Issue with binding itemsource using MVVM in WP7 - windows-phone-7

Question

I am using MVVM pattern and I am trying to bind a public ObservableCollection Friends property to LongListSelector
<toolkit:LongListSelector
ItemsSource="{Binding Friends}"
GroupHeaderTemplate="{StaticResource movieGroupHeader}"
ListHeaderTemplate="{StaticResource movieListHeader}">
<toolkit:LongListSelector.ItemTemplate>
<DataTemplate>
<Grid Margin="12,8,0,8">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<StackPanel Grid.Column="1" VerticalAlignment="Top">
<TextBlock Text="{Binding MyBoxName}" Style="{StaticResource PhoneTextLargeStyle}" FontFamily="{StaticResource PhoneFontFamilySemiBold}" Margin="12,-12,12,6"/>
<TextBlock Text="{Binding MyBoxID}" Style="{StaticResource PhoneTextNormalStyle}" TextWrapping="Wrap" FontFamily="{StaticResource PhoneFontFamilySemiBold}"/>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Status:" Style="{StaticResource PhoneTextSmallStyle}"/>
<TextBlock Text="{Binding Status}" Style="{StaticResource PhoneTextSmallStyle}" FontFamily="{StaticResource PhoneFontFamilySemiBold}"/>
</StackPanel>
</StackPanel>
</Grid>
</DataTemplate>
</toolkit:LongListSelector.ItemTemplate>
</toolkit:LongListSelector>
Now issues is when i try to add
_friends.Add(new Model.Friends
{
MyBoxID = e.RosterItem.Jid,
MyBoxName = e.RosterItem.Name,
Status = Matrix.Xmpp.PresenceType.unavailable
})
it gives InvalidCastException so i tried List instead of ObservableCollection. Now i don't get exception but nothing is displayed in LLS. How can i bind My ObservableCollection property to LLS with Grouping.

Answer 1

For LongListSelector you need not just a collection, but some more complex structure to support grouping
Look at LongListCollection. It's great class that should help you
Usage:
var list = new LongListCollection<Event, string>(Events, x => x.Date.ToLongDateString());