C# abstract class virtual method

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One of our YouTube channel subscribers faced this question in an interview for Dot Net developer role. The interview question was asked as follows.

An abstract class has a default implementation for a method. The method's default implementation is needed in some class but not in some other class. How can you achieve it?



The answer to this question is very simple. If you know what a virtual method is and it's purpose, then you already know the answer.

Let us understand the answer to this question with an example.

public abstract class AbstractClass

{

    public virtual void AbstractClassMethod()

    {

        Console.WriteLine("Default Implementation");

    }

}

public class SomeClass : AbstractClass

{ }

public class SomeOtherClass : AbstractClass

{

    public override void AbstractClassMethod()

    {

        Console.WriteLine("New Implementation");

    }

}

class Program

{

    public static void Main()

    {

        SomeClass sc = new SomeClass();

        sc.AbstractClassMethod();

        SomeOtherClass soc = new SomeOtherClass();

        soc.AbstractClassMethod();          

    }

}

Points to remember

• In the AbstractClass we created a virtual method with a default implementation. 
• Any class that wants the default implementation can simply use it by inheriting from the abstract class. In our case SomeClass inherits from AbstractClass and does not override the default implementation of the virtual method in the base class. This means when we create an instance of SomeClass and call the virtual method the class gets the default implementation of the virtual method.
• On the other hand if a class does not need the default implementation, that class can inherit from the abstract class and override the default implementation. In our example, SomeOtherClass overrides the virtual method it inherited from the base abstract class. This means when we create an instance of SomeOtherClass and call the virtual method the class gets the new implementation and not the default implementation.

Hope this answers the question. If you have any suggestions to improve this answer, please leave them as comments.