using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

 public static void Main()

 {

  var l = new List<int>(){1,2,3,4,5};

  l = l.Multiply(x => x%2 == 0, 10);

  Console.Write(String.Join(",", l));

 }

}

​

public static class ListExtensions{

    public delegate bool IsEqual(int x);

    public static List<int> Multiply(this IEnumerable<int> list, IsEqual func, int multiply){

        List<int> newList = new List<int>();

        foreach(var x in list){

            if(func(x)){

                newList.Add(x*multiply);

            }

        }

        return newList;

    }

}