CollectionViewSource Use Question

I am trying to do a basic use of CollectionViewSource and I must be missing something because it is just not working. Here is my XAML:

<Window.Resources>
  <CollectionViewSource Source="{Binding loc:MainVM.Instance.MapItems}" x:Key="MapCV">
     <CollectionViewSource.GroupDescriptions>
        <PropertyGroupDescription PropertyName="SourceProject" />
     </CollectionViewSource.GroupDescriptions>
  </CollectionViewSource>
</Window.Resources>

<ListBox ItemsSource="{StaticResource MapCV}" HorizontalContentAlignment="Stretch" ScrollViewer.HorizontalScrollBarVisibility="Disabled">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <Grid HorizontalAlignment="Stretch">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="2*"/>
                    <ColumnDefinition Width="2*"/>
                    <ColumnDefinition Width="50"/>
                </Grid.ColumnDefinitions>
                <TextBlock Grid.Column="0" Text="{Binding SourceType, Converter={StaticResource WorkItemTypeToStringConverter}}"/>
                <ComboBox Grid.Column="1" SelectedItem="{Binding DestType}" ItemsSource="{Binding Wo开发者_StackOverflow社区rkItemTypesForCurrentDestProject, Source={x:Static loc:MainMediator.Instance}, diagnostics:PresentationTraceSources.TraceLevel=High}" DisplayMemberPath="Name" />
                <Button Grid.Column="2" Content="{Binding PercentMapped}"/>
            </Grid>
        </DataTemplate>                                        
    </ListBox.ItemTemplate>
</ListBox>

This compiles fine, but when I run the app I get this error:

Cannot convert the value in attribute 'ItemsSource' to object of type 
'System.Collections.IEnumerable'. 'System.Windows.Data.CollectionViewSource' 
is not a valid value for property 'ItemsSource'.  Error at object 
'System.Windows.Controls.ListBox' in markup file 'WIAssistant;component/main.xaml

This is the collection I am attaching to:

// The mappings used to copy the values of the fields of one WorkItem to another.
public ObservableCollection<WorkItemTypeMapping> WorkItemTypeMappings
{
    get { return (ObservableCollection<WorkItemTypeMapping>)
          GetValue(WorkItemTypeMappingsProperty); }
    set { SetValue(WorkItemTypeMappingsProperty, value); }
}
public static readonly DependencyProperty WorkItemTypeMappingsProperty = 
    DependencyProperty.Register("WorkItemTypeMappings", 
    typeof(ObservableCollection<WorkItemTypeMapping>), typeof(MainMediator), 
    new UIPropertyMetadata(null));

I just want to do simple grouping on object Project SourceProject. I would rather not have to break out a tree view for this.

This should work for you

<ListBox ItemsSource="{Binding Source={StaticResource MapCV}}" ...