Question

I am creating a control that displays a summary of data. On this page I am binding a control to an ObservableColection that contains many items. I created a CollectionView that sorts the items by time but in this control I only want to show the top 3 items. I tried creating a converter to accomplish this but all ways I try to filter the list return a “new” list and the control is no longer updated when a new item is added to the master ObservableCollection.

Is there a clean way to accomplish this? I will have to do something similar for several of my collections so I would like to make something generic.

Answer 1

If you’re using the ViewModel pattern, the clean way is to have another ObservableCollection with the top N items. That way the binding is direct and you can change 3 to whatever N you want only by changing your model.

public class MyViewModel
{
    private ObservableCollection<string> myList;

    public ObservableCollection<string> MyList 
    { 
        get { return myList; }
        set { return myList; }
    }

    public Collection<string> MyListTop3 
    {
        get { return new Collection<string>(MyList.Take(3).ToList()); }
    }

    public MyViewModel() 
    {
        myList = new ObservableCollection<string>();

        myList.CollectionChanged += (sender, args) =>
        { 
            RaisePropertyChanged("MyListTop3");
        }; 
    } 
}

If you want to that care of it only through XAML (no the cleaneast way):

<ListBox>

    <ListBox.Resources>
        <ContentPresenter x:Key="value0" Content="{Binding MyList[0]}"/>
        <ContentPresenter x:Key="value1" Content="{Binding MyList[1]}"/>
        <ContentPresenter x:Key="value2" Content="{Binding MyList[2]}"/>
    </ListBox.Resources>

    <ListBoxItem Content="{DynamicResource value0}"/>
    <ListBoxItem Content="{DynamicResource value1}"/>
    <ListBoxItem Content="{DynamicResource value2}"/>

</ListBox>

This example shows a ListBox but you can use it on any other control.