I have a CheckBox in a ListBox. I set the ListBox ItemsSource to a List of Agency. Agency has a property
public class Agency
{
public bool isSelected { get; set;}
}
<ListBox> <!-- ItemsSource set in codebehind to List<Agency> -->
<CheckBox IsChecked="{Binding Path=isSelected, Mode=TwoWay}" />
</ListBox>
I have a function to check all the checkboxes
//SelectAll button
private void SelectAll_Click(object sender, RoutedEventArgs e)
{
List<Agency> list = this.AgencySubListBox.ItemsSource as List<Agency>;
for (int i = 0; i < list.Count; i++)
{
Agency d = list[i];
d.isSelected = true;
}
}
When I hit the select all button I expect the checkboxes to all be checked. But nothing happens.
You have to implement INotifyPropertyChanged for your Agency-class.
Then in your isSelected-Property, call PropertyChanged if the value of the property has been changed. Auto-properties as you used in your example do not support INotifiyPropertyChanged, therefore you can not use them for your purpose.
If you work with .net, I would also recommend to start the property names with upper case. This is the widely accepted standard.
You should make your model implement INotifyPropertyChanged, and implement it
There is nothing to tell the UI that the your checkbox binding has been invalidated, and needs rechecking. Therefore, implement INotifyPropertyChanged on your Agency type.
Related
I've a datagrid with an ItemsSource of ObservableCollection (OC) of objects. When an item's property changes, I want to work on the OC itself .
E.g. I've an item which is approved for uploading to our database. However, I need to loop through the OC to check if other items exist in the collection which already fit the set criteria, so that I may actually not have to upload the selected item.
On the datagrid, when I tick the checkbox of an item, it will change the boolean value (e.g. "IsToUpload") of the item, and an event should trigger on the property change.
I'm assuming I will then need to 'bubble up' my event notifications to the datagrid/mainwindow class, where I can then work on the OC. How may I do this, and if this is not the correct way, what should I be doing?
I've followed Aran Mulholland's class structure to colour my rows dynamically: Coloring WPF DataGridRows one by one
So my class structure is roughly as follows:
MainWindow -> DataGrid
-> ObservableCollection<ItemObjectViewModel:NotificationObject>
ItemObject : INotifyPropertyChanged //this class is where I
//store my item variables. It is referenced through properties
//in the ItemObjectViewModel.
Event bubling \ routing etc works for dependency objects in a visual \ logical tree. Your NotificationObject is not a dependency object and neither is it hosted in the visual tree.... What we have in visual tree are the checkboxes (that are bound to your NotificationObject).
Non MVVM
In you DataGrid you would have to Tag your Checkboxes with some identification and then use ButtonBase.Click="" event at datagrid level which will be handled for any click event bubbled for any button based eleemnt (such as buttons, menuitems, togglebuttons, checkboxes, radioboxes, comboboxes) that gets clicked in the entire visual tree of the datagrid.
In the handler verify if the e.OriginalSource is a checkbox and that its Tag is same as the identification value we have set in the XAML of the datagrid. That way we know that the CheckBox is clicked.
E.g.
<DataGrid AutogenerateColumns="False"
ItemsSource="{Binding NotificationObjectCollection}"
ButtonBase.Clicked="OnNotificationCheckBoxClicked">
<DataGrid.Columns>
<DataGridCheckBoxColumn Binding="{Binding IsClicked}"
Header="Click?">
<DataGridCheckBoxColumn.ElementStyle>
<Style TargetType="{x:Type CheckBox}">
<Setter Property="Tag" Value="IsClickCheckBox" />
</Style>
</DataGridCheckBoxColumn.ElementStyle>
</DataGridCheckBoxColumn>
</DataGrid.Columns>
</DataGrid>
private void OnNotificationCheckBoxClicked
(object sender, RoutedEventArgs e)
{
if (e.OriginalSource is CheckBox)
{
if (((CheckBox)e.OriginalSource).Tag == "IsClickCheckBox")
{
var notificationObject
= ((CheckBox)e.OriginalSource).DataContext
as NotificationObject;
if (notificationObject.IsClicked) { }
else { }
}
}
}
MVVM
The only way MVVM can notify the ancestor object in the visual is by using Command execution as the underlying NotificationObject gets checked (setter is called) we execute the command supplied to the NotificationObject.
