Listbox does not update when I change the connected list

Question

I have a List Box in WPF and I want the content of the list to change everytime the values are changed in code.

At the start of my program I insert the default values in the ListBox and this works well.

using System; using System.Collections.Generic; using System.Linq;
using System.Text; using System.Threading.Tasks; using System.ComponentModel;
using System.Collections.ObjectModel;

public partial class MainWindow : Window
{
    //here is my data which goes into the list
    private DataTable _dataTable1 = null;
    //list which goes into ListBox
    private List<CompareListItem> _compareListItems1 = null;
    public MainWindow()
    {
        InitializeComponent();
        // ..Code missing which writes data in _dataTable
        //ReloadCompareList fills the _compareListItems1 with data from _dataTable1
        _compareListItems1 = ReloadCompareList(_dataTable1);
        //here I do the binding to the ListBox
        compareSelectionList1.ItemsSource = _compareListItems1;
    }

But when I change the values here it does not affect the ListBox

        //this method is called when i want to replace the entire _compareListItems1 list
        private void tableList1_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            //this method write my data into _dataTable
            _dataTable1 = ReloadTableList(tableList1, _dataTable1, tableGrid1);
            if (_dataTable1 != null)
            {
                //the values of my compare list are replaced, but nothing happens with the ListBox
                _compareListItems1 = ReloadCompareList(_dataTable1);    // ESSENTIAL LINE

            }
        }
}

Each item in my ListBox will be a CompareListItem. I found here on stackoverflow the following topic about INotifyPropertyChanged and I implemented this here. It works when i update a single object in my list.

// Class for the items displayed in the Listbox of the compare list
public class CompareListItem : INotifyPropertyChanged
{
    private string itemTitle;
    public string ItemTitle
    {
        get{return itemTitle;}
        set{
            //this works when a single value in the list is changed, but not if i add or delete someting
            SetField(ref itemTitle, value, "ItemTitle");
        }
    }

    public CompareListItem(string title)    {
        //does not affect the data bindings, could be "itemTitle = title;" to
        SetField(ref itemTitle, title, "ItemTitle");
    }
    //this is from https://stackoverflow.com/questions/1315621/implementing-inotifypropertychanged-does-a-better-way-exist
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value))
            return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }
}

edit: here the XAML of my ListBox:

<ListBox x:Name="compareSelectionList1" Margin="10,0,10,10" IsSynchronizedWithCurrentItem="False"  Grid.Row="1" Height="100" VerticalAlignment="Bottom" SelectionMode="Multiple">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <Grid Margin="0,2">
                <TextBlock Text="{Binding ItemTitle, Mode=TwoWay, diag:PresentationTraceSources.TraceLevel=High}"></TextBlock>
            </Grid>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

When I add the following line it will work. But I think that is not the meaning of the approach with the DataBindings. I understood the approach like "you mention the view-element once in the code and then you never use the name "compareSelectionList1" again."

_compareListItems1 = ReloadCompareList(_dataTable1);    // ESSENTIAL LINE
compareSelectionList1.ItemsSource = _compareListItems1;

How can I replace my list so that the ListBox will be updated to via the data-binding?

score 2 · Accepted Answer · answered Jan 08 '14 at 14:31

You're setting _compareListItems1 to a new instance of List<CompareListItem>, but compareSelectionList1.ItemsSource still refers to the previous instance. That's why you need to reassign ItemsSource for it to work.

But I think that is not the meaning of the approach with the DataBindings

Currently, you're not using a binding to set the ItemsSource, so it can't be refreshed automatically. To do that, you need to expose the list as a property, and implement INotifyPropertyChanged in your Window (another option is to expose it as a dependency property). In XAML, bind ItemsSource to the list property, and it will work as expected.

I'll finish it tomorrow, but I'm pretty sure that this will work, thanks! Else I'd ask here again. — Marc Trittibach, Jan 08 '14 at 15:34

Listbox does not update when I change the connected list

1 Answers1