When ConvertBack is called for ItemsControl?

Question

I have the following item data class, as well as a converter.

class ListBoxViewItem
{
    public String Name { get; set; }
    public Boolean IsChecked { get; set; }
}

[ValueConversion(typeof(List<String>),typeof(List<ListBoxViewItem>))]
class ListToItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value == null) return null;
        IEnumerable<String> l = value as IEnumerable<String>;
        return (from n in l select new ListBoxViewItem() { IsChecked = true, Name = n });
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

class ListBoxData
{
    public List<String> AllData
    {
        get
        {
            return new List<string>()
            {
                "FOO",
                "BAR"
            };
        }
        set
        {

        }
    }
}

I bind the instance of the ListBoxData to a listbox control's ItemsSource. As below:

    <ListBox>
        <ListBox.ItemsSource>
            <Binding>
                <Binding.Path>AllData</Binding.Path>
                <Binding.Converter>
                    <local:ListToItemConverter />
                </Binding.Converter>
                <Binding.Mode>TwoWay</Binding.Mode>
            </Binding>
        </ListBox.ItemsSource>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <CheckBox IsChecked="{Binding IsChecked,Mode=TwoWay}"
                          Content="{Binding Name}"/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

So My question is, the Convert function is called when the listbox is showing, but as every items in this list box is a checkbox, although I use TwoWay binding to bind the instance and listbox, but ConvertBack is not called when I check/uncheck the checkbox in this listbox.

I am not sure if ConvertBack is designed to work as I expected. But if I want the ConvertBack can be triggered when check status is changed, what should I do ?

possible duplicate of [What is the use of ConvertBack method in IValueConverter interface?](http://stackoverflow.com/questions/6424074/what-is-the-use-of-convertback-method-in-ivalueconverter-interface) — Tilak, Dec 29 '12 at 15:11
No, I know when ConvertBack is called for a textbox control, but in my case, I want to know when ConvertBack is called for one of the item property changed in a itemscontrol. — winterTTr, Dec 29 '12 at 16:16

score 1 · Answer 1 · answered Dec 29 '12 at 15:45

The ConvertBack method will fire when the Converter is used in a Binding with Mode=TwoWay. Example is whe it's used in a binding to the Text property of a TextBox. The "Convert" method will be fired when the value changes when displaying the value in the TextBox, and the "ConvertBack" will fire when the user change the TextBox value.

Tilak · Answer 2 · 2012-12-29T15:23:03.877

0

Add UpdateSourceTrigger to Binding and Implement ConvertBack function with reverse functionality of Convert function.

Change following

<CheckBox IsChecked="{Binding IsChecked,Mode=TwoWay}">

To

 <CheckBox IsChecked="{Binding IsChecked,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}">

ConvertBack function is as follows

public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    if (value is IEnumerable<ListBoxViewItem>) 
    {
      var l = (IEnumerable<ListBoxViewItem>)value;
      return l.Select(l.IsChecked);
    }

   return null;
}

edited Dec 29 '12 at 15:23

answered Dec 29 '12 at 15:17

Tilak

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I tried in a WPF application, and add UpdateSourceTrigger, the break point in ConvertBack never triggered under debug mode. – winterTTr Dec 29 '12 at 16:13

score 0 · Answer 3 · answered Dec 29 '12 at 17:03

You are converting ListBox.ItemsSource, not CheckBox.IsChecked

I don't think ListBox.ItemsSource will even work with a TwoWay binding since by default ListBoxes do not make changes to their ItemsSource's source collection.

If you want a converter to get run when you change CheckBox.IsChecked, then you need to specify the converter to use in the IsChecked binding

<CheckBox IsChecked="{Binding IsChecked, 
                              Converter={StaticResource MyCheckboxConverter}}"
          Content="{Binding Name}"/>

But in this case, a converter should not be needed since IsChecked is already a boolean value, so you don't need to convert it at all.

If you're trying to trigger something when your CheckBox.IsChecked changes, you're better off making sure IsChecked triggers a PropertyChange notification, and executing code in the PropertyChanged event of the class containing the IsChecked property.

When ConvertBack is called for ItemsControl?

3 Answers3