using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var list = new  List<Sample>();

        list.Add(new SampleDerived(){ Age=2, DAge = 5 });

        list.Add(new SampleDerived(){ Age=3, DAge = 5 });

        list.Add(new SampleDerived(){ Age=4, DAge = 5 });

        list.Add(new SampleDerived(){ Age=5, DAge = 5 });

        list.Add(new SampleDerived(){ Age=6, DAge = 5 });

        var list2 = list.ToArray();

        Process(ref list2[2]);

        Console.WriteLine(list2[2].Age); // will print 10 not 4

    }

    public static void Process(ref Sample s)

    {

        s = new Sample(){Age=10};

    }

}

public class Sample

{

    public int Age {get; set;}

}

​

public class SampleDerived : Sample

{

    public int DAge {get; set;}

}