What are Virtual Methods?

Question

Why would you declare a method as \"virtual\".

What is the benefit in using virtual?

Answer 1

Here it is explained clearly with example C# Virtual Method

Answer 2

In C#, for overriding the base class method in derived class, you have to declare base class method as virtual and derived class method as override as shown below:

using System;
namespace Polymorphism
{
 class A
 {
 public virtual void Test() { Console.WriteLine("A::Test()"); }
 }

 class B : A
 {
 public override void Test() { Console.WriteLine("B::Test()"); }
 }

 class C : B
 {
 public override void Test() { Console.WriteLine("C::Test()"); }
 }

 class Program
 {
 static void Main(string[] args)
 {

 A a = new A();
 B b = new B();
 C c = new C();
 a.Test(); // output --> "A::Test()"
 b.Test(); // output --> "B::Test()"
 c.Test(); // output --> "C::Test()"

 a = new B();
 a.Test(); // output --> "B::Test()"

 b = new C();
 b.Test(); // output --> "C::Test()"

 Console.ReadKey();
 }
 }
}

You can also mix the method hiding and method overriding by using virtual and new keyword since the method of a derived class can be virtual and new at the same time. This is required when you want to further override the derived class method into next level as I am overriding Class B, Test() method in Class C as shown below:

using System;
namespace Polymorphism
{
 class A
 {
 public void Test() { Console.WriteLine("A::Test()"); }
 }

 class B : A
 {
 public new virtual void Test() { Console.WriteLine("B::Test()"); }
 }

 class C : B
 {
 public override void Test() { Console.WriteLine("C::Test()"); }
 }

 class Program
 {
 static void Main(string[] args)
 {

 A a = new A();
 B b = new B();
 C c = new C();

 a.Test(); // output --> "A::Test()"
 b.Test(); // output --> "B::Test()"
 c.Test(); // output --> "C::Test()"

 a = new B();
 a.Test(); // output --> "A::Test()"

 b = new C();
 b.Test(); // output --> "C::Test()"

 Console.ReadKey();
 }
 }
}

GOLDEN WORDS: The virtual keyword is used to modify a method, property, indexer, or event declared in the base class and allow it to be overridden in the derived class.

The override keyword is used to extend or modify a virtual/abstract method, property, indexer, or event of base class into derived class.

The new keyword is used to hide a method, property, indexer, or event of base class into derived class.

ENJOY :-)

Answer 3

The runtime takes place over compile time.
When you declare a method as virtual, declaring it in derived class require you to add a override or new modifier.
we can see that when TrySpeak. Passing in child and father, both call Speak of father, while TryScream, would call each method.
To understand this, there are some things we should know, in an instance of Child, There are two Scream methods from Child class or Father class. We could either call the Scream from Child class or Father class. Because Virtaul Modifier mark the method so it can be overriding by the derived class, which means even the Scream is called from Father class, it is overriden, it would be defferent if you use new modifier.

import system;
class Father
{
    Speak()
    {
        Console.Writeline("Father is speaking") 
    }
    virtual Scream()
    {
        Console.Writeline("Father is screaming")    
    }
}
class Child: father
{
    Speak()
    {
        Console.Writeline("Child is speaking")  
    }
    override Scream()
    {
        Console.Writeline("Child is screaming") 
    }
}
class APP
{
    public static void Main()
    {
        // We new two instances here
        Father father = new Father();
        Child child = new Child();        
        // Here we call their scream or speak through TryScream or TrySpeak
        TrySpeak(father);
        TrySpeak(child);
        //>>>"Father is speaking"
        //>>>"Father is speaking"
        TryScream(father);
        TryScream(child);
        //>>>"Father is screaming"
        //>>>"Child is screaming"
    }
    // when your method take an Parameter who type is Father
    // You can either pass in a Father instance or
    // A instance of a derived Class from Father
    // which could be Child
    public static void TrySpeak(Father person)
    {
        person.Scream();
    }
    public static void TryScream(Father person)
    {
        person.Speak();
    }
}

Answer 4

Needless to say, virtual methods come in handy when your code is trying to abide with the Open Closed Principle

Read More about the Open Closed Principle here, Uncle Bob's original OCP whitepaper.

Also pls be aware that methods are not virtual by default in C# unlike Java.

Answer 5

Even if you don't plan to derive from the class, marking the method virtual may be necessary in order to mock the class. Some mocking frameworks only allow you to mock virtual methods. Note that methods implementing an interface are virtual implicitly.

I use RhinoMocks which has this restriction and have taken to marking my methods virtual by default for just this reason. For me, this is probably the biggest reason to use virtual methods as the cases where inheritance comes into play are much less frequent.

Answer 6

A short question, a short answer! Qualify your method as "virtual" if you think you will inherit of the class it belongs to.

A longer answer: "virtual enables you to override, to give another meaning of your method in a derived class.