ContentControl与DataTemplateSelector和绑定内容

Question

I have the following ContentControl two-way binded with a Current property that shows the current selection of my ListView .

<ContentControl Content="{Binding Current}" ContentTemplateSelector="{StaticResource myTemplateSelector}">

It uses a DataTemplateSelector that returns 1 of 2 DataTemplates , one when the Content is null and the other one when it is not.

When I start my app the Current property is null and the correct DataTemplate is applied.

When I select one ListViewItem , Current property is not null and the ContentControl DataTemplate changes as it should.

When I unselect the ListViewItem , Current property is null again and the ContentControl disappears and none of the DataTemplates are applied.

I made breakpoints and output some data to the console and I discover that DataTemplateSelector does not runs when Current changes to null. However it runs the first time when it is also null. So I don't understand why is this happening. Anyone had this issue or a similar one?

Answer 1

When you deselect the item, selected item of the list box becomes null, thus the binding to Current cannot find the property.

In this case, trying to access Current doesn't return null it returns null reference exception which is swallowed by binding.

I'm assuming that you bind to the selected item (whether directly off the list box or routed through view model) in some way. Something like:

<Grid DataContext="{Binding SelectedItem}">
    <ContentControl ..../>
</Grid>

If so, you'd need to change bindings and the logic of template selector:

At ContentControl:

Content="{Binding }"

In related data templates, you'd need to add Current. at the beginning of all paths in all bindings.

In data template selector instead of checking the value, check the Current property (and if the casting of value fails, consider it null).