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());

        Console.WriteLine(sum);

    }

}