Method overriding in C#

Introduction:

Interviewer has ask about method overriding related tricky question in many interview. Definition is simple but many people get confused during interview.

I gets confused every time, even though I have read many times. So I thought of writing this article to clear all the doubts.

What is method overriding?

There are two type of polymorphism. Method overloading and Method overriding. Method overriding is important concept in C#.  Method having same signature and inherited from base class to derived classes is called method overriding. 

Here same signature means method name, number of arguments and type of arguments should be same.

Inheritance is required to achieve the method overriding.

How to achieve?

Virtual and override keyword is used for method overriding.

Override keyword is used with virtual and abstract method, property and event.

Inheritance:

In simple term we can say inheriting from parent class to derive class is called inheritance.

Three classes are mention here. Base, Derived and Subderived class. Base class has Show method. Derive class inherit base and subderive class inherit Derive class.

So Show() method is available for all classes.

Example:

Code for above diagram is mention here.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverriding
{

class Base
    {

        public  void  Show()
        {
            Console.WriteLine("Base");

        }

     
    }

    class Derived : Base
    {
      

    }

    class SubDerived : Derived
    {

    }
     class Program
    {
        static void Main(string[] args)
        {
           Base b = new Base();
           b.Show(); // Base

           b = new Derived();
           b.Show();//Drived

           SubDerived d = new SubDerived();
           d.Show();//SubDerive

           Console.ReadLine();

        }
    }

}

Output: Show() method is available for both derive and subderive class.

Output will be Base, Base, Base as shown below.

Hiding and Overriding Methods.

What will be output of this program.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverriding
{

 
class Base
    {

        public  void  Show()
        {
            Console.WriteLine("Base");

        }

       
    }

    class Derived : Base
    {
        public void Show()
        {
            Console.WriteLine("Derived");

        }

    }

    class SubDerived : Derived
    {
        public void Show()
        {
            Console.WriteLine("SubDerived");

        }
    }
      

    class Program
    {
        static void Main(string[] args)
        {
           Base b = new Base();
           b.Show(); // Base

           Derived d = new Derived();
           d.Show();//Drived

           SubDerived sd = new SubDerived();
           sd.Show();//SubDerive

           Console.WriteLine("---------------------------------");

           b = d;
           b.Show();//Base

           b = sd;
           b.Show();//Base

           Console.ReadLine();

        }
    }

}

Output: It will show warning for both Show method of Derived class and sub derived class as shown below.

But output will be as mention:

  

Method hiding(using new keyword)

Derive class overrides Base class. Show() method of Derive class hides the Show() method of Base class in above example. That’s way it is showing warning.

We can use new keyword if we want to hide Show() method of Base class in Derived class intentionally.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverriding
{

    
class Base
    {

        public  void  Show()
        {
            Console.WriteLine("Base");

        }

       
    }

    class Derived : Base
    {
        public new void Show()
        {
            Console.WriteLine("Derived");

        }

    }

    class SubDerived : Derived
    {
        public new void Show()
        {
            Console.WriteLine("SubDerived");

        }
    }
      

    class Program
    {
        static void Main(string[] args)
        {
           Base b = new Base();
           b.Show(); // Base

           Derived d = new Derived();
           d.Show();//Drived

           SubDerived sd = new SubDerived();
           sd.Show();//SubDerive

           Console.WriteLine("---------------------------------");

           b = d;
           b.Show();//Base

           b = sd;
           b.Show();//Base

           Console.ReadLine();

        }
    }

}
Output: Output will be same as above example without any warning.

  

Runtime polymorphism:

Main purpose of method overriding is to achieve runtime polymorphism. Virtual and override keyword is used for this.

Show() method of Base class is declare as virtual method. Derive class has derived from Base class using override keyword. SubDerived  class has overridden Derive class.

Here Base class instance b is consider as game changer. Create object of Base class like Base b=new Base(). Initialize derive class object d to b and SubDrive class object sd to b.

Then see the actual magic.

