using System;

using System.Linq;

using System.Collections.Generic;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        List<string> foo = new List<string>() {

            "1", "12", "123", "1234", "12345"

        };

        List<string> bar = new List<string>() {

            "123", "a", "b", "1", "12"

        };

        var foos = foo.Select((f, i) => new { idx = i, val = f });

        var bars = bar.Select((b, i) => new { idx = i, val = b });

        var indexes = foos.Join(bars, 

            f => f.val, 

            b => b.val, 

            (f, b) => new { idxA = f.idx, idxB = b.idx });

        foreach (var idxs in indexes) {

            Console.WriteLine("idxA: {0} idxB: {1}", idxs.idxA, idxs.idxB);

        }

    }

}

​

public class Foo {

    public void Do() { Console.WriteLine("Do"); }

}