This project shows a possible solution to update a ComboBox items when its linked ItemsSource it's modified elsewhere
If not explicitly called, a Control is not updated when its ItemsSource get modified. A workaround is to explicitly tell the Control's ItemsSource Binding that its source has changed, via INotifyPropertyChanged.
So every time an edit is done on the DataTable exposed by the DataGrid, I notify the ComboBox of the change.
The only requirement to implement this method is to create a binding between the ComboBox and The DataTable (DefaultView). To be able to do that, in case you have your code in the code-behind, you need to set the correct DataContext. In my case I set MainWindow.xaml DataContext to its code-behind via: 'DataContext="{Binding RelativeSource={RelativeSource Self}}"'
Hope it helps.