This is just a clarifying question:

Mainform.cs, starting at line 81

// get all order prices ResultsetFields orderPricesFields = new ResultsetFields(2); orderPricesFields.DefineField(OrderDetailsFieldIndex.OrderID, 0, "OrderID"); orderPricesFields.DefineField(OrderDetailsFieldIndex.ProductID, 1, "OrderPrice", "", AggregateFunction.Sum); IExpression productPriceExpression = new Expression( EntityFieldFactory.Create(OrderDetailsFieldIndex.Quantity), ExOp.Mul, EntityFieldFactory.Create(OrderDetailsFieldIndex.UnitPrice)); orderPricesFields[1].ExpressionToApply = productPriceExpression; IGroupByCollection groupBy = new GroupByCollection(); groupBy.Add(orderPricesFields[0]);

I think I follow all of this except for line 84. Why was ProductID selected? It doesn't seem to be used for anything, and I used Discount instead, and it doesn't appear to have altered the outcome at all. Am I correct in my assumption that the productPriceExpression overrides this value?

Thanks.

Any field will do, as the field itself is replaced by the expression assigned in the query. As fields are used for expression placeholders, this is the way to go. I also could have used a dummy field, I opted for this shortcut.