using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int[] terms = new int[]{2,4,6,9};   

        //First approach

        string result = String.Join(",", terms.ToList().ConvertAll(i => "["+i.ToString()+"]"));

        //Second approach

        string result1 = String.Join(",", terms.Select(x => "[" + x + "]"));

        //Third approach

        string result2 = string.Join(",", Array.ConvertAll(terms, s => $"[{s}]"));

        Console.WriteLine($"Result of first approach : {result}");

        Console.WriteLine($"Result of second approach : {result1}");

        Console.WriteLine($"Result of third approach : {result2}");

    }

}