Question

My WPF C# program has a listbox that holds items that the user can manipulate: changing the order, copy/paste, etc. Currently, when I select an item in the listbox, then click the move up button, the item will move up the list, but then the item is no longer highlighted or selected. So, I cannot do consecutive manipulations without reselecting the listbox item.

How can I force my listbox to retain it’s selection and highlighting?

Answer 1

I would bind the ListBox’s ItemsSource to an ObservableCollection. Then you can manipulate the ObservableCollection and the ListBox will be updated for you. Here’s an example:

The XAML:

<Window x:Class="ListBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:ListBox="clr-namespace:ListBox" Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <ListBox:ListBoxViewModel />
    </Window.DataContext>
    <StackPanel>
        <Button Command="{Binding Up}">Up</Button>
        <Button Command="{Binding Down}">Down</Button>
        <ListBox Grid.Column="0" ItemsSource="{Binding Items}" SelectedIndex="{Binding SelectedIndex, Mode=TwoWay}" />
    </StackPanel>
</Window>

and the code:

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows.Input;
using GalaSoft.MvvmLight.Command;

namespace ListBox
{
    public class ListBoxViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        public ObservableCollection<string> Items {get; private set; }

        private void ExecuteUp()
        {
            if (SelectedIndex == 0)
                return;
            Items.Move(SelectedIndex, SelectedIndex - 1);
        }
        private void ExecuteDown()
        {
            if (SelectedIndex >= Items.Count - 1)
                return;
            Items.Move(SelectedIndex, SelectedIndex + 1);
        }

        public ICommand Up { get; private set; }
        public ICommand Down { get; private set; }

        private int m_SelectedIndex = 0;
        public int SelectedIndex
        {
            get { return m_SelectedIndex; }
            set
            {
                m_SelectedIndex = value;
                OnPropertyChanged("SelectedIndex");
            }
        }

        public ListBoxViewModel()
        {
            Items = new ObservableCollection<string>() {"London", "Paris", "Berlin"};
            Up = new RelayCommand(ExecuteUp);
            Down = new RelayCommand(ExecuteDown);
        }

        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}