Polymorphism, Method Hiding and Overriding

Example Class Hierarchy

Let’s assume the following simple class hierarchy with classes A, B and C for the discussions in this text. A is the super- or base class, B is derived from A and C is derived from class B. In some of the easier examples, we will only refer to a part of this class hierarchy.

Inherited Methods

A method Foo() which is declared in the base class and not redeclared in classes or is inherited in the two subclasses:

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    using System;
    namespace Polymorphism
    {
        class A
        {
            public void Foo() { Console.WriteLine("A::Foo()"); }
        }

        class B : A {}

        class Test
        {
            static void Main(string[] args)
            {
                A a = new A();
                a.Foo();  // output --> "A::Foo()"

                B b = new B();
                b.Foo();  // output --> "A::Foo()"
            }
        }
    }

The method Foo() can be overridden in classes and C:

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    using System;
    namespace Polymorphism
    {
        class A
        {
              public void Foo() { Console.WriteLine("A::Foo()"); }
        }

        class B : A
        {
              public void Foo() { Console.WriteLine("B::Foo()"); }
        }

        class Test
        {
            static void Main(string[] args)
            {
                A a;
                B b;

                a = new A();
                b = new B();
                a.Foo();  // output --> "A::Foo()"
                b.Foo();  // output --> "B::Foo()"

                a = new B();
                a.Foo();  // output --> "A::Foo()"
            }
        }

There are two problems with this code.

  • The output is not really what we, say from Java, expected. The method Foo() is a non-virtual method. C# requires the use of the keyword virtual in order for a method to actually be virtual. An example usingvirtual methods and polymorphism will be given in the next section.
  • Although the code compiles and runs, the compiler produces a warning: …\polymorphism.cs(11,15): warning CS0108: The keyword new is required on ‘Polymorphism.B.Foo()‘ because it hides inherited member ‘Polymorphism.A.Foo()‘.

This issue will be discussed in  Hiding and Overriding Methods.

Virtual and Overridden Methods

Only if a method is declared virtual, derived classes can override this method if they are explicitly declared to override the virtual base class method with the override keyword.

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    using System;
    namespace Polymorphism
    {
        class A
        {
            public virtual void Foo() { Console.WriteLine("A::Foo()"); }
        }

        class B : A
        {
            public override void Foo() { Console.WriteLine("B::Foo()"); }
        }

        class Test
        {
            static void Main(string[] args)
            {
                A a;
                B b;

                a = new A();
                b = new B();
                a.Foo();  // output --> "A::Foo()"
                b.Foo();  // output --> "B::Foo()"

                a = new B();
                a.Foo();  // output --> "B::Foo()"
            }
        }
     }

Method Hiding

Why did the compiler in the second listing generate a warning? Because C# not only supports method overriding, but also method hiding. Simply put, if a method is not overriding the derived method, it is hiding it. A hiding method has to be declared using the new keyword. The correct class definition in the second listing is thus:

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    using System;
    namespace Polymorphism
    {
        class A
        {
            public void Foo() { Console.WriteLine("A::Foo()"); }
        }

        class B : A
        {
            public new void Foo() { Console.WriteLine("B::Foo()"); }
        }

        class Test
        {
            static void Main(string[] args)
            {
                A a;
                B b;

                a = new A();
                b = new B();
                a.Foo();  // output --> "A::Foo()"
                b.Foo();  // output --> "B::Foo()"

                a = new B();
                a.Foo();  // output --> "A::Foo()"
            }
        }
    }

Combining Method Overriding and Hiding

Methods of a derived class can both be virtual and at the same time hide the derived method. In order to declare such a method, both keywords virtual and new have to be used in the method declaration:

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            class A
            {
                public void Foo() {}
            }

            class B : A
            {
                public virtual new void Foo() {}
            }

A class can now declare a method Foo() that either overrides or hides Foo() from class B:

            class C : B
            {
                public override void Foo() {}
                // or
                public new void Foo() {}
            }
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