using System;
using System.Collections.Generic;
using System.Linq;
public class Program
{
public static void Main()
List<int> sequence = new List<int>{0,0,0,0,1,1,1,1,0,0,0,1,2,2,0,0,2,2};
int index = sequence.Select((x, ix) => (Item:x, Index:ix))
.Where(x => x.Item != 0)
.Skip(2) // you want the 3rd, so skip 2
.Select(x => x.Index)
.DefaultIfEmpty(-1) // if there is no third matching condition you get -1
.First(); // result: 6
Console.Write(index);
}
