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C# Polymorphism


What it is..
Polymorphism literally means existence of more than one form for a single entity.
In context of C# , a  Class can be used as more than one type; it can be used as its own type, any base types, or any  Interface type if it implements interfaces.
Every type in C# is polymorphic.

Polymorphism-Quick Review

A class derived from base class, inherits all the methods, fields, properties and events of the base class. The data and behavior of a base class can be changed using any one of the following two options

1. Use new keyword and replace the base class member in the derived class

For Example:
public class MyBase
{
    public void ReachOffice() { }
    public int Intime;
    public int OutTime
    {
        get { return 0; }
    }
}
 
public class MyDerived : MyBase
{
    public new void ReachOffice () { }
    public new int Intime;
    public new int OutTime
    {
        get { return 0; }
    }
}

Here the use of new keyword  hides the base class member and the new class members are called, when an object for the class is created.

MyDerived mdclass = new MyDerived ();
Mdclass. ReachOffice()

Hidden class members can still be called if an instance of the derived class is cast to an instance of the base class. For example:


MyBase Mbaseclass=( MyBase)mdclass;
Mbaseclass. ReachOffice();


2. Over-ride virtual base class member.

If an instance of a derived class has  to completely take over  from a base class members, the base class has to declare that member as virtual. Derived classes can then  override the base class member, to replace the base class implementation with its own.
For example:
public class MyBase
{
    public virtual void ReachOffice() { }
    public virtual int OutTime
    {
        get { return 0; }
    }
}
 
public class MyDerived : MyBase
{
    public override void ReachOffice () { }
    public override int OutTime
    {
        get { return 0; }
    }
}

When a derived class overrides a virtual member, that member is called even when an instance of that class is being accessed as an instance of the base class. For example:
The new overridden method is called in both the cases.
MyDerived mdclass = new MyDerived ();
Mdclass. ReachOffice()

MyBase Mbaseclass=( MyBase)mdclass;
Mbaseclass. ReachOffice();
As the overridden  virtual member is called regardless of which type(Base/Derived) the caller is using, it show that derived class has changed the initial basic behavior of the base class.(Polymorphism).

Virtual members remain virtual indefinitely. If class PP declares a virtual member, and class QQ derives from PP, and class RR derives from QQ, class RR inherits the virtual member, and has can override it, even if class QQ has  overridden that member. For example:
public class PP
{
    public virtual void MyVirtualMethod() { }
}
public class QQ : PP
{
    public override void MyVirtualMethod () { }
}
public class RR : QQ
{
    public override void MyVirtualMethod () { }
}
A derived class can stop virtual inheritance by declaring an override as sealed. For example:
public class RR : QQ
{
    public sealed override void MyVirtualMethod () { }
}
Now if a  class SS derives from RR,  it cannot override MyVirtualMethod.
Now if the requirement is there to replace MyVirtualMethod, it can be done using new keyword, but not to forget, it will not be overridden.
It will simply hide the base class implementation For Example:
public class SS: RR
{
    public new void MyVirtualMethod () { }
}
In this case, if MyVirtualMethod is called on SS using a variable of type SS, the new MyVirtualMethod is called. If a variable of type RR, QQ, or PP is used to access an instance of SS, a call to MyVirtualMethod will follow the rules of virtual inheritance, routing those calls to the implementation of MyVirtualMethod on class RR.
A derived class that has replaced or overridden a method or property can still access the method or property on the base class using the base keyword. For example:
C#
public class MyBase
{
    public virtual void BaseMethod () { }
}
public class MyDerived: MyBase
{
    public override void BaseMethod () { }
}
C#
public class Derived2: MyDerived
{
    public override void BaseMethod ()
    {
        // Call BaseMethod on Derived to get Derived 's behavior:
        base. BaseMethod ();
    }
}

This tutorial was a quick overview of Polymorphism as implemented in C#.

From now on, I’ll be starting a series of articles where I’ll cover the advanced topics of Threads.
Till then..Happy Coding!!                         


                                                            



























Note:
Fields cannot be virtual; only methods, properties, events and indexers can be virtual.

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