Use the weak reference based RelayCommand or DelegateCommand (as available on the internet) for this purpose.
Add a new NotificationObject constructor
private ICommand _isClickedCommand;
public NotificationObject(ICommand isClickedCommand)
{
_isClickedCommand = isClickedCommand;
}
private bool _isClicked;
public bool IsClicked
{
get
{
return _isClicked;
}
set
{
if (_isClicked != value)
{
_isClicked = value;
OnPropertyChanged("IsClicked");
isClickedCommand.Execute(this);
}
}
}
Using the notification object
public class ItemObjectViewModel
{
private DelegateCommand<NotificationObject>
_notificationObjectClickedCommand
= new DelegateCommand<NotificationObject>(
OnNotificationObjectCommandExecute);
....
private void PopulateCollection()
{
NotificationObjectCollection
= new ObservableCollection<NotificationObject>();
NotificationObjectCollection.Add(
new NotificationObject(_notificationObjectClickedCommand));
}
private void OnNotificationObjectCommandExecute(
NotificationObject notificationObject)
{
if (notificationObject.IsClicked) { }
else { }
}
}
You can also achieve the ICommand based behavior in non MVVM scenario by using 'RoutedCommand'
Let me know if this helps...
I would like to use a datatemplate for my datagrid columns and have a button for each item. I would like the item to be removed if the user clicks the button. I am using the MVVM pattern. How would I accomplish this?
<DataGridTemplateColumn>
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<Button Width="50" Content="Remove" Command="{Binding RemoveItemCommand}"/>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
public class ItemViewModel
{
public ItemViewModel()
{
RemoveCommand = new MyCommand(Remove);
}
public event EventHandler ItemRemoved;
public ICommand RemoveCommand { get; private set; }
private void Remove()
{
// Whatever it takes to remove item from your data store
service.Remove(this.Data);
var removeItem = ItemRemoved;
if (removeItem != null)
removeItem(this, EventArgs.Empty);
}
}
public class ListViewModel
{
public ListViewModel(IEnumerable<ItemViewModel> items)
{
ItemVMs=new ObservableCollection<ItemViewModel>(items);
foreach (var item in ItemVMs)
item.ItemRemoved += RemoveSelectedItem;
}
public ObservableCollection<ItemViewModel> ItemVMs { get; private set; }
private void RemoveSelectedItem(object sender, EventArgs e)
{
var item = sender as ItemViewModel;
item.ItemRemoved -= RemoveSelectedItem;
ItemVMs.Remove(item);
}
}
Each item's RemoveCommand would be bound to its button in your DataGrid. It sounds like you already have that part done. Make the ListViewModel's ItemVMs property the data source for your DataGrid.
The View is responsible for this. You can simply use codebehind to control the visibility of UI elements in response to user actions in the UI.
Sometimes, it is better to be practical than be rigidly dogmatic.
Well, now that you have edited your question, it becomes a completely different matter.
Your DataGrid should be bound to a collection of items.
Your button should be bound to a command on the ViewModel, and the CommandParameter should be the Model that particular row is bound to.
<DataTemplate>
<Button Content="Remove"
Command="{Binding DataContext.RemoveItemCommand,
ElementName=theWindow}"
CommandParameter="{Binding}" />
</DataTemplate>
Note some important things here. We need, from within the template, to bind to an ICommand on the ViewModel. The ViewModel is the DataContext of the Window. In this example, the window is named 'theWindow' (x:Name="theWindow"). Since the source of the Binding is the window, the Path must point to the ViewModel in the DataContext property on that Window.
