Instead of trying to sum the entire element, we can use the element selector to retrieve the member we are interested in summing.
using System; using System.Linq; using System.Collections; using System.Collections.Generic; class Program/*from w ww . ja v a 2 s . c o m*/ { static void Main(string[] args) { IEnumerable<StudentOptionEntry> options = StudentOptionEntry.GetStudentOptionEntries(); long optionsSum = options.Sum(o => o.optionsCount); Console.WriteLine("The sum of the Student options is: {0}", optionsSum); } } class StudentOptionEntry { public int id; public long optionsCount; public DateTime dateAwarded; public static StudentOptionEntry[] GetStudentOptionEntries() { StudentOptionEntry[] empOptions = new StudentOptionEntry[] { new StudentOptionEntry { id = 1, optionsCount = 2, dateAwarded = DateTime.Parse("1990/12/31") }, new StudentOptionEntry { id = 2, optionsCount = 10000, dateAwarded = DateTime.Parse("1992/06/30") }, new StudentOptionEntry { id = 2, optionsCount = 10000, dateAwarded = DateTime.Parse("1991/01/01") }, new StudentOptionEntry { id = 3, optionsCount = 5000, dateAwarded = DateTime.Parse("1997/09/30") }, new StudentOptionEntry { id = 2, optionsCount = 10000, dateAwarded = DateTime.Parse("2000/04/01") }, new StudentOptionEntry { id = 3, optionsCount = 7500, dateAwarded = DateTime.Parse("1998/09/30") }, new StudentOptionEntry { id = 3, optionsCount = 7500, dateAwarded = DateTime.Parse("1998/09/30") }, new StudentOptionEntry { id = 4, optionsCount = 1500, dateAwarded = DateTime.Parse("1997/12/31") }, new StudentOptionEntry { id = 101, optionsCount = 2, dateAwarded = DateTime.Parse("1998/12/31") } }; return (empOptions); } }