using System;
using System.Linq;
public class Program
{
public static void Main()
int ? [] stuff = new int ? []{1, 2, 3, 4, 5, null};
int sum = 0;
sum = stuff.Select((t, i) => stuff.ElementAt(i)).Where(x => x != null).Sum(x => x.GetValueOrDefault()); // sum = stuff.Sum(x => x.GetValueOrDefault());
Console.WriteLine(sum);
sum = stuff.Sum(x=>x.GetValueOrDefault());
}