C# | Method Overriding

Method Overriding in C# is similar to the virtual function in C++. Method Overriding is a technique that allows the invoking of functions from another class (base class) in the derived class. Creating a method in the derived class with the same signature as a method in the base class is called as method overriding. 

In simple words, Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes. When a method in a subclass has the same name, same parameters or signature and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class. Method overriding is one of the ways by which C# achieve Run Time Polymorphism(Dynamic Polymorphism).

The method that is overridden by an override declaration is called the overridden base method. An override method is a new implementation of a member that is inherited from a base class. The overridden base method must be virtual, abstract, or override.

Example:

class base_class
{
    public void gfg();
}

class derived_class : base_class
{
    public void gfg();
}

class Main_Method
{
 static void Main()
 {
    derived_class d = new derived_class();
    d.gfg();
 }
}

Here the base class is inherited in the derived class and the method gfg() which has the same signature in both the classes, is overridden. 

In C# we can use 3 types of keywords for Method Overriding:

• virtual keyword: This modifier or keyword use within base class method. It is used to modify a method in base class for overridden that particular method in the derived class.
• override: This modifier or keyword use with derived class method. It is used to modify a virtual or abstract method into derived class which presents in base class.

class base_class
{
    public virtual void gfg();
}

class derived_class : base_class
{
    public override void gfg();
}
class Main_Method
{
 static void Main()
 {
    derived_class d = new derived_class();
    d.gfg();
    
      base_class b = new derived_class();
        b.gfg();
 }
}

Here first, d refers to the object of the class derived_class and it invokes gfg() of the class derived_class then, b refers to the reference of the class base and it hold the object of class derived and it invokes gfg() of the class derived. Here gfg() method takes permission from base class to overriding the method in derived class.

Example 1: Method Overriding without using virtual and override modifiers

C#

// C# program to demonstrate the method overriding
// without using 'virtual' and 'override' modifiers
using System;

// base class name 'baseClass'
class baseClass

{
    public void show()
    {
        Console.WriteLine("Base class");
    }
}

// derived class name 'derived'
// 'baseClass' inherit here
class derived : baseClass
{

    // overriding
    new public void show()
    {
        Console.WriteLine("Derived class");
    }
}

class GFG {

    // Main Method
    public static void Main()
    {

        // 'obj' is the object of
        // class 'baseClass'
        baseClass obj = new baseClass();


        // invokes the method 'show()'
        // of class 'baseClass'
        obj.show();

        obj = new derived();

        // it will invokes the method
        // 'show()' of class 'baseClass'
        obj.show();

    }
}

Output:

Base class
Base class

Explanation: In this program, the object obj invokes class baseClass two times and call the method show() of class baseClass. To avoid this problem we use virtual and override keyword.

Example 2: Method overriding using virtual and override modifiers.

C#

// C# program to illustrate the use of
//'virtual' and 'override' modifiers
using System;

class baseClass {

    // show() is 'virtual' here
    public virtual void show()
    {
        Console.WriteLine("Base class");
    }
}


// class 'baseClass' inherit
// class 'derived'
class derived : baseClass
{

    //'show()' is 'override' here
    public override void show()
    {
        Console.WriteLine("Derived class");
    }
}

class GFG {

    // Main Method
    public static void Main()
    {

        baseClass obj;

        // 'obj' is the object
        // of class 'baseClass'
        obj = new baseClass();

        // it invokes 'show()'
        // of class 'baseClass'
        obj.show();


        // the same object 'obj' is now
        // the object of class 'derived'
        obj = new derived();

        // it invokes 'show()' of class 'derived'
        // 'show()' of class 'derived' is overridden
        // for 'override' modifier
        obj.show();

    }
}

Output:

Base class
Derived class

• base Keyword: This is used to access members of the base class from derived class. It basically used to access constructors and methods or functions of the base class. The base keyword cannot use within a static method. Base keyword specifies which constructor of the base class should be invoked while creating the instances of the derived class.

Use of Base keyword:

• Call methods or functions of base class from derived class.
• Call constructor internally of base class at the time of inheritance.

Example 3:

C#

// C# program to show the use of 'base'
// keyword in method overriding
using System;

// base class
public class web {


    string name = "GeeksForGeeks";

    // 'showdata()' is member method,
    // declare as virtual
    public virtual void showdata()
    {
        Console.WriteLine("Website Name: " + name);
    }
}

// derived class
// class 'web' is inherits
// class 'stream'
class stream : web {


    string s = "Computer Science";

    //'showdata()' is overridden
    // in derived class
    public override void showdata()
    {

        // Calling 'showdata()' of base
        // class using 'base' keyword
        base.showdata();

        Console.WriteLine("About: " + s);
    }
}

class GFG {

    // Main Method
    static void Main()
    {

        // 'E' is object of class stream
        // also works as object of
        // class 'web'
        stream E = new stream();


        // it first invokes 'showdata()'
        // of class 'web' then it invokes
        // 'showdata()' of class 'stream'
        E.showdata();

    }
}

Output:

Website Name: GeeksForGeeks
About: Computer Science

Example 4: How the base keyword specifies the calling of base-class constructor from derived class when derived class instances are created.

