using System;

using System.Linq;

using System.Collections.Generic;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        string[] foo = {"aaaa", "bbb","ccc"};

        var bar = foo.Where(x => x == "bbb").Select((y,i) => new {Index = i, Value = y}).FirstOrDefault();

        Console.WriteLine(bar);

​

        foo[bar.Index] = "fff";

        foreach(var i in foo)

        {

            Console.WriteLine(i);

        }

    }

}

​