使用ItemsSource时，操作无效。 使用ItemsControl.ItemsSource访问和修改元素

Question

I'm trying to make 2 list boxes where I can press on a button to add an item selected from the left listbox to the right listbox. Here's the XAML for the listboxes:

        <ListBox
        x:Name="LeftList"
        Foreground="{StaticResource Foreground}"
        HorizontalAlignment="Left" 
        Height="237" Margin="15,103,0,0" 
        VerticalAlignment="Top" 
        Width="128">
        <ListBoxItem>360T</ListBoxItem>
        <ListBoxItem>BARX</ListBoxItem>
        <ListBoxItem>BNP</ListBoxItem>
        <ListBoxItem>BOA</ListBoxItem>
        <ListBoxItem>CITI</ListBoxItem>
        <ListBoxItem>CS</ListBoxItem>
        <ListBoxItem>DB</ListBoxItem>
        <ListBoxItem>GS</ListBoxItem>
        <ListBoxItem>JPM</ListBoxItem>
        <ListBoxItem>RBS</ListBoxItem>
        <ListBoxItem>UBS</ListBoxItem>
    </ListBox>

    <ListBox 
        x:Name="RightList"
        Foreground="{StaticResource Foreground}"
        HorizontalAlignment="Left" 
        Height="237" Margin="257,103,0,0" 
        VerticalAlignment="Top" 
        Width="128"/>

C#:

List<string> leftSideList = new List<string>();
List<string> rightSideList = new List<string>();

    public ChooseLPWindow()
    {
        InitializeComponent();

        //Add to the collection leftside list
        leftSideList.Add("360T");
        leftSideList.Add("BARX");
        leftSideList.Add("BNP");
        leftSideList.Add("BOA");
        leftSideList.Add("CITI");
        leftSideList.Add("CS");
        leftSideList.Add("DB");
        leftSideList.Add("GS");
        leftSideList.Add("JPM");
        leftSideList.Add("RBS");
        leftSideList.Add("UBS");

    }

    private void AddBtn_Click(object sender, RoutedEventArgs e)
    {
        if (LeftList.SelectedIndex > -1)
        {
            int SelectedIndex = LeftList.SelectedIndex;
            string SelectedItem = LeftList.SelectedValue.ToString();

            //Add the selected item to the right side list
            RightList.Items.Add(SelectedItem);
            rightSideList.Add(SelectedItem);

            if (leftSideList != null)
            {
                //Remove the item from the collection list 
                leftSideList.RemoveAt(SelectedIndex);

                //Update the left side list
                LeftList.Items.Clear();
                LeftList.ItemsSource = leftSideList;
            }
        }
    }

I get the exception on:

LeftList.Items.Clear();

This happens when I try to add a second item the first one gets added but then the exception occurs when you try to add another item. The error is:

Operation is not valid while ItemsSource is in use. Access and modify elements with ItemsControl.ItemsSource

Any suggestions?

Answer 1

You can't modify ListBox's Items when the items populated through ItemsSource . In that case you suppose to modify items in the ItemsSource collection instead.

I'd suggest to change your List to ObservableCollection . With that removing item from collection is enough, because ObservableCollection has built-in mechanism to notify UI to refresh whenever item added or removed from collection :

ObservableCollection<string> leftSideList = new ObservableCollection<string>();
ObservableCollection<string> rightSideList = new ObservableCollection<string>();

public ChooseLPWindow()
{
    InitializeComponent();

    leftSideList.Add("360T");
    leftSideList.Add("BARX");
    leftSideList.Add("BNP");
    leftSideList.Add("BOA");
    leftSideList.Add("CITI");
    leftSideList.Add("CS");
    leftSideList.Add("DB");
    leftSideList.Add("GS");
    leftSideList.Add("JPM");
    leftSideList.Add("RBS");
    leftSideList.Add("UBS");

    LeftList.ItemsSource = leftSideList;
}

private void AddBtn_Click(object sender, RoutedEventArgs e)
{
    if (LeftList.SelectedIndex > -1)
    {
        int SelectedIndex = LeftList.SelectedIndex;
        string SelectedItem = LeftList.SelectedValue.ToString();

        //Add the selected item to the right side list
        RightList.Items.Add(SelectedItem);
        rightSideList.Add(SelectedItem);

        if (leftSideList != null)
        {
            //Remove the item from the ItemsSource collection 
            //instead of removing it from ListBox.Items
            leftSideList.RemoveAt(SelectedIndex);
        }
    }
}

Answer 2

I fixed the issue by doing this:

    private void AddBtn_Click(object sender, RoutedEventArgs e)
    {
        if (LeftList.SelectedIndex > -1)
        {
            int SelectedIndex = LeftList.SelectedIndex;
            string SelectedItem = LeftList.SelectedValue.ToString();

            //Add the selected item to the right side list
            RightList.Items.Add(SelectedItem);
            rightSideList.Add(SelectedItem);

            //Delete the item from the left side list
            //ListLps.Items.RemoveAt(SelectedIndex);

            if (leftSideList != null)
            {
                //Remove the item from the collection list 
                leftSideList.RemoveAt(SelectedIndex);
                LeftList.Items.RemoveAt(SelectedIndex);

            }
        }

    }