How to modify method return value in all derived classes?

Question

Imagine you have a class hierarchy as:

class Base
{
    public virtual string GetName()
    {
        return "BaseName";
    }
}

class Derived1 : Base
{
    public override string GetName()
    {
        return "Derived1";
    }
}

class Derived2 : Base
{
    public override string GetName()
    {
        return "Derived2";
    }
}

In most appropriate way, how can I write the code in a way that all "GetName" methods adds "XX" string to return value in derived class?

For example:

         Derived1.GetName returns "Derived1XX"

         Derived2.GetName returns "Derived2XX"

Changing the code of GetName method implementation is not good idea, because there may exist several derived types of Base.

Answer 1

Leave GetName non-virtual, and put the "append XX" logic in that function. Extract the name (without "XX") to a protected virtual function, and override that in the child classes.

class Base
{
    public string GetName()
    {
        return GetNameInternal() + "XX";
    }

    protected virtual string GetNameInternal() 
    {
        return "BaseName";
    }
}

class Derived1 : Base
{
    protected override string GetNameInternal()
    {
        return "Derived1";
    }
}

class Derived2 : Base
{
    protected override string GetNameInternal()
    {
        return "Derived2";
    }
}