To achieve your goal, first of all you need to store all expanded groups - RadGridView.GroupRowIsExpanded is the perfect event for that. I have pasted a sample code below although it is not the "full" solution, since one group could be expanded and collapsed several times. You will need to handle all the possible cases.
.radGridView.GroupRowIsExpandedChanged += newEventHandler<Telerik.Windows.Controls.GridView.GroupRowEventArgs>(radGridView_GroupRowIsExpandedChanged);
> expandedGroups =
sender, Telerik.Windows.Controls.GridView.GroupRowEventArgs e)
Then, based on the values you have saved in the expandedGroups collection, you should expand each group with the GridView.ExpandGroup method. This should be done after the data has been reloaded.
Please try this approach and let me know if you have met any problems.
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