grouping in itemscontrol using ItemsSource from ViewModel
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I've defined ItemsControl like that:

    <ItemsControl Style="{StaticResource SellingDashboardToDosList}" Grid.Row="2" BorderThickness="1" Background="#C7E8F8" HorizontalAlignment="Stretch" ItemsSource="{Binding Path=ToDoList}">
                        <ItemsControl.GroupStyle>
                            <GroupStyle>
                                <GroupStyle.ContainerStyle>
                                    <Style TargetType="GroupItem">
                                        <Setter Property="Template">
                                            <Setter.Value>
                                                <ControlTemplate TargetType="GroupItem">
                                                    <GroupBox Header="{Binding Path=Model.TodoType}" >
                                                        <ItemsPresenter />
                                                    </GroupBox>
                                                </ControlTemplate>
                                            </Setter.Value>
                                        </Setter>
                                    </Style>
                                </GroupStyle.ContainerStyle>
                            </GroupStyle>
                        </ItemsControl.GroupStyle>
                    </ItemsControl>

The ItemsSource is a SynchronisedObservableCollection<T> in the ViewModel. But this XAML doesn't produce any grouping. I assume that I should specify somehow that ItemsSource is groupable. But where should I specify it? 

If I would use an XmlDataProvider with some static data, then I could do it in a CollectionViewSource element like in following example: http://cromwellhaus.com/2010/03/grouping-is-crazy-easy-in-wpf/ . I've tried to do it like that:

    <CollectionViewSource x:Key="CollectionViewSource1" Source="{Binding Path=ToDoList}">
                <CollectionViewSource.GroupDescriptions>
                    <PropertyGroupDescription PropertyName="TodoType" />
                </CollectionViewSource.GroupDescriptions>
            </CollectionViewSource>

But then I get a runtime binding error that ItemsSource is of invalid type.