System Collections Generic C# by API

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Text;
public class MainClass
{
    public static void Main()
    {
        List intList = new List(new int[] { 3, 5, 15, 1003, 25 });
        ReadOnlyCollection roList = intList.AsReadOnly();
    }
}