using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public class Test{

        public int Id {get;set;}

    }

    public static void Main()

    {

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

        var b = new List<int>();

        b.AddRange(a.Select(f=>f.Id));

        foreach (int id in b) {

            Console.WriteLine(id);

        }

​

    }

}