virtual ...

using System.Collections.Generic;

namespace test
{
    public class Something
    {
        public Something()
        {
            OrderableItems c1 = new Car(200);
            OrderableItems b1 = new Bus(31);
            List<OrderableItems> items = new List<OrderableItems> { c1, b1 };

            // c1 is an object, it needs cast to show value of BHP
            // but if I use my custom ToString method I will be able to get the value
            SomethingCommon bhp = items[0].GetSomethingCommon();
        }
    }
}


class OrderableItems
{
    public virtual SomethingCommon GetSomethingCommon()
    {
        // default for all OrderableItems.
        // Check abstract too.
        return new SomethingCommon(0);
    } 
}

public class SomethingCommon
{
    private int _someMeasurer;

    public SomethingCommon(int someMeasurer)
    {
        _someMeasurer = someMeasurer;
    }
}

class Car: OrderableItems
{

    public Car(int bhp)
    {
        BHP = bhp;
    }

    public int BHP { get; private set; }

    public override SomethingCommon GetSomethingCommon()
    {
        // fun
        return new SomethingCommon(BHP);
    }
}