Practical usage of virtual functions in c#

So basically if in your ancestor class you want a certain behaviour for a method. If your descendent uses the same method but has a different implementation you can override it, If it has a virtual keyword.

using System;
class TestClass 
{
   public class Dimensions 
   {
      public const double pi = Math.PI;
      protected double x, y;
      public Dimensions() 
      {
      }
      public Dimensions (double x, double y) 
      {
         this.x = x;
         this.y = y;
      }

      public virtual double Area() 
      {
         return x*y;
      }
   }

   public class Circle: Dimensions 
   {
      public Circle(double r): base(r, 0) 
      {
      }

      public override double Area() 
      { 
         return pi * x * x; 
      }
   }

   class Sphere: Dimensions 
   {
      public Sphere(double r): base(r, 0) 
      {
      }

      public override double Area()
      {
         return 4 * pi * x * x; 
      }
   }

   class Cylinder: Dimensions 
   {
      public Cylinder(double r, double h): base(r, h) 
      {
      }

      public override double Area() 
      {
         return 2*pi*x*x + 2*pi*x*y; 
      }
   }

   public static void Main()  
   {
      double r = 3.0, h = 5.0;
      Dimensions c = new Circle(r);
      Dimensions s = new Sphere(r);
      Dimensions l = new Cylinder(r, h);
      // Display results:
      Console.WriteLine("Area of Circle   = {0:F2}", c.Area());
      Console.WriteLine("Area of Sphere   = {0:F2}", s.Area());
      Console.WriteLine("Area of Cylinder = {0:F2}", l.Area());
   }
}

Edit: Questions in comment
If I don't use virtual keyword in base class, will it work?

If you use the override keyword in your descendent classes it will not work. You will generate compiler error CS0506 'function1' : cannot override inherited member 'function2' because it is not marked "virtual", "abstract", or "override"

If you don't use the override You'll get the CS0108 warning 'desc.Method()' hides inherited member 'base.Method()' Use the new keyword if hiding was intended.

To get around this put the new keyword in front of the method you are hiding.

e.g.

  new public double Area() 
  {
     return 2*pi*x*x + 2*pi*x*y; 
  }

..and is it compulsory to override a virtual method in derived class?
No, if you don't override the method, the descendent class will use method it is inheriting from.

Like any other language..when you want polymorphism. There are tons of usage for this. For example you want to abstract the way input is read from a console or a file or some other device. You can have a generic reader interface followed by multiple concrete implementations using virtual functions.

e.g. proxying methods. i.e. overwriting methods at runtime. For example, NHibernate uses this to support lazy loading.

This allows to achieve late binding, meaning to determine at runtime rather than at compile-time which object's member will be invoked. See Wikipedia.

Basically virtual members allow you to express polymorphism, a derived class can have a method with the same signature as the method in its base class, and the base class will call the derived class's method.

A basic example:

public class Shape
{
    // A few example members
    public int X { get; private set; }
    public int Y { get; private set; }
    public int Height { get; set; }
    public int Width { get; set; }

    // Virtual method
    public virtual void Draw()
    {
        Console.WriteLine("Performing base class drawing tasks");
    }
}

class Circle : Shape
{
    public override void Draw()
    {
        // Code to draw a circle...
        Console.WriteLine("Drawing a circle");
        base.Draw();
    }
}
class Rectangle : Shape
{
    public override void Draw()
    {
        // Code to draw a rectangle...
        Console.WriteLine("Drawing a rectangle");
        base.Draw();
    }
}
class Triangle : Shape
{
    public override void Draw()
    {
        // Code to draw a triangle...
        Console.WriteLine("Drawing a triangle");
        base.Draw();
    }
}

From here:

In object-oriented programming, a virtual function or virtual method is a function or method whose behavior can be overridden within an inheriting class by a function with the same signature.

For example you have a base class Params and a set of derived classes. You want to be able to perform the same operation on an array that stores all possible classes derived from params.

No problem - declare the method virtual, add some basic implementation to Params class and override it in derived classes. Now you can just traverse the array and call the method through the reference - the correct method will be called.

class Params {
public:
   virtual void Manipulate() { //basic impl here }
}

class DerivedParams1 : public Params {
public:
   override void Manipulate() {
      base.Manipulate();
      // other statements here
   }
};

// more derived classes can do the same

void ManipulateAll( Params[] params )
{
    for( int i = 0; i < params.Length; i++ ) {
       params[i].Manipulate();
    }
 }

Example

Let's consider the ToString() method in System.Object. Because this method is a member of System.Object it's inherited in all classes and will provide the ToString() methods to all of them.

namespace VirtualMembersArticle
{
    public class Company
    {
        public string Name { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Company company = new Company() { Name = "Microsoft" };
            Console.WriteLine($"{company.ToString()}");
            Console.ReadLine();
        }   
    }
}

The output of the previous code is:

VirtualMembersArticle.Company

Let's consider that we want to change the standard behavior of the ToString() methods inherited from System.Object in our Company class. To achieve this goal it's enough to use the override keyword to declare another implementation of that method.

public class Company
{
    ...
    public override string ToString()
    {
        return $"Name: {this.Name}";
    }         
}

Now, when a virtual method is invoked, the run-time will check for an overriding member in its derived class and will call it if present. The output of our application will then be:

Name: Microsoft

In fact, if you check the System.Object class you will find that the method is marked as virtual.

namespace System
{
    [NullableContextAttribute(2)]
    public class Object
    {
        ....
        public virtual string? ToString();
        ....
    }
}