virtual methods

In this chapter you will learn:

What are virtual methods and how to create virtual methods
virtual methods from parent classes

Get to know virtual methods

A virtual member, marked by the virtual keyword, can be overriden by its subclasses. The method, property, indexer and event can be virtual.

A virtual method is declared as virtual in a base class and it is redefined in one or more derived classes. When redefining a virtual method in the derived class, the override modifier is used.

When overriding a method, the type signature of the method cannot be changed. A virtual method cannot be static or abstract.

The following code shows how to use the virtual method to provide the different implementation of the Area.

using System;// j ava  2  s  .c  o m

class Shape{
  public virtual double Area{
     get{
        return 0;
     }
  }
}


class Rectangle:Shape{
   public int width;
   public int height;
   
   public override double Area{
      get{
         return width * height;
      }
   }
}

class Circle:Shape{
   public int radius;
   
   public override double Area{
      get{
         return 3.14 * radius * radius;
      }
   }
}


class Program
{
    static void Main(string[] args)
    {
        Rectangle r = new Rectangle();
        r.width = 4;
        r.height = 5;
        Circle c = new Circle();
        c.radius = 6;
        Console.WriteLine(r.Area);
        Console.WriteLine(c.Area);

    }
}

The output:

C# requires that the virtual and override must have the same signature, accessor and return type.

virtual methods from parent classes

When a virtual method is not overridden, the base class method is used.

using System; /*from  j a v a 2  s . com*/
 
class BaseClass { 
  // Create virtual method in the base class.  
  public virtual void who() { 
    Console.WriteLine("who() in BaseClass"); 
  } 
} 
 
class Derived1 : BaseClass { 
  // Override who() in a derived class. 
  public override void who() { 
    Console.WriteLine("who() in Derived1"); 
  } 
} 
 
class Derived2 : BaseClass { 
  // This class does not override who(). 
} 
 
class MainClass { 
  public static void Main() { 
    BaseClass baseOb = new BaseClass(); 
    Derived1 dOb1 = new Derived1(); 
    Derived2 dOb2 = new Derived2(); 
 
    BaseClass baseRef; // a base-class reference 
 
    baseRef = baseOb;  
    baseRef.who(); 
 
    baseRef = dOb1;  
    baseRef.who(); 
 
    baseRef = dOb2;  
    baseRef.who(); // calls BaseClass's who() 
  } 
}

The code above generates the following result.

Next chapter...

What you will learn in the next chapter:

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