Virtual methods without body

Question

I was looking at some code in an abstract class:

public virtual void CountX(){}

public virtual void DoCalculation() { ...code}

Why should I declare an empty virtual method in an abstract class if it is not mandatory to override it in derived types?

Answer 1

Because if the default behaviour is to do nothing, but derived classes might want to do something. It's a perfectly valid structure.

It allows your base code to call it. You tend to see similar designs when there is "BeforeXXX" and "AfterXXX" code, at the base class this code is empty, but the method needs to be there to compile. In derived classes, this code is optional, but needs to be virtual to be overridden.

The fact that it is in an abstract class shouldn't confuse its behaviour.

An example:

  abstract class Base
    {
        public void ProcessMessages(IMessage[] messages)
        {
            PreProcess(messages);

            // Process.

            PostProcess(messages);
        }

        public virtual void PreProcess(IMessage[] messages)
        {
            // Base class does nothing.
        }

        public virtual void PostProcess(IMessage[] messages)
        {
            // Base class does nothing.
        }
    }

    class Derived : Base
    {
        public override void PostProcess(IMessage[] messages)
        {
            // Do something, log or whatever.
        }

        // Don't want to bother with pre-process.
    }

If these methods (Pre, Post) were abstract, then all derived classes would need to implement them (likely as empty methods) - code litter that can be removed using empty virtual methods at the base.