Base b = new Base()

b.Show();//”Base”

b= new Derive();

b.Show();//”Drive”

b = new SubDerived();

b.Show();//”SubDerived”

Here you can notice that, what ever object is assign to Base class object, respective method of that class is called.

Example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverriding
{

    
class Base
    {

        public virtual void  Show()
        {
            Console.WriteLine("Base");

        }

       
    }

    class Derived : Base
    {
        public override void Show()
        {
            Console.WriteLine("Derived");

        }

    }

    class SubDerived : Derived
    {
        public override void Show()
        {
            Console.WriteLine("SubDerived");

        }
    }
    

    class Program
    {
        static void Main(string[] args)
        {
           Base b = new Base();
           b.Show(); // Base

           Derived d = new Derived();
           d.Show();//Drived

           SubDerived sd = new SubDerived();
           sd.Show();//SubDerive

           Console.WriteLine("---------------------------------");

           b = d;
           b.Show();//Drived

           b = sd;
           b.Show();//SubDerive

           Console.ReadLine();

        }
    }

}

Output:

 

Some Tricky Question:

1. What will be output of following program.

class Base
   {

       public virtual void Show()
       {
           Console.WriteLine("Base");

       }

   }

   class Derived : Base
   {
       public void Show()
       {
           Console.WriteLine("Derived");

       }

   }

   class SubDerived : Derived
   {
       public new void Show()
       {
           Console.WriteLine("SubDerived");

       }
   }

   class Program
   {
       static void Main(string[] args)
       {
           Base b = new Base();
           b.Show(); // Base

           Derived d = new Derived();
           d.Show();//Drived

           SubDerived sd = new SubDerived();
           sd.Show();//SubDerive

           Console.WriteLine("---------------------------------");

           b = d;
           b.Show();//Drived

           b = sd;
           b.Show();//SubDerive

           Console.ReadLine();

       }
   }

Output: There will be compilation warning.

but output will be like this screen.

 

2. What will be output of following program.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverridingAndHidingTrickyQuestion
{
    class Base
    {

        public virtual void Show()
        {
            Console.WriteLine("Base");

        }

    }

    class Derived : Base
    {
        public override void Show()
        {
            Console.WriteLine("Derived");

        }

    }

    class SubDerived : Derived
    {
        public new void Show()
        {
            Console.WriteLine("SubDerived");

        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Base b = new Base();
            b.Show(); // Base

            Derived d = new Derived();
            d.Show();//Drived

            SubDerived sd = new SubDerived();
            sd.Show();//SubDerive

        

            Console.WriteLine("---------------------------------");

            b = d;
            b.Show();//Derived

            b = sd;
            b.Show();//Derived

         
            Console.ReadLine();

        }
    }
}

Output: Output is strange. See the output screen. This is left to readers to find the reason

behind this.

 

3. What will be output of below program.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MethodOverridingAndHidingTrickyQuestion
{
    class Base
    {

        public void Show()
        {
            Console.WriteLine("Base");

        }

    }

    class Derived : Base
    {
        public new virtual void Show()
        {
            Console.WriteLine("Derived");

        }

    }

    class SubDerived : Derived
    {
        public override void Show()
        {
            Console.WriteLine("SubDerived");

        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Base b = new Base();
            b.Show(); // Base

            Derived d = new Derived();
            d.Show();//Drived

            SubDerived sd = new SubDerived();
            sd.Show();//SubDerive
                    

            Console.WriteLine("---------------------------------");

            b = d;
            b.Show();//Base

            b = sd;
            b.Show();//Base

         
            Console.ReadLine();

        }
    }
}

Output: Here output is little strange for these 2 lines.

      b = d;
      b.Show();//Base

     Base class doesn’t has virtual function. Derive class has implemented virtual method.

    derive class object d is assign to base class object b and method show() gets called but same method with same signature is present in Base class also

   so it calls Show() method of base class.

      b = sd;
       b.Show();//Base

  Same happens for SubDrived class also.