仅在XAML中对组合框中的项进行分组

我正在尝试对CollectionViewSource做一个基本的使用,我一定是遗漏了一些东西,因为它根本就不起作用。这是我的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 WorkItemTypesForCurrentDestProject, Source={x:Static loc:MainMediator.Instance}, diagnostics:PresentationTraceSources.TraceLevel=High}" DisplayMemberPath="Name" />
                <Button Grid.Column="2" Content="{Binding PercentMapped}"/>
            </Grid>
        </DataTemplate>                                        
    </ListBox.ItemTemplate>
</ListBox>

这编译得很好,但当我运行应用程序时,会出现以下错误:

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

// 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));

我只想对对象进行简单的分组 Project SourceProject . 我宁愿不必为此打破树状视图。