public abstract class computercomp
{
    public string Description{get;set;}
    public abstract double cost()
    {
        return 0;
    }
}

public class CPU1 : computercomp
{
    computercomp _comp;
    public CPU1(computercomp comp)
    {
         _comp = comp;
    }
    
    public override double Cost()
    {
         return 200 + _comp.Cost();
    }
}

public class HD500: computercomp
{
    computercomp _comp;
    public HD500(computercomp comp)
    {
         _comp = comp;
    }
    
    public override double Cost()
    {
         return 700 + _comp.Cost();
    }
}

public class Memory256: computercomp
{
    computercomp _comp;
    public Memory256(computercomp comp)
    {
         _comp = comp;
    }
    
    public override double Cost()
    {
         return 50 + _comp.Cost();
    }
}

public class Case: computercomp
{
    public override double Cost()
    {
         return 200;
    }
}

//...
public static void main()
{
     computercomp comp = new Case();
     comp = new Memory256(comp);
     comp = new CPU1(comp);
     comp = new HD500(comp);

     double price = comp.Cost();
}