How to use a RibbonGallery with a RibbonComboBox - wpf

Question

So I wanted to add a RibbonCombobox to my Ribbon in WPF. For some reason, RibbonCombobox does not have a selectionchanged event. I read that you should use a RibbonGallery for selection change event so I implemented this
<RibbonComboBox Label="Equations" x:Name="EquationListComboToolbar" ItemsSource="{Binding}">
<RibbonGallery x:Name="EquationListComboboxGallery" SelectedValue="{Binding XPath=.}" />
</RibbonComboBox>
Behind the scene the binding is done like this.
EquationListComboToolbar.DataContext = ViewModel.EquationNames;
this.Bind(ViewModel, vm => vm.SelectedEquation, v => v.EquationListComboboxGallery.SelectedItem).DisposeWith(cleanup);
Observable.FromEventPattern(EquationListComboboxGallery, nameof(EquationListComboboxGallery.SelectionChanged)).Subscribe(e => ViewModel.SelectEquation(EquationListComboboxGallery.SelectedItem?.ToString()));
At runtime I get the following error
"An unhandled exception of type 'System.InvalidOperationException' occurred in WindowsBase.dll
Items collection must be empty before using ItemsSource." When the app initalizez. I know it's something about the Gallery but I can't figure out what is the problem and how can I achieve this.
As I was suggested, I already tried the answer that was suggested
<RibbonComboBox Label="Equations" x:Name="EquationListComboToolbar" ItemsSource="{Binding}">
<RibbonComboBox.ItemTemplate>
<DataTemplate>
<RibbonGallery x:Name="EquationListComboboxGallery" SelectedValue="{Binding XPath=.}" />
</DataTemplate>
</RibbonComboBox.ItemTemplate>
</RibbonComboBox>
Doing this, will make by binding imposible

Answer 1

Ah, yes. The Microsoft ribbon library is lots of fun. Luckily I've been down this road before. Here's a working example of a RibbonComboBox from one of my applications, complete with RibbonGallery:
<RibbonComboBox DropDownHeight="400">
<RibbonGallery MaxColumnCount="1" ScrollViewer.VerticalScrollBarVisibility="Auto" SelectedItem="{Binding MySelectedItemProperty}">
<RibbonGalleryCategory ItemsSource="{Binding MyItemsSourceProperty}"/>
</RibbonGallery>
</RibbonComboBox>
I'm not entirely sure this is the only way to do things, but I know this way works. Note that I set ItemsSource on the RibbonGalleryCategory, not the RibbonComboBox itself. It might be possible to use the RibbonGallery without a RibbonGalleryCategory, in which case you would set ItemsSource on RibbonGallery, but I've not tested this.
Note you also have the ability to add multiple galleries categories to a single RibbonComboBox like so:
<RibbonComboBox DropDownHeight="400">
<RibbonGallery MaxColumnCount="1" ScrollViewer.VerticalScrollBarVisibility="Auto" SelectedItem="{Binding MySelectedItemProperty}">
<RibbonGalleryCategory ItemsSource="{Binding MyFirstItemsSourceProperty}"/>
<Separator/>
<RibbonGalleryCategory ItemsSource="{Binding MySecondItemsSourceProperty}"/>
</RibbonGallery>
</RibbonComboBox>
The above lets you show multiple lists in the same drop down and allows the user to select a single item from any list. Functionality like this is probably why RibbonGalleryCategory exists in the first place.