We pass the current Model the DataGrid row is bound to into the command. This way, it is triival to remove it from the collection in the ViewModel.
public ObservableCollection<Model> Items {get;set;}
public ICommand RemoveItemCommand {get;set;}
// this method is called when RemoveItemCommand.Execute is called!
public void Execute(object parameter)
{
Items.Remove(parameter as Model);
}
This assumes you're using one of the standard delegated ICommand implementations out there. You can see how this is trivial to implement, and since the collection is an observable one, once you click the button and the Model is removed, the DataGrid will be notified of the change in the collection and remove that row.
You're probably better off using the standard routed events on the Click event of the button instead of a Command. The click event will allow you to retrieve the information about what control was clicked, and then you can also easily retrieve the parent of the button, to delete that item.
I have a datagrid that is multi-select enabled. I need to change the selection in the viewmodel. However, the SelectedItems property is read only and can't be directly bound to a property in the viewmodel. So how do I signal to the view that the selection has changed?
Andy is correct. DataGridRow.IsSelected is a Dependency Property that can be databound to control selection from the ViewModel. The following sample code demonstrates this:
<Window x:Class="DataGridMultiSelectSample.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:tk="clr-namespace:Microsoft.Windows.Controls;assembly=WPFToolkit"
Title="Window1" Height="300" Width="300">
<StackPanel>
<tk:DataGrid AutoGenerateColumns="False" ItemsSource="{Binding}" EnableRowVirtualization="False">
<tk:DataGrid.Columns>
<tk:DataGridTextColumn Header="Value" Binding="{Binding Value}" />
</tk:DataGrid.Columns>
<tk:DataGrid.RowStyle>
<Style TargetType="tk:DataGridRow">
<Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
</Style>
</tk:DataGrid.RowStyle>
</tk:DataGrid>
<Button Content="Select Even" Click="Even_Click" />
<Button Content="Select Odd" Click="Odd_Click" />
</StackPanel>
</Window>
using System.ComponentModel;
using System.Windows;
namespace DataGridMultiSelectSample
{
public partial class Window1
{
public Window1()
{
InitializeComponent();
DataContext = new[]
{
new MyViewModel {Value = "Able"},
new MyViewModel {Value = "Baker"},
new MyViewModel {Value = "Charlie"},
new MyViewModel {Value = "Dog"},
new MyViewModel {Value = "Fox"},
};
}
private void Even_Click(object sender, RoutedEventArgs e)
{
var array = (MyViewModel[]) DataContext;
for (int i = 0; i < array.Length; ++i)
array[i].IsSelected = i%2 == 0;
}
private void Odd_Click(object sender, RoutedEventArgs e)
{
var array = (MyViewModel[])DataContext;
for (int i = 0; i < array.Length; ++i)
array[i].IsSelected = i % 2 == 1;
}
}
public class MyViewModel : INotifyPropertyChanged
{
public string Value { get; set; }
private bool mIsSelected;
public bool IsSelected
{
get { return mIsSelected; }
set
{
if (mIsSelected == value) return;
mIsSelected = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("IsSelected"));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
}
Be sure to set EnableRowVirtualisation="False" on the DataGrid element, else there's a risk that the IsSelected bindings fall out of kilter.
I haven't worked with the DataGrid much, but one technique that works for the ListView is to bind to the IsSelected property of the individual ListViewItem. Just set this to true for each object in your list, and then it will get selected.
Maybe the object that represents a row in the DataGrid also has an IsSelected property, and can be used in this way as well?
Guys, thanks for the help. My problem was solved. I think the problem is pretty common for new WPF developers, so I will restate my problem and as well as the solution in more details here just in case someone else runs into the same kind of problems.
The problem: I have a multi-select enabled datagrid of audio files. The grid has multiple column headers. The user can multi-select several row. When he clicks the Play button, the audio files will be played in the order of one the columns headers (say column A). When playback starts, the multi-select is cleared and only the currently playing file is highlighted. When playback is finished for all files, the multi-selection will be re-displayed. The playback is done in the viewmodel. As you can see, there are two problems here: 1) how to select the currently playing file from the viewmodel, and 2) how to signal to the view from the viewmodel that playback is finished and re-display the multi-selection.
