DataGrid - 折叠除第一个组之外的所有组

I have a DataGrid with grouped ItemsSource. There are an expander on each group, so I can expand/collapse all the groups. Now, I'm trying to collapse all groups by default, but leave the first group expanded. The items source is dynamic, so I can't build any converter to check the group name. I must do it by group index.I have a DataGrid with grouped ItemsSource. The