Question

If I inherit from a base class, is there a rule which states “only inherit where you need to implement/use all (or X%) of the base classes functionality/methods”? I understand that if I inherit then I have inherited the lot, but that doesn’t mean I have to use it all.

Assume the following pattern:

public abstract class Template
{   
    public void LoadCustomer()
    {
        //Load customer logic
    }
    public virtual void LoadGrid()
    {
        //Logic   
    }
    public virtual void other()
   { //logic }

    public virtual void other2()
   { //logic }

    public virtual void other3()
   { //logic }

    public string WelcomeMessage()
    {
         //perform lots of complex logic
         return "Hello and welcome";
    }
}

If part of my application wants to only show the WelcomeMessage(), would it be wrong to inherit from the Template class above (I know I can only inherit once but that isn’t factor for this question) (also note, I’ve purposely not included any abstract methods/fields).

Answer 1

I went down this road on quite a few of my projects before I “saw the light.”

is there a rule which states “only inherit where you need to implement/use all (or X%) of the base classes functionality/methods”?

Yes and no. No that there is no rule, but yes that there is a way to do what you want to do. It’s call favoring composition over inheritance and what that means is that you need to break off each block of functionality into a single class, and then (in what is currently your sub-classes) create an instance of that class and then delegate the needed work to it. Using this technique will allow you to easily make modifications to what is currently your super-class without having ripple effects on your current sub-classes.

Bottom line, when in doubt (or if it just feels funny) don’t use inheritance. This is embodied by the Liskov substitution principle which states that any sub-class should be a full and proper substitute for it’s super class. (i.e. don’t have Boat subclass Plane just because they can both carry people).

For more fun reading, check out the SOLID software engineering paradigm.