Question

Should I always need to use FirstOrDefault to show columns not included in aggregate function?

 var creditos = from c in context.creditos
                       where c.Status == status
                       join a in context.acreditados on c.IDCredito equals a.IDCredito
                       group a by a.IDCredito into g
                       select new
                       {
                           Id = g.FirstOrDefault().creditos.IDCredito,
                           Expediente = g.FirstOrDefault().creditos.Expediente,
                           Status = (_Credito.Status)g.FirstOrDefault().creditos.Status,
                           Producto = (_Credito.Producto)g.FirstOrDefault().creditos.Producto,
                           Monto = g.Sum(Monto => Monto.Cantidad),
                           Fecha = g.FirstOrDefault().creditos.FechaInicio,
                           Tasa = g.FirstOrDefault().creditos.TasaInteres,
                           Plazo = g.FirstOrDefault().creditos.Plazo,
                           Periodo = g.FirstOrDefault().creditos.Periodo
                       };

Answer 1

No, not really: you could use LastOrDefault. The important thing is to get a single value to “pair up” with scalars produced in the aggregation.

An alternative is to put these values in the key at the group by stage: if you know they would be the same, add them to the key of the group, and pull them from there during the aggregation stage.

var creditos = from c in context.creditos
                   where c.Status == status
                   join a in context.acreditados on c.IDCredito equals a.IDCredito
                   group a by new {a.IDCredito, a.FechaInicio, a.TasaInteres, a.Plazo, a.Periodo } into g
                   select new
                   {
                       Id = g.FirstOrDefault().creditos.IDCredito,
                       Expediente = g.FirstOrDefault().creditos.Expediente,
                       Status = (_Credito.Status)g.FirstOrDefault().creditos.Status,
                       Producto = (_Credito.Producto)g.FirstOrDefault().creditos.Producto,
                       Monto = g.Sum(Monto => Monto.Cantidad),
                       Fecha = g.Key.FechaInicio,
                       Tasa = g.Key.TasaInteres,
                       Plazo = g.Key.Plazo,
                       Periodo = g.Key.Periodo
                   };