combobox с checkbox с# wfp

Как-то так (использовал PropertyChanged.Fody, поэтому в коде нет явных вызовов OnPropertyChanged и OnIsSelectedChanged):

public class CalendarItem: INotifyPropertyChanged
{
  public CalendarItem(Calendar owner) => Owner = owner;

  public Calendar Owner { get; }
  public bool IsSelected { get; set; }
  void OnIsSelectedChanged() => Owner.UpdateSelection(this);

  public string Name { get; set; }
  public event PropertyChangedEventHandler PropertyChanged;
  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
}

public class Calendar : INotifyPropertyChanged
{
  readonly ObservableCollection<CalendarItem> _items = new ObservableCollection<CalendarItem>();
  public ICollectionView Items { get; }
  public Calendar()
  {
    var dtf = CultureInfo.InvariantCulture.DateTimeFormat;
    foreach(var s in Enumerable.Range(1,12).Select(n=>dtf.GetMonthName(n)))
      _items.Add(new CalendarItem(this){Name = s});
    Items = CollectionViewSource.GetDefaultView(_items);
  }

  public void UpdateSelection(CalendarItem selectedItem)
  {
    var selected = _items.Where(i => i.IsSelected).ToList();
    SelectedItem = selected.Any() ? selectedItem : null;
    Selection = selected.Any() ? selected.Select(i => i.Name).Aggregate((a, b) => $"{a}, {b}") : string.Empty;
  }

  public CalendarItem SelectedItem { get; set; }
  public string Selection { get; private set; }
  public event PropertyChangedEventHandler PropertyChanged;

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
}

Разметка:

<Window.Resources>
  <local:Calendar x:Key="Calendar" />
  <DataTemplate DataType="{x:Type local:CalendarItem}" x:Key="CalendarItemTemplate">
    <CheckBox
      IsChecked="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
      Content="{Binding Name, Mode=OneWay}" />
  </DataTemplate>
  <DataTemplate x:Key="SelectionBoxTemplate">
    <TextBlock
      Text="{Binding RelativeSource={RelativeSource FindAncestor, ComboBox, 1}, Path=DataContext.Selection}" />
  </DataTemplate>
  <DataTemplate x:Key="CombinedTemplate">
    <ContentPresenter
      x:Name="Presenter"
      Content="{Binding}"
      ContentTemplate="{StaticResource CalendarItemTemplate}" />
    <DataTemplate.Triggers>
      <DataTrigger
        Binding="{Binding RelativeSource={RelativeSource FindAncestor,ComboBoxItem,1}}"
        Value="{x:Null}">
        <Setter
          TargetName="Presenter" Property="ContentTemplate"
          Value="{StaticResource SelectionBoxTemplate}" />
      </DataTrigger>
    </DataTemplate.Triggers>
  </DataTemplate>
</Window.Resources>

<Window.DataContext>
  <StaticResource ResourceKey="Calendar" />
</Window.DataContext>

<ComboBox
  VerticalAlignment="Center"
  Margin="10"
  ItemsSource="{Binding Items}"
  SelectedItem="{Binding SelectedItem, Mode=OneWay}"
  ItemTemplate="{StaticResource CombinedTemplate}" />