WPF: Bind to element in DataTemplate in ResourceDictionary

Question

I'd like to bind to a element's property (a ListBox's SelectedItems.Count in my specific case) that's dynamically inserted into my window from a DataTemplate located in a ResourceDictionary. I'd like to enable/disable a button when the count reaches a certain number of ListBoxItems are selected. I thought this would work:

<Style TargetType="Button">
  <Style.Triggers>
    <DataTrigger Binding="{Binding Source={StaticResource myResourceKey}, Path=myListBox.SelectedItems.Count}" Value="25">
      <Setter Property="IsEnabled" Value="False"/>
    </DataTrigger>
  </Style.Triggers>
</Style>

But I'm getting the following error:

System.Windows.Data Error: 40 : BindingExpression path error: 'myListBox' property not found on 'object' ''DataTemplate' (HashCode=50217655)'. BindingExpression:Path=aoiListBox.SelectedItems.Count; DataItem='DataTemplate' (HashCode=50217655); target element is 'Button' (Name='myBtn'); target property is 'NoTarget' (type 'Object')

How can I achieve this binding? Thanks in advance.

Answer 1

Well you can write a workaround, but I strongly recomment not to implement it that way. Consider, that a style in a ResourceDictionary is an empty resource, which should be decoupled from any specific instance (in your case myListBox) in your application. Problem is, that you cannot use this malformed style on another Button. So you don't need to, better you shouldn't, declare it as resource.

I definitely recomment to declare this Style directly in the Button. E.g.

<ListBox x:Name="myListBox" />
<Button>
    <Button.Style>
        <Style TargetType="Button">
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=myListBox, 
                             Path=SelectedItems.Count}" Value="25">
                    <Setter Property="IsEnabled" Value="False"/>
                </DataTrigger>
            </Style.Triggers>
        </Style> 
    </Button.Style>
</Button>

Additionally, I would use a Binding via the ElementName property.