XAML ListBox Items binding to an Activity

Question

I'm using Microsoft Activity Library Designer; For some reasons I need to use ListBox to show some information in it.But I have a problem with it's ItemsSource binding.My Activity side property is like this:

    private ObservableCollection<string> _selectedItems;
    public ObservableCollection<string> SelectedItems
    {
        get 
        {
            if (_selectedItems == null)
            {
                ObservableCollection<string> items = new ObservableCollection<string>();
                return items;
            }
            return _selectedItems;
        }
        set 
        { 
            _selectedItems = value;
        }
    }

And my XAML side code is like this:

....
<Button Content="Add Item" HorizontalAlignment="Stretch" Grid.Column="0"
        Click="Button_Click" Margin="5, 0, 5, 5"/>

<Button Content="Remove Item" HorizontalAlignment="Stretch" Grid.Column="1"
        Click="DelButton_Click" Margin="5, 0, 5, 5"/>
....
<ListBox x:Name="LstSelectedPosts" MinHeight="20" ItemsSource="{Binding Path=ModelItem.Selecteditems, Mode=TwoWay}"/>
....

Now when I try to Add/Remove an item to/from this ListBox in Add Item and Remove Item buttons click event, debugger shows me an error that tells I can't modify the ListBox binding source. So how can I change this Listbox's Items?

Answer 1

Ok there are some errors in your code that could cause the problem. In the getter, I think you should have this.

if (_selectedItems == null)
{
   _selectedItems = new ObservableCollection<string>();
}
return _selectedItems;

In your version, _selectedItems never get initialized.

In the Xaml code, when you set the ItemSource, you wrote Seleceteditems instead of SelectedItems this error doesn't cause an error when you compile but your listBox doesn't have its itemSource setted to the correct element.

And then, you didn't specify the source in

ItemsSource="{Binding Path=ModelItem.Selecteditems, Mode=TwoWay}

that means the source is by default, the DataContext of your object and that DataContext should be initialized with an object that has a public property named ModelItem which has in turn a public property named Selecteditems. Hope it works.

Here is a small example. in my xaml file

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <ListBox Height="287" HorizontalAlignment="Left" Margin="12,12,0,0" x:Name="LstSelectedPosts" VerticalAlignment="Top" Width="294" 
             ItemsSource="{Binding Path=SelectedItems, Mode=TwoWay}"/>
    <Button Content="Add Item" HorizontalAlignment="Stretch" Click="Button_Click" Margin="321,110,68,170"/>
    <Button Content="Remove Item" HorizontalAlignment="Stretch" Click="DelButton_Click" Margin="321,147,68,129"/>
</Grid>

in my xaml.cs file

public partial class MainWindow : Window
{

    private CDataContext _myCDataContext;

    public MainWindow()
    {
        InitializeComponent();
        _myCDataContext = new CDataContext();
        DataContext = _myCDataContext;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        _myCDataContext.Add();
    }

    private void DelButton_Click(object sender, RoutedEventArgs e)
    {
       _myCDataContext.Remove(LstSelectedPosts.SelectedItem.ToString());
    }


}

and my CDataContext class

 class CDataContext
{
    private int _count = 0;
    private ObservableCollection<string> _selectedItems;
    public ObservableCollection<string> SelectedItems
    {
        get
        {
            if (_selectedItems == null)
            {
                _selectedItems = new ObservableCollection<string>();

            }
            return _selectedItems;
        }
        set
        {
            _selectedItems = value;
        }
    }

    public void Remove(string s)
    {
        SelectedItems.Remove(s);
    }

    public void Add()
    {
        SelectedItems.Add(_count.ToString());
        _count++;
    }
}