The solution: To solve the first problem, I created a property in the viewmodel that is bound to the view's SelectedIndex property to select the currently playing file. To solve the second problem, I created a boolean property in the view model to indicate playback is finished. In the view's code behind, I subscribed the the boolean property's PropertyChanged event. In the event handler, the view's SelectedItems property is re-created from the saved multi-selection (the contents of SelectedItems was saved into a list and SelectedItems was cleared when playback started). At first, I had trouble re-creating SelectedItems. It turned out the problem was due to the fact that re-creation was initiated through a second thread. WPF does not allow that. The solution to this is to use the Dispatcher.Invoke() to let the main thread do the work. This may be a very simple problem for experienced developers, but for newbies, it's a small challenge. Anyway, a lot of help from different people.
Just use SelectedItems on any MultiSelector derived class , and use methods Add, Remove, Clear on IList it returns .
I'm horrible at this WPF thing, so bear with me.
I'm using the Xceed DataGrid for WPF, and I need to know when someone selects a row, but I can't figure out how to do it. I'm sure I need to add some XAML to enable this, but I can't figure out what I should do.
I use a MVVM approach and therefor favor data binding. I will bind the SelectedItem property to a SelectedItem property on my ViewModel object for the grid.
<xcdg:DataGridControl x:Name="grid" SelectedItem="{Binding SelectedItem}">
</xcdg:DataGridControl>
Then on your property setter can do what ever is necessary upon change in the SelectedItemChanged() method.
private IMyItem _selectedItem;
public IMyItem SelectedItem
{
get { return _selectedItem; }
set {
_selectedItem = value;
OnPropertyChanged("SelectedItem");
SelectedItemChanged();
}
}
I'm actually struggling a bit with the same thing myself, except I have a prerequisite that the selection notification be done via an ICommand; however, if you do not have this need, you can wire up the SelectionChanged event handler. It's pretty elementary stuff, but I'll include the code just in case:
XAML:
<Grid>
<DataGrid:DataGridControl x:Name="gridControl" SelectionChanged="gridControl_SelectionChanged">
<!-- Content -->
</DataGrid:DataGridControl>
</Grid>
Code-behind:
private void gridControl_SelectionChanged(object sender, Xceed.Wpf.DataGrid.DataGridSelectionChangedEventArgs e)
{
var selectedIndex = gridControl.SelectedIndex; // int index
var selectedItem = gridControl.SelectedItem; // instance of bound object
var selectedItems = gridControl.SelectedItems; // IList of bound objects
}
All that said, I'm very interested to hear if there are any elegant solutions for getting the selected row from an Xceed DataGrid with an ICommand (in my case, I'm using anonymous types, which can make a difference)...
You don't have to write complicated code for something simple... although it can become tedious, here is some code for you. I hope this helps:
<Style TargetType="xcdg:DataRow">
<EventSetter Handler="dr_PreviewMouseDown" Event="PreviewMouseDown" />
</Style>
void dr_PreviewMouseDown(object sender, MouseButtonEventArgs e)
{
DataRow dr = sender as DataRow;
Debug.WriteLine(sender);
}
So here's what I came up with
System.ComponentModel.DependencyPropertyDescriptor gridItemsSourceDescriptor = System.ComponentModel.DependencyPropertyDescriptor.FromProperty(DataGridControl.SelectedItemProperty, typeof(DataGridControl));
gridItemsSourceDescriptor.AddValueChanged(dgBaxRuns, HandleSelectionChanged);
I made for me a easiest way.
<xctk:MaterialButton Margin="5,0,5,0" Grid.Column="3" Content="Szűrt sorok kijelölése" Command="{Binding SelectFilteredRowsCommand}" CommandParameter="{Binding ElementName=MyDataGrid}" />
So, i send my datagrid with my commandparameter to the viewmodel.
public RelayCommand<object> SelectFilteredRowsCommand { get; set; }
SelectFilteredRowsCommand = new RelayCommand<object>((o) =>
{
var datagrid = o as DataGridControl;
if (datagrid != null)
{
var datagriditems = datagrid.Items.Cast<SelectableProduct>();
foreach (SelectableProduct selectableProduct in datagriditems)
{
selectableProduct.IsSelect = true;
}
}
});
And convert back to datagrid itemsoruce type.
