using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int sum = 0;

        var numbers = new[]{-2, -1, 0, 1, 2, 3};

        var sumPositiveNumbers = numbers.Where(x => x > 0).Select(x => sum += x);

        Console.WriteLine($"Not yet evaluated sum of positive integers: {sum}.");

        Evaluate(sumPositiveNumbers);

        Console.WriteLine($"Evaluated sum of positive integers: {sum}.");

    }

​

    private static IEnumerable<T> Evaluate<T>(IEnumerable<T> source)

        => source.Aggregate(Enumerable.Empty<T>(), (evaluated, s) => evaluated.Append(s));

}