在使用 ItemsSource 之前，项目集合必须为空

问题描述

如果我将 DataTrigger 放在一个简单的列表框中，我会得到这个运行时异常:

If I put a DataTrigger in a simple Listbox I get this runtime exception:

Items 集合在使用 ItemsSource 之前必须为空

Items collection must be empty before using ItemsSource

我的列表框没有数据触发器(也不例外):

My listbox without datatrigger (no exception):

<ListBox ItemsSource="{Binding EdgedBoards}" SelectedItem="{Binding SelEdgedBoard, Mode=TwoWay}" DisplayMemberPath="Name">
    <ListBox.ItemContainerStyle>
        <Style TargetType="{x:Type ListBoxItem}" 
               BasedOn="{StaticResource {x:Type ListBoxItem}}">

            <Setter Property="IsSelected"
                    Value="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

我的列表框与 DataTrigger:

<ListBox ItemsSource="{Binding EdgedBoards}" SelectedItem="{Binding SelEdgedBoard, Mode=TwoWay}" DisplayMemberPath="Name">
    <Style TargetType="{x:Type ListBox}" BasedOn="{StaticResource {x:Type ListBox}}">
        <Setter Property="Focusable" Value="True" />

        <Style.Triggers>
            <DataTrigger Binding="{Binding ElementName=EdgedBoardsAdd_UC, Path=Visibility}" Value="Visible">
                <Setter Property="Focusable" Value="False" />
            </DataTrigger>
        </Style.Triggers>
    </Style>

    <ListBox.ItemContainerStyle>
        <Style TargetType="{x:Type ListBoxItem}" BasedOn="{StaticResource {x:Type ListBoxItem}}">
            <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

后面的代码有什么问题?

What's wrong with the latter code?

推荐答案

你没有正确声明样式，所以它被设置为列表框的 content - 你手动声明了一个包含单个样式的列表.

You aren't declaring the style correctly, and so it's being set as the content of the listbox - you are manually declaring a list that contains a single style.

你应该用 <ListBox.Style> 元素包裹你现有的 Style 元素来解决这个问题.

You should wrap your existing Style element with the <ListBox.Style> element to fix this problem.

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