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Understanding polymorphism with abstract, virtual and override keyword in C#

The Abstract Modifier

The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events.

The Virtual Keyword

The virtual keyword is used to modify a method, property, indexer, or event declaration and allow for it to be overridden in a derived class.

The Override Modifier

The override modifier is required to extend or modify the abstract or virtual implementation of an inherited method, property, indexer, or event.

Override Rules

  • The overridden base method is a virtual, abstract, or override method. In other words, the overridden base method cannot be static or non-virtual.
  • The overridden base method is not a sealed method.
  • The override method and the overridden base method have the same return type and same signature.
  • The override declaration and the overridden base method have the same declared accessibility (i.e public, internal etc).
  • The override declaration does not specify type-parameter-constraints-clauses. Instead the constraints are inherited from the overridden base method.

Reference: https://docs.microsoft.com/

Code example

In following example override modifier is used to implement the abstract and extend virtual implementation of an inherited methods Print () and Display (), always it will call extended/ modified members.

   public abstract class OverrideParent
    {
        //Concreate implementation, child allowed to override
        public virtual void Print()
        {
            Console.WriteLine("Parent virtual print");
        }
 
        //pure abstract member, child must override
        public abstract void Display();
    }

 

    public class ChildOverride :OverrideParent
    {
        public override void Print()
        {
            Console.WriteLine("1st level print override");
        }
        public override void Display()
        {
            Console.WriteLine("1st level implementation");
        }
    }

 

    public class NestedChildOverride: ChildOverride
    {
        public override void Print()
        {
            Console.WriteLine("2nd level print override");
        }
        public override void Display()
        {
            Console.WriteLine("2nd level override/implementation");
        }
    }

 

Client implementation 

   //How it works?
    class Program
    {
        static void Main(string[] args)
        {
            OverrideParent ParentTypeChildImpn = new ChildOverride();
            ChildOverride ChildTypeChildImpn = new ChildOverride();
            ChildOverride ChildTypeNestedChldImpl = new NestedChildOverride();
            NestedChildOverride NestedChildTypeNestedImpl= new NestedChildOverride();
 
            //parent abstract type and child implementation
            ParentTypeChildImpn.Print();
            ParentTypeChildImpn.Display();
 
            //child type and child implementations
            ChildTypeChildImpn.Print();
            ChildTypeChildImpn.Display();
 
            //Child type and derived nested implementations
            ChildTypeNestedChldImpl.Print();
            ChildTypeNestedChldImpl.Display();
 
            //Nested child type and nested implementations
            NestedChildTypeNestedImpl.Print();
            NestedChildTypeNestedImpl.Display();
 
 
          }
    }


Output:

 

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