C#

// C# program to show how base keyword
// specifies the calling of base-class
// constructor from the derived class
// when derived class instances are created
using System;

// base class
public class clssA {

    int n1, n2;

    // default constructor
    public clssA()
    {
        Console.WriteLine("Default Constructor Invoked");
    }

    // parameterized constructor
    public clssA(int i, int j)
    {

        // construct values
        n1 = i;
        n2 = j;
        Console.WriteLine("Parameterized Constructor Invoked");
        Console.WriteLine("Invoked Values are: " + n1 + " and " + n2);
    }
}

// derived class
public class DerivedClass : clssA
{

    // This constructor will instantiate
    // 'clssA()' [no argument constructor]
    // using 'base' keyword
    public DerivedClass() : base() { }


    // This constructor will instantiate
    // 'clssA(int i, int j)' [parameterized
    // constructor] using 'base' keyword
    public DerivedClass(int i, int j) : base(i, j) { }

// Main Method
static void Main()
{

    // invoke no argument constructor
    DerivedClass d1 = new DerivedClass();

    Console.WriteLine();

    // invoke parameterized constructor
    DerivedClass d2 = new DerivedClass(10, 20);

}
}

Output:

Default Constructor Invoked

Parameterized Constructor Invoked
Invoked Values are: 10 and 20

Example 5: It shows how base keyword specifies the base-class constructor called from derived class and also calling of a method using the base keyword from the derived class.

C#

// C# program to show how 'base' keyword specifies
// the base-class constructor that called from
// derived class and also calling a method 'swap'
// from derived class using base keyword
using System;

// base class
public class clssA {

    public int n1, n2;

    // default constructor
    public clssA()
    {
        Console.WriteLine("In clssA 'no argument constructor' invoked");
    }

    // parameterized constructor
    public clssA(int i, int j)
    {

        // construct values
        n1 = i;
        n2 = j;
        Console.WriteLine("in clssA 'parameterized constructor' invoked");
        Console.WriteLine("the invoked values are " + n1 + " and " + n2);
        Console.WriteLine();
    }

    public virtual void swap()
    {
        Console.WriteLine("swap function of base class(clssA) invoked");
        Console.WriteLine("Before swap num1 = {0} and num2 = {1}", n1, n2);

        // swapping
        int t = n1;
        n1 = n2;
        n2 = t;
        Console.WriteLine("After swap num1 = {0} and num2 = {1}", n1, n2);
    }
}

// derived class
public class DerivedClass : clssA {

    // This constructor will instantiate
    // 'clssA' [no argument constructor]
    // using 'base' keyword
    public DerivedClass() : base() { }

    // This constructor will instantiate
    // 'clssA' [parameterized constructor]
    // using 'base' keyword
    public DerivedClass(int i, int j) : base(i, j) { }

    public override void swap()
    {

        // it access the swap function of
        // 'clssA' using 'base' keyword
        base.swap();

        Console.WriteLine();

        Console.WriteLine("Swap function of derived class invoked");
        Console.WriteLine("Before swap num1 = {0} and num2 = {1}", n1, n2);

        // swapping
        int t = n1;
        n1 = n2;
        n2 = t;
        Console.WriteLine("After swap num1 = {0} and num2 = {1}", n1, n2);
    }

// Main Method
static void Main()
{

    // invoke no argument constructor
    DerivedClass d1 = new DerivedClass();

    Console.WriteLine();

    // invoke parameterized constructor
    DerivedClass d2 = new DerivedClass(10, 20);

    // calling swap function
    d2.swap();

}
}

Output:

In clssA 'no argument constructor' invoked

in clssA 'parameterized constructor' invoked
the invoked values are 10 and 20

swap function of base class(clssA) invoked
Before swap num1 = 10 and num2 = 20
After swap num1 = 20 and num2 = 10

Swap function of derived class invoked
Before swap num1 = 20 and num2 = 10
After swap num1 = 10 and num2 = 20

Note:

• Method overriding is possible only in derived classes. Because a method is overridden in the derived class from the base class.
• A non-virtual or a static method can’t be overridden.
• Both the override method and the virtual method must have the same access level modifier.