C# redefine sub-method in child class without redefining calling method in parent class

Question

I have a parent class with many methods in it that are all called by a single top-level method.

Conceptually the parent class looks like this:

class ParentClass
{
    void TopMethod(){ Lots of code and calls Methods1-N defined below}

    void Method1(){}
    void Method2(){}
    ...
    void MethodN(){}
}

I have many other classes that I want to be just minor variations on this base class. So I declare them as inheriting from ParentClass. Say all I need to do is change the definition of Method1 in the Child Classes. But how do I tell the child class to use everything else from the ParentClass, just with the new definition for Method1. In particular I don’t want the duplicate code of overriding TopMethod in the child class just so I can make it use the redefined Method1 in the child class instead of the Method1 in the ParentClass.

Well if Method1, Method2 etc are all virtual, you just override Method1 in the derived class... — Jon Skeet, Feb 28 '19 at 15:49
You make your method virtual in parent class and override them in inherited classes wherever needed — Haseeb Asif, Feb 28 '19 at 15:50
Also consider the appropriate access modifiers on your members (methods in this case). In your example code everything is `private` by default which means a base class could not see those members (or override them). If you want access inside a derived class to a member of the base class you need `protected` at a minimum. If you want to define a new implementation it needs to also be marked as `virtual`. — Igor, Feb 28 '19 at 15:52
See also [What are Virtual Methods?](https://stackoverflow.com/q/622132/1260204) and [What is the difference between an abstract function and a virtual function?](https://stackoverflow.com/q/391483/1260204) and [Practical usage of virtual functions in c#](https://stackoverflow.com/q/1062102/1260204) — Igor, Feb 28 '19 at 15:53
The post by DavidG is what I was trying to do - thanks! I was using just the "new" keyword in the child class, instead of "virtual" in the parent and "override" in the child. — John B, Feb 28 '19 at 16:24

score 1 · Answer 1 · answered Feb 28 '19 at 15:51

1

From the docs: virtual keyword

answered Feb 28 '19 at 15:51

Schwarzie2478

2,186
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score 1 · Accepted Answer · answered Feb 28 '19 at 15:53

You need to make Method1, Method2 etc. virtual and override them in the child class. For example:

public class ParentClass
{
    public void TopMethod()
    {
        Console.WriteLine("Top method in parent");
        Method1();
    }

    public virtual void Method1()
    {
        Console.WriteLine("Method1 in parent");
    }
}

public class ChildClass : ParentClass
{
    public override void Method1()
    {
        Console.WriteLine("Method1 in child");
    }
}

Now calling each class:

var parent = new ParentClass();
var child = new ChildClass();

parent.TopMethod();
child.TopMethod();

Will give you this output:

Top method in parent
Method1 in parent
Top method in parent
Method1 in child

C# redefine sub-method in child class without redefining calling method in parent class

2 Answers2