I'm trying manual source operations with grouping and aggregates, but I get: InvalidOperationException: No generic method 'Sum' on type 'System.Linq.Enumerable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic

I've edited the sample in the docs of manual source operations with grouping and added some aggregates.

Since some feats with the grid are not compatible with others, it is not easy to understand what is feasible and what is not. I know from the docs that virtual scolling is not compatible with many feats, or that group load on demand is not comaptible with aggregates, but I don't found any about aggregates with manual operations.

Please find the attached demo project