using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<string> list = new List<string>{"a", "b", "c", "d","c"};

        List<string> list2 = new List<string>{"a", "c"};

        var result = list.Select((a, b) => new {Value = a, Index = b})

                          .Where(x => list2.Any(d => d == x.Value))

                          .Select(c => c.Index).ToArray();

        foreach (var item in r)

        {

            Console.WriteLine(item.ToString());

        }

    }

}