Code error in Decorator design pattern..

Question

Hello everyone,

 

It's the first time I publish something in the forum, so I apologize if the code is not presented in the appropriate font for a code.. :/

 

I tried to implement the decorator design pattern, but for some reason something goes wrong during the process and the compiler throws NullReferenceException for a reason I can't figure out.

I'd be grateful for your help.

 

I have this as my basic item:

public class Hambruger
   {
   public virtual int PattySizeInGrams { get; set; }
   public virtual string Bun { get; set; }
   public Hambruger()
      {
         PattySizeInGrams = 200;
         Bun = "Ordinary bun";
      }
   }

public virtual void GetDescription()        {               Console.WriteLine("Hamburger:\n {0}, {1} grams", Bun,PattySizeInGrams );        }

 

 The exception happens when "new ExtraMeatDecorator(ham) is called. Before it calls the ExtraMeatDecorator constructor, it has to call its base constructor - HamburgerDecorator, which in turn, has to call first to the Hamburger constructor. The PattySizeInGrams

and Bun properties are null and thus, cannot make the assignment in the Hamburger constructor.

 

I just can't seem to figure out the reason. I've made a simpler example in which this problem didn't happen:

public class Parent
    {
        public  int Age { get; set; }
        public Parent()
        {
            Age = 25;
        }
    }

    public class Child : Parent
    {
        public Child(Parent par) : base() { }
    }