Linq with Grouping And Sum

var myList= (from t1 in liste
             group t1 by t1.ApprovalDate into x
             select new Invoice
             {
                CustomerID = x.FirstOrDefault().CustomerID,
                CustomerName = x.FirstOrDefault().CustomerName,
                CustomerSurname = x.FirstOrDefault().CustomerSurname,
                ApprovalDate  = x.FirstOrDefault().ApprovalDate,
                Total = x.Sum(t => t.Total)
              }
              ).ToList();