I've got a WPF app using the Model-View-ViewModel pattern.
In my ViewModel I've got a ListCollectionView to keep a list of items.
This ListCollectionView is bound to a ListBox in my View.
<ListBox Grid.Row="1" ItemsSource="{Binding Useragents}" SelectionMode="Multiple"/>
The ListBox has SelectionMode=Multiple, so you can select more items at one time. Now the ViewModel needs to know which items has been selected.
The problem is: in the View-Model-ViewModel pattern the ViewModel has no access to the View, so I can't just ask the ListBox which items has been selected. All I have is the ListCollectionView, but I can't find a way to find which items has been selected in there.
So how do I find which items has been selected in the ListBox? Or a trick to achieve this (maybe bind something to a Boolean 'IsSelected' in my items? But what? How?)
Maybe someone who is using this pattern, too, can help me here?
You need to create a ViewModel that has the concept of IsSelected on it and is bound to the IsSelected property of the actual ListBoxItem that represents it in the View using the standard WPF bindings architecture.
Then in your code, which knows about your ViewModel, but not the fact that it's represented by any specific View, can just use that property to find out which items from the Model are actually selected irrespective of the designers choice for how its represented in the View.
PRISM MVVM Reference Implementation has a behaviour called SynchronizeSelectedItems, used in Prism4\MVVM RI\MVVM.Client\Views\MultipleSelectionView.xaml, which synchronizes checked items with the ViewModel property named Selections:
<ListBox Grid.Column="0" Grid.Row="1" IsTabStop="False" SelectionMode="Multiple"
ItemsSource="{Binding Question.Range}" Margin="5">
<ListBox.ItemContainerStyle>
<!-- Custom style to show the multi-selection list box as a collection of check boxes -->
<Style TargetType="ListBoxItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBoxItem">
<Grid Background="Transparent">
<CheckBox IsChecked="{Binding IsSelected, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
IsHitTestVisible="False" IsTabStop="True"
AutomationProperties.AutomationId="CheckBoxAutomationId">
<ContentPresenter/>
</CheckBox>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ListBox.ItemContainerStyle>
<i:Interaction.Behaviors>
<!-- Custom behavior that synchronizes the selected items with the view models collection -->
<Behaviors:SynchronizeSelectedItems Selections="{Binding Selections}"/>
</i:Interaction.Behaviors>
</ListBox>
Go to http://compositewpf.codeplex.com/ and grab it all or use this:
//===================================================================================
// Microsoft patterns & practices
// Composite Application Guidance for Windows Presentation Foundation and Silverlight
//===================================================================================
// Copyright (c) Microsoft Corporation. All rights reserved.
// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY
// OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT
// LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
// FITNESS FOR A PARTICULAR PURPOSE.
//===================================================================================
// The example companies, organizations, products, domain names,
// e-mail addresses, logos, people, places, and events depicted
// herein are fictitious. No association with any real company,
// organization, product, domain name, email address, logo, person,
// places, or events is intended or should be inferred.
//===================================================================================
using System;
using System.Collections;
using System.Collections.Specialized;
using System.Diagnostics.CodeAnalysis;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;
namespace MVVM.Client.Infrastructure.Behaviors
{
/// <summary>
/// Custom behavior that synchronizes the list in <see cref="ListBox.SelectedItems"/> with a collection.
/// </summary>
/// <remarks>
/// This behavior uses a weak event handler to listen for changes on the synchronized collection.
