using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var c = new[]{1, 2, 3, };

        var s = c.Sumi((x, i) => i % 2 == 0 ? x * 2 : x);

        // Getting the value out

        Console.WriteLine(s); // 10

    }

}

​

public static class LinqExtensions

{

    public static int Sumi(this IEnumerable<int> source, Func<int, int, int> selector) => source.Select(selector).Sum();

}