What is the difference between an abstract functio

Question

What is the difference between an abstract function and a virtual function? In which cases is it recommended to use virtual or abstract? Which one is the best approach?

Answer 1

Abstract methods are always virtual. They cannot have an implementation.

That's the main difference.

Basically, you would use a virtual method if you have the 'default' implementation of it and want to allow descendants to change its behaviour.

With an abstract method, you force descendants to provide an implementation.

Answer 2

An abstract method is a method that must be implemented to make a concrete class. The declaration is in the abstract class (and any class with an abstract method must be an abstract class) and it must be implemented in a concrete class.

A virtual method is a method that can be overridden in a derived class using the override, replacing the behavior in the superclass. If you don't override, you get the original behavior. If you do, you always get the new behavior. This opposed to not virtual methods, that can not be overridden but can hide the original method. This is done using the new modifier.

See the following example:

public class BaseClass
{
    public void SayHello()
    {
        Console.WriteLine("Hello");
    }


    public virtual void SayGoodbye()
    {
        Console.WriteLine("Goodbye");
    }

    public void HelloGoodbye()
    {
        this.SayHello();
        this.SayGoodbye();
    }
}


public class DerivedClass : BaseClass
{
    public new void SayHello()
    {
        Console.WriteLine("Hi There");
    }


    public override void SayGoodbye()
    {
        Console.WriteLine("See you later");
    }
}

When I instantiate DerivedClass and call SayHello, or SayGoodbye, I get "Hi There" and "See you later". If I call HelloGoodbye, I get "Hello" and "See you later". This is because SayGoodbye is virtual, and can be replaced by derived classes. SayHello is only hidden, so when I call that from my base class I get my original method.

Abstract methods are implicitly virtual. They define behavior that must be present, more like an interface does.

Answer 3

You basically use a virtual method when you want the inheritors to extend the functionality IF they want to.

You use abstract methods when you want the inheritors to implement the functionality (and in this case they have no choice)

Answer 4

There are nothing call virtual class in C#.

For functions

1. Abstract function only have signature only,the drive class should override with functionality.
2. Virtual function will hold the part of functionality the drive class may or may not override it according to the requirement

You can decide with your requirement.

Answer 5

An abstract function is "just" a signature, without an implementation. It is used in an interface to declare how the class can be used. It must be implemented in one of the derived classes.

Virtual function (method actually), is a function you declare as well, and should implemented in one of the inheritance hierarchy classes.

The inherited instances of such class, inherit the implementation as well, unless you implement it, in a lower hierarchy class.

Answer 6

Virtual Method:

• Virtual means we CAN override it.

• Virtual Function has an implementation. When we inherit the class we can override the virtual function and provide our own logic.

• We can change the return type of Virtual function while implementing the
  function in the child class(which can be said as a concept of
  Shadowing).

Abstract Method

• Abstract means we MUST override it.

• An abstract function has no implementation and must be in an abstract class.

• It can only be declared. This forces the derived class to provide the implementation of it.

• An abstract member is implicitly virtual. The abstract can be called as pure virtual in some of the languages.

  public abstract class BaseClass
  { 
      protected abstract void xAbstractMethod();
  
      public virtual void xVirtualMethod()
      {
          var x = 3 + 4;
      }
  }