Question

I have a ObservableCollection which I need to bind to 2 labels, first to show count of items in the collection and second to show the sum of values.

First label is bound to collections count property and second label is bound directly to ObservableCollection with a convertor to calculate total of all items

XAML looks something like this

<Grid>
    <ListBox Name="itemList" ItemsSource="{Binding DataList}"/>
    <Label Name="lblcount" Content="{Binding DataList.Count}" />
    <Label Name="lblTotal" Content="{Binding DataList, Converter={StaticResource calculateTotalConvertor}" />
</Grid>

My VM has a collection like this

    ObservableCollection<int> data = new ObservableCollection<int>();

    public ObservableCollection<int> DataList
    {
        get { return data; }
        set { data = value; }
    }

My convertor code is

public class CalculateTotalConvertor : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        ObservableCollection<int> collection = value as ObservableCollection<int>;

        return collection.Sum();
    }

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

Issue is on adding new items in DataList, ListView and label showing count of items gets updated but “lblTotal” doesnt get updated with total count.

Basically how to force your binding to be evaluated on ObservableCollection changes ? How does it work directly for ListView or DataGrid but not for label ?

I know this problem can be solved by creating a property in VM to show total and raise property change when collection gets updated but is there is any better solution than that ?

Of-course this is simplified form of my actual problem, I dont have access to the ViewModel and the collection, its a third party control. I am creating a wrapper user control and have a relative binding with the view to its inner collection.

Answer 1

The other answers correctly explain why it is not updating. To force it to update you can change your converter to an IMultiValueConverter:

public class CalculateTotalConvertor : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        ObservableCollection<int> collection = values.FirstOrDefault() as ObservableCollection<int>;

        return collection.Sum();
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Then change your binding to a MultiBinding which also pulls in the Count:

<Label Name="lblTotal">
    <Label.Content>
        <MultiBinding Converter="{StaticResource calculateTotalConvertor}">
            <Binding Path="DataList"/>
            <Binding Path="DataList.Count"/>
        </MultiBinding>
    </Label.Content>
</Label>

Now the second binding will notify that the binding needs to update when items are added or removed, but you can just ignore the count value and not use it.