A nice and flexible way to introduce filtering and sorting on a view model data collection is to use a CollectionViewSource. Here’s a quick example:

Starting with a bound collection property like this…

        public ObservableCollection<Item> Items { get; set; }

        // Constructor
        public MainPage()
        {
            Items = new ObservableCollection<Item>
                        {
                            new Item { Date = DateTime.Now.AddDays(-4), Name = "Item 1"},
                            new Item { Date = DateTime.Now.AddDays(-3), Name = "Item 2"},
                            new Item { Date = DateTime.Now.AddDays(-5), Name = "Item 3"},
                            new Item { Date = DateTime.Now.AddDays(-8), Name = "Item 4"},
                            new Item { Date = DateTime.Now.AddDays(-1), Name = "Item 5"},
                        };

            InitializeComponent();
            DataContext = this;
        }

Which is bound to a ListBox like the following:

            <ListBox x:Name="ListBox"
                     Margin="0,0,-12,0"
                     ItemsSource="{Binding Items}">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Vertical" >
                            <TextBlock Text="{Binding Date}"
                                       TextWrapping="Wrap"
                                       Style="{StaticResource PhoneTextExtraLargeStyle}" />
                            <TextBlock Text="{Binding Name}"
                                       TextWrapping="Wrap"
                                       Style="{StaticResource PhoneTextSubtleStyle}" />
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>

We simply need to insert a CollectionViewSource object into the data binding chain. To do this we can instantiate a CollectionViewSource, set the Items collection to it’s Source property and bind to the CollectionViewSource’s View property. See below:

public System.Windows.Data.CollectionViewSource Source { get; set; }

        // Constructor
        public MainPage()
        {
            Items = new ObservableCollection<Item>
                        {
                            new Item { Date = DateTime.Now.AddDays(-4), Text = "Item 1"},
                            new Item { Date = DateTime.Now.AddDays(-3), Text = "Item 2"},
                            new Item { Date = DateTime.Now.AddDays(-5), Text = "Item 3"},
                            new Item { Date = DateTime.Now.AddDays(-8), Text = "Item 4"},
                            new Item { Date = DateTime.Now.AddDays(-1), Text = "Item 5"},
                        };

            Source = new System.Windows.Data.CollectionViewSource();
            Source.Source = Items;

(Note: I am using the code behind here just for illustrative purposes. I would usually expose the CollectionViewSource from a View Model).

and the xaml changes to:

<ListBox Grid.Row="1" x:Name="ListBox"
                     Margin="0,0,-12,0"
                     ItemsSource="{Binding Source.View}">

Now, as a simple example of what we can do I attach two buttons with click handlers to enable sorting of the ListBox. So,

           <StackPanel Grid.Row="0"Orientation="Horizontal">
                <ButtonContent="Asc"Click="AscButtonClick"></Button>
                <ButtonContent="Desc"
                       Click="DescButtonClick"></Button>
            </StackPanel>

with the click handlers defined like this…

        private void DescButtonClick(object sender, RoutedEventArgs e)
        {
            Source.SortDescriptions.Clear();
            Source.SortDescriptions.Add(new SortDescription("Date", ListSortDirection.Descending));
        }

        private void AscButtonClick(object sender, RoutedEventArgs e)
        {
            Source.SortDescriptions.Clear();
            Source.SortDescriptions.Add(new SortDescription("Date", ListSortDirection.Ascending));
        }

So, what has this achieved? A simple sort of the Listbox, but also since I don’t need to sort my original list a ui-agnostic way to maintain the original data.

The project source can be downloaded here https://skydrive.live.com/?cid=4f1b7368284539e5&sc=documents&id=4F1B7368284539E5%21352

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