C# Mongodb. Campo de soma para todos os documentos

var Total = _collection.AsQueryable().Where(x => x.ClientId == 2).Sum(x => x.Cash);

var x = MongoCollection.Aggregate()
                    .Group(
                            doc => doc.clientId,
                            group => new
                            {
                                clientId = group.Key,
                                Total = group.Sum(y => y.sum)
                            }
                    ).ToList().FirstOrDefault(c => c.clientId == 2).Total;