WPF VB.net ListBox how to get selected item index or a sorted & grouped list relative to item source

listboxvb.netwpf

Good evening all,

Firstly thank you for the time taken to read this.
I'm having some difficulty with a sorted & grouped listbox in WPF vb.net 3.5 that has an items source bound to an ObservableCollection.

What I want to be able to do is to retrieve a piece of data from the ObservableCollection items source depending on what item in the listbox the user has selected.

I've almost got it working but because the listbox is sorted it does not match the index of the items source.

Here is the code that I have so far:

Dim i As Integer = lstBox1.Items.IndexOf(lstBox1.SelectedItem)
MessageBox.Show(myListSource.Item(i).Description.ToString, "Source Description")

As I previously mentioned, because the lstBox is sorted and also grouped the index's don't match up.

Does anyone know how I can get the correct information I want from the List Source depending on the selected item in the list box?

Again, thank you very much for your time,

Rob

Best Answer

You can bind the ItemsSource to the ObservableCollection<Foo> and bind the SelectedItem to an instance of Foo.

That way, you have removed the dependency on the index of the list - you can group and sort as you wish - by selecting an item in the list, the current instance in you backing class (probably a ViewModel) will update through the Binding.

It should look something like this

<ListBox
    ItemsSource="{Binding MyCollection}"
    SelectedItem="{Binding CurrentSelection}" />

and in the code (ViewModel) acting as the DataContext for the view...

Private _myCollection As ObservableCollection(Of Foo)
Public Property MyCollection As ObservableCollection(Of Foo)
Get
    Return _myCollection
End Get

Private _currentItem As Foo
Public Property CurrentItem As Foo
Get
    Return _currentItem
End Get
Set(ByVal value As Foo)
    Me._currentItem = value
End Set

(with apologies if I have the vb syntax wrong)

So if you need to access the ListBox's SelectedItem in your ViewModel, you can just use the CurrentItem property...

MessageBox.Show(CurrentItem.Description.ToString, "Source Description")

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C# – Listbox scrollbar doesn’t follow selected item (with ICollectionView)

I've found the answer. I needed to use

myListBoxItem.BringIntoView();

The problem was that I didn't wanted to add any code-behind, as I am implementing MVVM.

The solution is using Attached Behaviors. Josh Smith has a great article about this: Introduction to Attached Behaviors in WPF.

I adedd a Setter to the style of the items in the ListBox:

<ListBox.ItemContainerStyle>
    <Style TargetType="{x:Type ListBoxItem}">
        <Setter
            Property="custom:ListBoxItemBehavior.IsBroughtIntoViewWhenSelected"
            Value="True" />
    </Style>
</ListBox.ItemContainerStyle>

And added the following class (only changed TreeView from Josh's article to ListBox):

public static class ListBoxItemBehavior
{
    #region IsBroughtIntoViewWhenSelected

    public static bool GetIsBroughtIntoViewWhenSelected(ListBoxItem listBoxItem)
    {
        return (bool)listBoxItem.GetValue(IsBroughtIntoViewWhenSelectedProperty);
    }

    public static void SetIsBroughtIntoViewWhenSelected(
      ListBoxItem listBoxItem, bool value)
    {
        listBoxItem.SetValue(IsBroughtIntoViewWhenSelectedProperty, value);
    }

    public static readonly DependencyProperty IsBroughtIntoViewWhenSelectedProperty =
        DependencyProperty.RegisterAttached(
        "IsBroughtIntoViewWhenSelected",
        typeof(bool),
        typeof(ListBoxItemBehavior),
        new UIPropertyMetadata(false, OnIsBroughtIntoViewWhenSelectedChanged));

    static void OnIsBroughtIntoViewWhenSelectedChanged(
      DependencyObject depObj, DependencyPropertyChangedEventArgs e)
    {
        ListBoxItem item = depObj as ListBoxItem;
        if (item == null)
            return;

        if (e.NewValue is bool == false)
            return;

        if ((bool)e.NewValue)
            item.Selected += OnListBoxItemSelected;
        else
            item.Selected -= OnListBoxItemSelected;
    }

    static void OnListBoxItemSelected(object sender, RoutedEventArgs e)
    {
        // Only react to the Selected event raised by the ListBoxItem
        // whose IsSelected property was modified.  Ignore all ancestors
        // who are merely reporting that a descendant's Selected fired.
        if (!Object.ReferenceEquals(sender, e.OriginalSource))
            return;

        ListBoxItem item = e.OriginalSource as ListBoxItem;
        if (item != null)
            item.BringIntoView();
    }

    #endregion // IsBroughtIntoViewWhenSelected
}

It works!!

Wpf – Listbox with checkbox not triggering selected item when checkbox is checked/unchecked

Because you set SelectionMode="Multiple", you cannot use SelectedItem.
You also cannot bind to SelectedItems because this property is read-only.

Not all is lost because in your code you bind IsSelected to IsOperationSelected

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

So now you can process the selected items using IsOperationSelected as indicated in the following example in your ViewModel:

foreach (var operationsInfo in OperationsInfoCol)
{
  if ( operationsInfo.IsOperationSelected)
  {
    // do something...
  }
}

Best Answer

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