using System;

using System.Collections;

​

public class Program

{

    public static void Main()

    {

        var a = new int[] { 1, 2, 3, 4 };

        IncrementItem(a, 3);

    }

    static void IncrementItem(IList list, int n)

    {

        var index = list.IndexOf(n);

        if (index >= 0)

        {

            list[index] = (((int)list[index]) + 1).ToString();

        }

    }

}