Silverlight 4: ListBoxItem Selection Problem

Question

I need to programatically select a subset of ListBoxItems in a ListBox (SelectedMode=Multiple) control.

<Grid x:Name="LayoutRoot" Background="White">
    <ListBox Height="238" HorizontalAlignment="Left" Margin="26,41,0,0" Name="listBox1" VerticalAlignment="Top" Width="349" SelectionMode="Multiple" />
    <Button Content="Fill" Height="23" HorizontalAlignment="Left" Margin="26,12,0,0" Name="buttonFill" VerticalAlignment="Top" Width="75" Click="buttonFill_Click" />
    <Button Content="Randomly Select" Height="23" HorizontalAlignment="Left" Margin="116,12,0,0" Name="buttonSelectRandom" VerticalAlignment="Top" Width="104" Click="buttonSelectRandoml_Click" />
</Grid>


    private void buttonFill_Click(object sender, RoutedEventArgs e)
    {
        for (int i = 0; i < 100; i++)
            listBox1.Items.Add(new ListBoxItem { Content = i.ToString()});
    }

    private void buttonSelectRandom_Click(object sender, RoutedEventArgs e)
    {
        var rand = new Random();

        foreach (ListBoxItem item in listBox1.Items)
            if (rand.Next(2)==1) item.IsSelected = true; 
    }

However it seems that only the currently visible items show as selected when I run the code (click the "Fill" button and then the "Randomly Select" button). Scrolling through the ListBox shows no other ListBoxItems as selected even though a check of their "IsSelected" state in code will show them set to "true".

Interestingly, if I manually scroll to the end of the ListBox (or part way) first and then click the "Randomly Select" button then the ListBox will draw all the selected items correctly . I have tried many workarounds but can't seem to find one that works. Is this a bug? Any workarounds?

Thanks for your help.

Jink

Justin XL · Accepted Answer · 2011-04-22 01:35:56Z

3

this might be because the ListBox's using VirtualizingStackPanel. Can you test it with a normal StackPanel?

    <ListBox>
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel/>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
    </ListBox>

Edit:

Another solution is, instead of doing item.IsSelected = true, you do

        foreach (int item in listBox1.Items)
        {
            if (rand.Next(2) == 1)
            {
                this.listBox1.SelectedItems.Add(item);
            }
        }

I have tested it and it works. :)

edited Apr 22, 2011 at 1:35

answered Apr 21, 2011 at 3:22

Justin XL

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6 Comments

Kimberly Over a year ago

+1 This worked for me. It also explains why scrolling through the list before setting the selections causes the items to display properly.

Kimberly Over a year ago

@Xin, I can't accept the answer, as it's not my question. :) I bumped it because it's not clear from your answer whether you had tried that code yourself, but it does work to fix that particular issue. Anyway, in case the OP actually needs a virtualizing items panel because of memory-intensive items, I hope they will leave the question open for a few days and give many people a chance to weigh in with other possible workarounds.

Jink Over a year ago

+1 Thanks @Xin, that works. Hmmm, the VirtualizingStackPanel - I'll have to take a look at that. I'll take @Kimberly 's advice and leave it open for a while before accepting your answer to see if there is a good way to do it with the VirtualizingStackPanel or some such in place because I could have a lot of items in the list and performance might benefit.

Justin XL Over a year ago

Hi, I have updated the answer and hopefully this time it will work even with the VirtualizingStackPanel. Interestingly, I have found your original code works in WPF. That means once those invisible ListBoxItems are scrolled into view, they will be selected automatically. However this is not the case in Silverlight.

Jink Over a year ago

Excellent @Xin. Tested your second code batch and can confirm that it does work with the VirtualizingStackPanel in my Silverlight project. I would be interested to know what's behind this WPF\Silverlight behavioral difference.

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