Error, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var employees = new Employees();
            employees.Add( new Employee(new object[] { new EmployeeID(1), new EmployeeName("Initech") }));
            employees.Add(new Employee(new object[] { new EmployeeID(2), new EmployeeName("Foobar Inc") }));
            
            var employeeIds = employees.Items
                     .SelectMany(e => e.EmployeeFields )
                     .OfType<EmployeeID>()
                     .Select(item => item.ToString())
                     .ToList();
            
            foreach(var employeeId in employeeIds)
            {
                Console.WriteLine(employeeId);
            }
        }
    }
    
    public class Employees
    {
        private List<Employee> items;
        public Employees()
        {
            this.items = new List<Employee>();
        }
        public IEnumerable<Employee> Items { get { return items; } }
        
        public void Add(Employee item)
        {
            this.items.Add(item);
        }
    }
    
    public class Employee
    {
        private object[] employeeFields;
        public Employee(object[] employeeFields)
        {
            this.employeeFields = employeeFields;
        }
        
        public Object[] EmployeeFields{ get { return employeeFields; }}
    }
    
    
    public class EmployeeID
    {
        private int id;
        public EmployeeID(int id)
        {
            this.id = id;
        }
        
        
    }
    
    public class EmployeeName
    {
        private string name;
        public EmployeeName(string name)
        {
            this.name = name;
        }
        
     }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,13 sec, cpu time: 0,12 sec, average memory usage: 14 Mb, average nr of threads: 3