Silverlight 5 ListBox IsSelected style binding broken?

Question

i see Silverlight 5 bought style binding. Tried to apply it in a ListBox control, for multiple selection. I have the following XAML ListBox (the code works in a WPF application).

     <ListBox ItemsSource="{Binding Values}" SelectionMode="Multiple">
        <ListBox.ItemContainerStyle>
            <Style TargetType="ListBoxItem">
                <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}"/>
            </Style>
        </ListBox.ItemContainerStyle>
        <ListBox.ItemTemplate>
            <DataTemplate DataType="ListBoxItem">
                <TextBlock Text="{Binding DisplayValue}"/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

When i run this i get a binding error, it seems that it tries to find the IsSelected property on the type of "Values" collection instead of each individual item from that collection. Has anyone else experience this?

Update Added full code to reproduce, you need to scroll the listbox to see the error in the output log

public class ValueViewModel : INotifyPropertyChanged
{
    private bool _isSelected;
    public bool IsSelected
    {
        get { return _isSelected; }
        set
        {
            _isSelected = value;
            OnPropertyChanged("IsSelected");
        }
    }

    private string _displayValue;
    public string DisplayValue
    {
        get { return _displayValue; }
        set
        {
            _displayValue = value;
            OnPropertyChanged("DisplayValue");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

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

public class MainPageViewModel : INotifyPropertyChanged
{

    public ObservableCollection<ValueViewModel> _values;
    public ObservableCollection<ValueViewModel> Values
    {
        get { return _values; }
        set
        {
            _values = value;
            OnPropertyChanged("Values");
        }
    }

    public MainPageViewModel()
    {
        Values = new ObservableCollection<ValueViewModel>();

        for (var i = 0; i < 50; i++)
            Values.Add(new ValueViewModel() { DisplayValue = i.ToString(), IsSelected = (i % 5) == 0 });
    }

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

And the XAML:

<Grid x:Name="LayoutRoot" Background="White" >
    <Grid.Resources>
        <viewmodels:MainPageViewModel x:Key="vmMainPage"/>
    </Grid.Resources>
    <Grid x:Name="workGrid" DataContext="{Binding Source={StaticResource vmMainPage}}">
        <ListBox ItemsSource="{Binding Values}" SelectionMode="Multiple" Height="100">
            <ListBox.ItemContainerStyle>
                <Style TargetType="ListBoxItem">
                    <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}"/>
                </Style>
            </ListBox.ItemContainerStyle>
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Margin="5" Text="{Binding DisplayValue}"/>
                        <TextBlock Margin="5" Text="{Binding IsSelected}"/>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Grid>

Update 2 It seems the problem around the error is that in a scrollable situation if you select items 1 and then scroll down and select item 49 (in the above example), the 1st selection is lost.

Answer 1

So i've managed to find a workarround that seems to do the job for my needs. It will set the already loaded values once the Loaded event gets triggered. And it wraps the MouseDown event to set the selection status. It's not a true databind but gets the job done, and still keeps the View clean of code.

    <ListBox ItemsSource="{Binding Values}" SelectionMode="Multiple" Height="100">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Margin="2" Text="{Binding DisplayValue, Mode=TwoWay}">
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="Loaded">
                            <ei:ChangePropertyAction 
                                        TargetObject="{Binding RelativeSource={RelativeSource AncestorType=ListBoxItem}}"
                                        PropertyName="IsSelected" 
                                        Value="{Binding IsSelected}"/>
                        </i:EventTrigger>
                        <i:EventTrigger EventName="MouseLeftButtonDown">
                            <ei:ChangePropertyAction 
                                        TargetObject="{Binding}"
                                        PropertyName="IsSelected" 
                                        Value="{Binding IsSelected, Converter={StaticResource invertBooleanConverter}}"/>
                        </i:EventTrigger>
                    </i:Interaction.Triggers>  
                </TextBlock>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

Answer 2

I cannot reproduce it. It works fine for me. Here is a full working example based on your code. One issue that I did notice though is that when a ListBoxItem is rendered it automatically sets the property on the data object to false, regardless of whether it was true to begin with. So if you load up a list and set some of it's items to be pre-selected, all the items will be unselected when the ListBoxItems are rendered. One way to prevent this is to use Dispatcher.BeginInvoke and set the selected items there. See my comments in the code below.

XAML:

<UserControl x:Class="SilverlightApplication12.MainPage"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             d:DesignHeight="300"
             d:DesignWidth="400">

    <Grid x:Name="LayoutRoot"
          Background="White">

        <ListBox ItemsSource="{Binding Entities}"
                 SelectionMode="Multiple">
            <ListBox.ItemContainerStyle>
                <Style TargetType="ListBoxItem">
                    <Setter Property="IsSelected"
                            Value="{Binding Path=IsSelected, Mode=TwoWay}" />
                </Style>
            </ListBox.ItemContainerStyle>
            <ListBox.ItemTemplate>
                <DataTemplate DataType="ListBoxItem">
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding Name}" />
                        <TextBlock Margin="10 0 0 0"
                                   Text="IsSelected:" />
                        <TextBlock Margin="5 0 0 0"
                                   Text="{Binding IsSelected}"></TextBlock>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

    </Grid>
</UserControl>

Code-behind + entity class:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace SilverlightApplication12
{
    public partial class MainPage : UserControl, INotifyPropertyChanged
    {
        private ObservableCollection<MyEntity> _Entities;

        public ObservableCollection<MyEntity> Entities
        {
            get { return _Entities; }
            set
            {
                _Entities = value;
                OnPropertyChanged("Entities");
            }
        }

        public MainPage()
        {
            InitializeComponent();

            Entities = new ObservableCollection<MyEntity>();
            Entities.Add(new MyEntity()
            {
                Name = "One",
                IsSelected = false,
            });
            Entities.Add(new MyEntity()
            {
                Name = "Two",
                IsSelected = true,
                //Even though this is initially true it does not matter. 
                //When the ListBoxItem is rendered it sets the property to false. 
            });
            Entities.Add(new MyEntity()
            {
                Name = "Three",
                IsSelected = false,
            });

            LayoutRoot.DataContext = this;

            //Enable the following line to set the 2nd item to selected when the page is loaded. 
            //Dispatcher.BeginInvoke(() => Entities[1].IsSelected = true);
        }

        public event PropertyChangedEventHandler PropertyChanged;

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

    public class MyEntity : INotifyPropertyChanged
    {
        private string _Name;

        public string Name
        {
            get { return _Name; }
            set
            {
                _Name = value;
                OnPropertyChanged("Name");
            }
        }

        private bool _IsSelected;

        public bool IsSelected
        {
            get
            {
                return _IsSelected;
            }
            set
            {
                _IsSelected = value;
                OnPropertyChanged("IsSelected");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

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

Answer 3

Everything except this <DataTemplate DataType="ListBoxItem"> looks fine to me.

---

If the Values collection is a collection of ListBoxItems, you don't need the IsSelected binding.

Otherwise, the DataType on your DataTemplate is wrong and should probably be left blank.