using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        var myArray = new int[] {1, 2, 3, 4, 2, 4, 6, 7, 8};

        var newArray = post4e(myArray, whereFunction);

        foreach(var e in newArray)

        {

            Console.WriteLine(e);

        }

    }

    private static bool whereFunction(int value)

    {

        return value == 4;

    }   

    public static IEnumerable<T> post4e<T>(IEnumerable<T> set, Func<T, bool> pred)

    {

        if (set == null)

        {

            throw new ArgumentNullException("set");

        }

        if (pred == null)

        {

            throw new ArgumentNullException("pred");

        }

        var search = set

            .Select((v, idx) => new { v, idx })

            .Where(x => pred(x.v))

            .OrderByDescending(x => x.idx)

            .FirstOrDefault();

        if (search == null)

        {

            return Enumerable.Empty<T>();

        }

        return set.Skip(search.idx + 1);

    }

}