Question

Ok, I have a singleton class GraphMaster which contains a number of system-wide values. I have a subclass GraphObject : GraphMaster which has graph specific data. By subclassing, I can access members of either the global class or subclass. And by using a singleton class, I can change the global variables anywhere and have them be reflected in all the subclasses.

However, I’m getting stuck because the base class’s constructor wants to call the singleton class’s constructor, but it can’t as it’s marked private.

how do I get around this? Is what I’m trying to do possible? I went down this path due to responses to this post: Can I make a "global" object to store variables for multiple objects?

For example,

public class GraphMasterObject {
    private static GraphMasterObject instance;
    private GraphMasterObject() {   }
    }

    public static GraphMasterObject Instance {
        get {
            if (instance == null) instance = new GraphMasterObject();
            return instance;
        }
    }
    public int globalVar=10;
}

public class GraphObject : GraphMasterObject {

   public GraphObject() {
   }

   public int localVar=20;
}

I want to be able to do

GraphObject go = new GraphObject();
go.globalVar <- this is 10
GraphMasterObject.Instance.globalVar = 20;
go.globalVar <- now this is 20 

Answer 1

Ok, I have a singleton class GraphMaster which contains a number of system-wide values. I have a subclass GraphObject : GraphMaster which has graph specific data.

That’s a problem to start with. As soon as you have a class which has subclasses, that it by definition not a singleton. Someone can add another subclass at any point, and even if you only have one instance of each subclass, you’ll have two distinct instances which are compatible with the base class.

You could add something in the base class constructor to throw an exception if there’s already an instance, but it would be pretty smelly. Fundamentally, singletons are incompatible with subclassing. Rethink your design. (Ideally, avoid the singleton pattern in the first place, but that’s another matter…)