from с in список
                group c by new { c.paramA, c.paramB } into cg
                orderby cg.Key.paramA, cg.Key.paramB
                select new
                {
                    ParamA = cg.Key.paramA,
                    ParamB = cg.Key.paramB,
                    Total = cg.Sum(c => c.val)
                };