WPF ComboBox SelectedItem 在 TabControl 开关上设置为 Null [英] WPF ComboBox SelectedItem Set to Null on TabControl Switch

问题描述

我的 WPF 应用程序有一个简单的问题，让我把头撞在桌子上.我有一个 TabControl，其中每个 TabItem 都是使用类似于以下的 DataTemplate 为 ViewModel 生成的视图:

I've got a simple problem in my WPF application which has me banging my head on the table. I have a TabControl, where every TabItem is a View generated for a ViewModel using a DataTemplate similar to this:

<DataTemplate DataType="{x:Type vm:FooViewModel}">
    <vw:FooView/>
</DataTemplate>

FooView 包含一个组合框:

FooView contains a ComboBox:

<ComboBox ItemsSource="{Binding Path=BarList}" DisplayMemberPath="Name" SelectedItem="{Binding Path=SelectedBar}"/>

和 FooViewModel 包含一个简单的属性: public Bar SelectedBar { get;放;}.我的问题是，当我为 ComboBox 设置值时，更改到另一个选项卡，然后再改回来，ComboBox 再次为空.如果我在我的属性的 setter 上设置断点，当我切换到另一个选项卡时，我会看到该属性被分配给 null.

and FooViewModel contains a simple Property: public Bar SelectedBar { get; set; }. My problem is that when I set the value for my ComboBox, change to another tab, then change back, the ComboBox is empty again. If I set a breakpoint on the setter for my property, I see that the property is assigned to null when I switch to another tab.

据我所知，当一个选项卡被切换时，它会从 VisualTree 中删除 - 但为什么它将我的 ViewModel 的属性设置为 null?这让我很难保持持久状态，并且检查 value != null 似乎不是正确的解决方案.任何人都可以摆脱这种情况吗?

From what I understand, when a tab is switched, it is removed from the VisualTree - but why is it setting my ViewModel's property to null? This is making it very difficult for me to hold persistent state, and checking value != null does not seem like the right solution. Can anyone shed some like on this situation?

setter 断点处的调用堆栈仅显示 [外部代码] - 那里没有提示.

The call stack at the setter breakpoint only shows [External Code] - no hints there.

推荐答案

我们刚刚遇到了同样的问题.我们找到了一个描述该问题的博客条目.看起来它是 WPF 中的一个错误，并且有一个解决方法:在 ItemsSource 绑定之前指定 SelectedItem 绑定，问题应该消失了.

we just ran into the same problem. We found a blog entry describing the problem. It looks like it is a bug in WPF and there is a workaround: Specify the SelectedItem binding before the ItemsSource binding and the problem should be gone.

博客文章链接:

http://www.metanous.be/pharcyde/post/Bug-in-WPF-combobox-databinding.aspx

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