using System;

using System.Linq;

using System.Collections.Generic;

​

public class HelloWorld

{

    public class MyExpression {

        public string Value;

        public MyExpression(string v) { this.Value = v;}

    }

    public static void Update(MyExpression e) {

        e.Value += "_Suffix";

    }

    public static void UpdateMany(List<MyExpression> e) {

        for (var i = 0; i < e.Count; i++) {

            if(i %3 == 0) {

                Update(e[i]);

            }

        }

    }

    public static void Main(string[] args)

    {

        var v = Enumerable.Range(1, 10).Select(i => new MyExpression("col" + i)).ToList();

        Console.WriteLine("Before: " + string.Join(",", v.Select(e => e.Value)));

​

        UpdateMany(v);

        Console.WriteLine("After: " + string.Join(",", v.Select(e => e.Value)));

    }

}