Question

I have a list box with a bunch of values in it. I also have an UP button and a DOWN button.
With these buttons, I would like to move the selected item in the list box up/down. I am having trouble doing this.

Here is my code so far:

    private void btnDataUp_Click(object sender, RoutedEventArgs e)
    {

        int selectedIndex = listBoxDatasetValues.SelectedIndex; //get the selected item in the data list

        if (selectedIndex != -1 && selectedIndex != 0) //if the selected item is selected and not at the top of the list
        {
            //swap items here
            listBoxDatasetValues.SelectedIndex = selectedIndex - 1; //keep the item selected
        }


    }

I do not know how to swap the values! Any help would be GREATLY appreciated!

Answer 1

Since you have populated the listbox by binding to a ObservableCollection using ItemsSource, you cant modify the Items property of the listbox.

ItemsSource can be set only when the Items collection is empty, and Items can be modified only when ItemsSource is null.

Otherwise you will get the error “Operation is not valid while ItemsSource is in use…”

What you have to do, is modify the underlying collection, and because it’s an ObservableCollection, the ListBox will reflect the changes.

The following code shows how you can move an item up and down by swapping the item in the collection.

The corresponding XAML just contains a listbox called lbItems and 2 buttons which hook up the eventhandlers.

public partial class MainWindow : Window
{
    private ObservableCollection<string> ListItems = new ObservableCollection<string>  
    { 
        "Item 1", "Item 2", "Item 3", "Item 4", "Item 5", "Item 6"
    };

    public MainWindow()
    {
        InitializeComponent();
        lbItems.ItemsSource = this.ListItems;
    }

    private void up_click(object sender, RoutedEventArgs e)
    {
        var selectedIndex = this.lbItems.SelectedIndex;

        if (selectedIndex > 0)
        {
            var itemToMoveUp = this.ListItems[selectedIndex];
            this.ListItems.RemoveAt(selectedIndex);
            this.ListItems.Insert(selectedIndex - 1, itemToMoveUp);
            this.lbItems.SelectedIndex = selectedIndex - 1;
        }
    }

    private void down_click(object sender, RoutedEventArgs e)
    {
        var selectedIndex = this.lbItems.SelectedIndex;

        if (selectedIndex + 1 < this.ListItems.Count)
        {
            var itemToMoveDown = this.ListItems[selectedIndex];
            this.ListItems.RemoveAt(selectedIndex);
            this.ListItems.Insert(selectedIndex + 1, itemToMoveDown);
            this.lbItems.SelectedIndex = selectedIndex + 1;
        }
    }
}