using System;

​

public class Program

{

    public static void Main()

    {

        var h = new TwoStepHistory<Baka>();

        h.Value = new Baka{Val = "test1"};

        h.Value = new Baka{Val = "test2"};

        Console.WriteLine(h.OldValue.Val);

        Console.WriteLine(h.Value.Val);

        h.Value.Val = "test3";

        Console.WriteLine(h.OldValue.Val);

        Console.WriteLine(h.Value.Val);

    }

}

​

struct TwoStepHistory<T>

{

    private T _v0;

    public T Value

    {

        get

        {

            return _v0;

        }

​

        set

        {

            OldValue = _v0;

            _v0 = value;

        }

    }

​

    public T OldValue

    {

        get;

        private set;

    }

}

​

class Baka

{

    public string Val;

}