How to program "Selection Changing" event in ListBox in WPF?

Question

I have a listbox and I want to prevent changing of ListBox's selection if user has not finished certain tasks, this is the best explanation i can provide at this time, in WinForms there used to be selection changing and it has cancellable event arguement, where in we could trap and cancel even of changing the selection.

I thought I will inherit listbox and do something but internally all functionalities in Selector class are hidden that I can see in reflector but there is no way i can inherit and override any methods !!

Answer 1

I know this doesn't directly answer your question, but in most cases (I'd need to be convinced of a reason not to), I would simply not enable to control until the criteria for selection is met.

This simple step removes most of the complexity of figuring out that the value changed, and if it was a valid change, etc. If you allow the comboxbox to be edited (i.e. a value typed in), that adds another level of complexity.

Otherwise, here is a related discussion: http://stackoverflow.com/questions/314503/how-to-prevent-cancel-a-comboboxs-value-change-in-c

Answer 2

One solution would be to make the ListBox and ListBoxItems not focusable until you are ready for the user to change them. Here is a quick mock-up that accomplished this:

XAML:

<StackPanel>
        <ListBox x:Name="LB">
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}"  Content="item 1"/>
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}" Content="item 2"/>
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}" Content="item 3"/>
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}" Content="item 4"/>
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}" Content="item 5"/>
            <ListBoxItem Focusable="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=Focusable}" Content="item 6"/>
        </ListBox>
        <Button Content="Lock/Unlock" Click ="Button_Click"/>
    </StackPanel>

Code:

   private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (LB.Focusable == true)
            LB.Focusable = false;
        else
            LB.Focusable = true;
    }

Answer 3

Bind IsSelected to a property in your view model class, and handle the case in the property's setter, e.g.:

public bool IsSelected
{
   get { return _IsSelected; }
   set
   {
       if (value && DisableSelection)
       {
          AlertUser();
       }
       else
       {
          _IsSelected = value;
       }
       OnPropertyChanged("IsSelected");
   }
}

Note that you raise the PropertyChanged event even if the property didn't change, because from the view's perspective it actually did change.

Answer 4

I derived a class MyListBox from ListBox and added event called SelectionChanging which is cancellable event. Then I used MyListBoxItem as ItemContainer in MyListBox which handles Preview Left Mouse Up event and raises Selection Changing event, on cancel value, I mark event as handled, which prevents new selection as well as it allows me to notify user to do someting.