ListBox Index returns negative value

Question

I'm creating a windows phone app using MVVM light pattern. I'm having trouble on my list box as it always returning a negative value (-1) for selected index. Does anyone knows how to resolve it?

here is my code in View Model, do i missed anything?Thanks!

 public void OnViewListSelectedItem(SelectionChangedEventArgs e)
    {

        ListBox lb = new ListBox();

        if (e.AddedItems.Count == 1)
        {
            if (lb.SelectedIndex == 0)
            {
                _navigationService.NavigateTo(new Uri(ViewModelLocator.ByVendorUrl, UriKind.Relative));
            }

            if (lb.SelectedIndex == 1)
                {
                    _navigationService.NavigateTo(new Uri(ViewModelLocator.ByVendorUrl, UriKind.Relative));
                }
            if (lb.SelectedIndex == 2)
                {
                    _navigationService.NavigateTo(new Uri(ViewModelLocator.ByCombinationUrl, UriKind.Relative));
                }
            }
    }

XAML code Here

 <ListBox x:Name="lbviewlist">
                <i:Interaction.Triggers>
                        <i:EventTrigger EventName="SelectionChanged">
                            <Command:EventToCommand Command="{Binding ViewListCommand}" PassEventArgsToCommand="True"/>
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                    <ListBox.Items>
                    <ListBoxItem Content="By Product" FontSize="35" Margin="10,12,12,0"/>
                    <ListBoxItem Content="By Vendor" FontSize="35" Margin="10,12,12,0"/>
                    <ListBoxItem Content="By Best Combination" FontSize="35" Margin="10,12,12,0"/>
                    </ListBox.Items>
                </ListBox>

Answer 1

You are creating a new ListBox() (called lb) in your code. You don't populate it, so it will be empty and will always have a SelectedIndex of -1

Then check the 'Source' property of 'e' and cast it to a ListBox

ListBox myList = (ListBox) e.Source;

You can then access the properties on myList.

Answer 2

based on my research SelectedIndex property of listbox is not bindable, when you use the get accessor for the SelectedIndex property, it always returns -1. An attempt to use the set accessor for the SelectedIndex property raises a NotSupportedException. -- MSDN List selectedporperty

I also updated my code since my first code is wrong that creates new listbox and that results to empty/null. Also selectionchanged event does not have a problem to be used as event.

    public void method (SelectionChangedEventArgs e)
    {


        {                
            if (e.AddedItems.Count == 1)
            {
                var listBoxItem = (ListBoxItem)e.AddedItems[0];
                string _string1 = "Test";
                if ((string)listBoxItem.Content == _string1)
                {
                    navigationService.NavigateTo(new Uri(ViewModelLocator.page1, UriKind.Relative));
                }
            }
         }
}

Thats it. Hope it helps! :)