Xamarin: Inheritance and Polymorphism
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Inheritance and polymorphism are key object-oriented programming concepts supported in C#. Inheritance allows a child class to inherit properties and behaviors from a parent class. Polymorphism refers to classes having different implementations of the same method, allowing the method to behave differently depending on the object that calls it. Virtual methods allow subclasses to override behaviors of parent classes, while the new keyword is used to hide inherited methods instead of overriding them.
Xamarin: Inheritance and Polymorphism
- 1. Inheritance & Polymorphism Eng Teong Cheah Microsoft MVP Windows Development
- 2. Inheritance
- 3. Inheritance C# is an object oriented programming language so it supports one key feature of OOPS; Inheritance. In general, Inheritance can be explained as acquiring the properties of the parent. For this, we will use the most common example of the car class. You can see Car is an Object who has basic feature like name, height, no of wheels. It has two specialized version one is sedan and other is hatchback. Both sedan and hatch back has all the basic feature of car and some specialized property of their own. This feature is known as inheritance.
- 4. Inheritance When a child class has all the features of base class plus its own specialized properties. Then this phenomenon is known as inheritance.
- 5. Inheritance Types of inheritance supported by C# are follows: - Single inheritance Single base class and single derived class. - Multilevel inheritance B derive from A and C derive from B. - Hierarchical inheritance B and C both derived from A. - Multiple inheritance using Interfaces Class D derived from Interface A, Interface B and Class C.
- 6. Inheritance Preventing a Class from Being Inherited C# We can prevent the class from Being Inherited by Other Class by putting Sealed Keyword in class. See Example Below:- sealed class stringHandler { public string inputString { get; set; } public string OutputString { get; set; } public stringHandler(string inputstring) { this.inputString = inputString; } public String FormatString() { // Some Code here } }
- 7. Inheritance Constructor initialization C# Inheritance See the car example on top of this page, if we create an object of HatchBack class . The constructor initialization series is given below – HatchBack hb = new HatchBack("hatchback",4,125,40,true); 1. First Car constructor is initialized public Car(string name, int wheelcount, float length, float breadth) { this.Name = name; this.WheelCount = wheelcount; this.length = length; this.breadth = breadth; }
- 8. Inheritance 2. Second Hatchback Constructor will be initialized public HatchBack(string name, int wheelcount, float length, float breadth, bool isconvertible) : base(name, wheelcount, length, breadth) { this.IsConvertible = isconvertible; } So, from these examples we can say that constructor initialization in inheritance will always follow from base to derived downwards.
- 9. Inheritance Calling Base class’s Constructor in Inherited C# Classes : Base class constructor is called in derived class using base Keyword. See Car example on top of the page Calling a Base class’s Hidden members in inherited class C# : Base class hidden member function can be called by using base.functionname syntax. See Example below -
- 10. Inheritance Type Casting in Inheritance C# Parent class reference can hold child class reference but opposite is not true. Casting an object to a parent class is called upcasting. Casting an object to child class is called downcasting. See Example below - We have Draw() method both in base class and derived class. But the see which one is called when. If the Shape class’s Draw() Method is declared as virtual and overridden in Oval class’s Draw() method then result would be different. Result would be. Output when Draw() method will be declared as Virtual and overridden in derived class : Shape Drawn Oval Drawn Oval Drawn
- 11. Polymorphism
- 12. Polymorphism Polymorphism is one of the key features of OOPS. Polymorphism can be broken as below. Polymorphism =Poly + morphism =Multiple + forms. Polymorphism is simply calling derived class methods from base class reference during runtime. It is the behavioural change of methods depending upon the instance called at runtime.
- 13. Polymorphism Before we understand it in polymorphism in C# language, let’s understand it with a simple fantasy example. A mermaid, yes a mermaid. Viola……But how a mermaid is related to Polymorphism see image below. Mermaid shows a polymorphic behavior according to environment. Mermaid behaves like human while on earth and as a fish while in water.
- 14. 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.
- 15. Inherited Methods A method Foo() which is declared in the base class A and not redeclared in classes B or C is inherited in the two subclasses
- 16. Inherited Methods The method Foo() can be overridden in classes B and C:
- 17. Inherited Methods 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 using virtual 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 section Hiding and Overriding Methods.
- 18. 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.
- 19. 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:
- 20. 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:
- 21. Combining Method Overriding and Hiding A class C can now declare a method Foo() that either overrides or hides Foo() from class B:
- 22. Demo