var query = (from t in Transactions
group t by new {t.MaterialID, t.ProductID}
into grp
select new
{
grp.Key.MaterialID,
grp.Key.ProductID,
Quantity = grp.Sum(t => t.Quantity)
}).ToList();
group x by new { x.Column1, x.Column2 }
.GroupBy(x => new { x.Column1, x.Column2 })