Setting a read-only object from a derived class

Question

I am writing a library that other developers in our company will use. A state machine base class has a ReadOnlyCollection<T> of allowed states, etc. Developers need to inherit from this class and set the allowed states.

I want to limit them to initialize the ReadOnlyCollection<T> in the constructor of their derived classes and not be able to modify it later.

If I declare the ReadOnlyCollection<T> as a read-only property in the base class, that does not work since it cannot be modified in the constructor of the derived class.

I imagine this to be a not-so-uncommon scenario. Any elegant way to achieve this short of having developers override the ReadOnlyCollection<T>?

Answer 1

Don't let them initialize by themselves. Make your base class constructor to take the collection as an argument:

public class BaseClass
{
      protected readonly Collection someObject;
      public BaseClass(Collection object)
      {
            someObject = object
      }
}

So now when the derived class constructor is called it should call base class constructor also with the collection object otherwise it will be a compile time error. This will make sure that the collection is initialized in the constructor and no where else.

public class Derivedclass : BaseClass
{
      public DerivedClass() : base(/*pass the collection object here*/)
      {
      }
}

Still there is a pit fall in this, if you get the reference of a collection, you can still modify the collection by calling Add or remove method of the collection in the derived class only thing is you cant reinitialize if its readonly.