using System;

using System.Linq;

using System.Collections.Generic;

​

class Program

{

    static IEnumerable<string> Data () {

        Console.WriteLine("data: evaluate");

        yield return "red";

        Console.WriteLine("data: evaluate");

        yield return "green";

        Console.WriteLine("data: evaluate");

        yield return "blue";

        Console.WriteLine("data: complete");

    }

    static (T value, int index) TraceSelect<T> (T value, int index) {

        Console.WriteLine("select: evaluate");

        return (value, index);

    }

    static void Main()

    {

        Console.WriteLine("---- foreach loop");

        foreach ((var value, int index) in Data().Select(TraceSelect))

            Console.WriteLine($"foreach: {index} {value}");

        Console.WriteLine("---- tolist + foreach method");

        Data().Select(TraceSelect).ToList().ForEach(e => {

            Console.WriteLine($"foreach: {e.index} {e.value}");

        });

        Console.WriteLine("----");

    }

}