Keywords"
                        Grid.Column="2"
                        Width="300"
                        Margin="5,0,0,0"
                        HorizontalAlignment="Left"
                        ItemsSource="{Binding Source={StaticResource AllBooks}}"
                        DataContext="{Binding ElementName=listBoxBooks,Path=SelectedItem,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}">

                        <ComboBox.ItemTemplate>
                            <DataTemplate>
                                <StackPanel Orientation="Horizontal">
                                    <CheckBox Width="200">
                                        <CheckBox.IsChecked>
                                            <MultiBinding Converter="{StaticResource TextInListTrueFalseConverter}" >
                                                <Binding Path="KeywordsForTextbox"></Binding>
                                                <Binding RelativeSource="{RelativeSource Self}" Path="Content"></Binding>
                                            </MultiBinding>
                                        </CheckBox.IsChecked>
                                    </CheckBox>
                                </StackPanel>
                            </DataTemplate>
                        </ComboBox.ItemTemplate>
                    </ComboBox>

当我运行程序时，当我单击一本书时似乎还可以，但出现异常当我单击组合框时：不可能从'MS.Internal.NamedObject'转换为'System.String'类型。我看到value [0]是UnsetValue。

When I run my program, is seems ok when I click on a book, but I get an exception when I click on the combobox : impossible cast from 'MS.Internal.NamedObject' to 'System.String' type. I saw that value[0] is UnsetValue.

在调试时，当我使用间谍跟踪WpfApp1.App.Books [0] .KeywordsForTextbox的值时，我的价值很高（一个字符串，它是Book [0]的关键字的列表。也许问题出在listboxBooks.SelectedItem.KeywordsForTextBox？我无法在VS中窥探 listboxBooks的值。

At debugging, when I use spies to track the value of WpfApp1.App.Books[0].KeywordsForTextbox, it gives me the good value (a string which is the list of the keywords of Book[0]. maybe the problem comes from listboxBooks.SelectedItem.KeywordsForTextBox? I can't spy in VS the value of 'listboxBooks'.

一些相关内容...
MainWindow的构造函数的开头：

some related content... the beginning of the constructor of MainWindow:

public MainWindow()
        {


            InitializeComponent();
            listBoxBooks.ItemsSource = App.Books;

转换器的转换方法：

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var check = false;
            if ((values != null && values.Length == 2))
            {
                string listString = (string)values[0];
                string wordToFind = (string) values[1];
                if ((listString != null))
                {
                    List<string> keywordsList = listString.Split(',').ToList();
                    if (keywordsList.Contains(wordToFind)) check = true;
                }

            }

            return check;


        }

KeywordsForTextbox方法：

the KeywordsForTextbox method:

public string KeywordsForTextbox
        {
            get { return string.Join(",", _keywords); }

        }

最终实现AllBooks ：（作为一个窗口资源）

and finally the implementation of AllBooks:(as a window resource)

<ObjectDataProvider
            x:Key="AllBooks"
            MethodName="listOfAllKeywords"
            ObjectType="{x:Type mangmt:BookManagement}" />

谢谢。

推荐答案

Multi的第一个绑定应该是到Books ListBox中的SelectedItem。我在适当的地方添加了< CheckBox.IsChecked> ，并将Content = {Binding}添加到CheckBox中：

The first Binding of the Multi should be to be to the SelectedItem in the ListBox of Books. I have added in the <CheckBox.IsChecked> where appropriate, and Content="{Binding}" to the CheckBox:

                <ComboBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <CheckBox Width="200" Content={Binding}>
                              <CheckBox.IsChecked>
                                <MultiBinding Converter="{StaticResource TextInListTrueFalseConverter}" >
                                    <Binding ElementName=listBoxBooks, Path=SelectedItem.KeywordsForTextbox"></Binding>
                                    <Binding RelativeSource="{RelativeSource Self}" Path="Content"></Binding>
                                </MultiBinding>
                              </CheckBox.IsChecked>
                            </CheckBox>
                        </StackPanel>
                    </DataTemplate>
                </ComboBox.ItemTemplate>

您可能还希望向IMultiValueConverter添加一些验证以确保传递的值未设置，以避免发生异常：如果VB中的not values（0）是DependencyProperty.UnsetValue而不是values（1）是DependencyProperty.UnsetValue然后

You may also wish to add some validation to the IMultiValueConverter to make sure the passed values are not unset, to avoid an exception: If Not values(0) Is DependencyProperty.UnsetValue And Not values(1) Is DependencyProperty.UnsetValue Then in VB.

关于选中复选框的行为，我猜这是因为ConvertBack IMul​​tiValueConverter的方法。您可以删除抛出异常代码，并编写一种方法来将选中/未选中框的文本添加/删除到关键字列表中。

Regarding the behaviour on checking the checkbox, I am guessing this is because of the ConvertBack Method of the IMultiValueConverter. You can remove the 'Throw Exception' code, and write a method to add/remove the text of the checked/unchecked box to your keyword list.

这篇关于多重绑定中的DependencyProperty.unsetValue的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！