/// </remarks>
public class SynchronizeSelectedItems : Behavior<ListBox>
{
public static readonly DependencyProperty SelectionsProperty =
DependencyProperty.Register(
"Selections",
typeof(IList),
typeof(SynchronizeSelectedItems),
new PropertyMetadata(null, OnSelectionsPropertyChanged));
private bool updating;
private WeakEventHandler<SynchronizeSelectedItems, object, NotifyCollectionChangedEventArgs> currentWeakHandler;
[SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly",
Justification = "Dependency property")]
public IList Selections
{
get { return (IList)this.GetValue(SelectionsProperty); }
set { this.SetValue(SelectionsProperty, value); }
}
protected override void OnAttached()
{
base.OnAttached();
this.AssociatedObject.SelectionChanged += this.OnSelectedItemsChanged;
this.UpdateSelectedItems();
}
protected override void OnDetaching()
{
this.AssociatedObject.SelectionChanged += this.OnSelectedItemsChanged;
base.OnDetaching();
}
private static void OnSelectionsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var behavior = d as SynchronizeSelectedItems;
if (behavior != null)
{
if (behavior.currentWeakHandler != null)
{
behavior.currentWeakHandler.Detach();
behavior.currentWeakHandler = null;
}
if (e.NewValue != null)
{
var notifyCollectionChanged = e.NewValue as INotifyCollectionChanged;
if (notifyCollectionChanged != null)
{
behavior.currentWeakHandler =
new WeakEventHandler<SynchronizeSelectedItems, object, NotifyCollectionChangedEventArgs>(
behavior,
(instance, sender, args) => instance.OnSelectionsCollectionChanged(sender, args),
(listener) => notifyCollectionChanged.CollectionChanged -= listener.OnEvent);
notifyCollectionChanged.CollectionChanged += behavior.currentWeakHandler.OnEvent;
}
behavior.UpdateSelectedItems();
}
}
}
private void OnSelectedItemsChanged(object sender, SelectionChangedEventArgs e)
{
this.UpdateSelections(e);
}
private void UpdateSelections(SelectionChangedEventArgs e)
{
this.ExecuteIfNotUpdating(
() =>
{
if (this.Selections != null)
{
foreach (var item in e.AddedItems)
{
this.Selections.Add(item);
}
foreach (var item in e.RemovedItems)
{
this.Selections.Remove(item);
}
}
});
}
private void OnSelectionsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
this.UpdateSelectedItems();
}
private void UpdateSelectedItems()
{
this.ExecuteIfNotUpdating(
() =>
{
if (this.AssociatedObject != null)
{
this.AssociatedObject.SelectedItems.Clear();
foreach (var item in this.Selections ?? new object[0])
{
this.AssociatedObject.SelectedItems.Add(item);
}
}
});
}
private void ExecuteIfNotUpdating(Action execute)
{
if (!this.updating)
{
try
{
this.updating = true;
execute();
}
finally
{
this.updating = false;
}
}
}
}
}
Look at this blogpost by Josh Smith The Initially Selected Item when Binding to a Grouped ICollectionView
The solution of Drew Marsh works very well, I recommend it. And I have another solution !
Model View ViewModel is a Passive View, you can also use a Presentation Model to access some datas of your presentation without being coupled with WPF
(this pattern is used in the Stocktrader example of PRISM).
Drew Marsh's answer is fine if you have a small list, if you have a large list the performance hit for finding all your selected items could be nasty!
My favorite solution is to create an attached property on your ListBox that then binds to an ObservableCollection which contains your selected items.
Then with your attached property you subscribe to the items SelectionChanged event to add/remove items from your collection.
For me the best answer is to break a little the principle of MVVM.
On the code behind
1. Instanciate your viewModel
2. add an event handler SelectionChanged
3. iterate through your selected items and add each item to your list of the viewModel
ViewModel viewModel = new ViewModel();
viewModel.SelectedModules = new ObservableCollection<string>();
foreach (var selectedModule in listBox1.SelectedItems)
{
viewModel.SelectedModules.Add(selectedModule.ToString());
}
Here is another variant of the View-Model-ViewModel Pattern where the ViewModel has access to the view through an IView interface.
I encountered quite a lot scenarios where you can't use WPF binding and then you need a way in code to synchronize the state between the View and the ViewModel.
How this can be done is shown here:
WPF Application Framework (WAF)
Have a look over here
http://blog.functionalfun.net/2009/02/how-to-databind-to-selecteditems.html
David Rogers' solution is great and is detailed at the below related question:
Sync SelectedItems in a muliselect listbox with a collection in ViewModel