Inheritance Base class with one instance

Question

I want to know if it's possible having something like this:

• basic class A1
• derived class A2 : A1
• derived class A3 : A1

to have only one instance of A1? Because when I use ctor(params):base(params) for each derived class A2 , A3 create a new instance of A1 and I want to avoid this.

Answer 1

Via inheritance, no. This isn't possible. If you chose composition though, this can be accomplished.

A1 becomes a Singleton.

A2 and A3 both have a reference to the Singleton instance of A1 and proxies the appropriate calls through to that instance.