Question

Whenever i override a method of a base class, other than my implementation of this method, i seem to have 3 choices.

1) Call base.Method(), and then provide my implementation.

2) Provide my implementation and then call base.Method()

3) Just provide my implementation.

Recently while using a library i have realized few bugs that were introduced because of not implementing the method as expected by the library. I am not sure if that is bad on part of library, or something wrong in my understanding.

I will take one example.

public class ViewManager {
     public virtual void Customize(){
        PrepareBaseView();
     }
}

public class PostViewManager {
     public override void Customize(){
        base.Customize();
        PreparePostView();
     }
}


public class PreViewManager {
     public override void Customize(){
        PreparePreView();
        base.Customize();
     }
}


public class CustomViewManager {
     public override void Customize(){
        PrepareCustomView();
     }
}

My question here is that how could a child class know (without taking a look at base class implementation) which order (or option) is being expected by the parent class?
Is there a way in which parent class could enforce one of the three alternates to all the deriving classes?

Answer 1

how could a child class know (without taking a look at base class implementation) which order (or option) is being expected by the parent class?

There is no way to “know” this when you are subclassing and overriding a method. Proper documentation is really the only option here.

Is there a way in which parent class could enforce one of the three alternates to all the deriving classes?

The only option here is to avoid the issue. Instead of allowing the subclass to override the method, it can be declared non-virtual, and call a virtual method in the appropriate place. For example, if you want to enforce that subclasses “call your version first”, you could do:

public class BaseClass {
    public void Method() // Non-virtual
    {
          // Do required work

          // Call virtual method now...
          this.OnMethod();
    }

    protected virtual void OnMethod()
    { // Do nothing
    }
 }

The subclasses can then “override” OnMethod, and provide functionality that happens after “method”‘s work.

The reason this is required is that virtual methods are designed to allow a subclass to completely replace the implementation of the parent class. This is done on purpose. If you want to prevent this, it’s better to make the method non-virtual.