does ArrayList and random numbers needs a library in c#?

I am unable to declare random numbers and ArrayList. It shows a red line beneath these key words. even though I am able to print things like how c# does.

What could be the possible problem? I installed vs2017 and started using it yesterday. wrote a simple program and it worked fine.

here is the code abd the error

code:

using System;
using System.Text;
namespace Question3
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] rand_array = new int[2000000];
            int num = random.Next();
            ArrayList arrlist = new ArrayList();
        }
    }
}