Your friends
is a field, not a property.
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
//public int[] friends = new int[100];
public int[] Friends {get; set; }
}
since you fill it later with a ToArray()
you don't really need to initialize it. When you still want to, use a constructor or write the long form of the property.