| title | Polymorphism (C# Programming Guide) | ||
|---|---|---|---|
| ms.date | 07/20/2015 | ||
| ms.prod | .net | ||
| ms.technology |
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| ms.topic | article | ||
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| ms.assetid | 086af969-29a5-4ce8-a993-0b7d53839dab | ||
| caps.latest.revision | 31 | ||
| author | BillWagner | ||
| ms.author | wiwagn |
Polymorphism is often referred to as the third pillar of object-oriented programming, after encapsulation and inheritance. Polymorphism is a Greek word that means "many-shaped" and it has two distinct aspects:
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At run time, objects of a derived class may be treated as objects of a base class in places such as method parameters and collections or arrays. When this occurs, the object's declared type is no longer identical to its run-time type.
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Base classes may define and implement virtual methods, and derived classes can override them, which means they provide their own definition and implementation. At run-time, when client code calls the method, the CLR looks up the run-time type of the object, and invokes that override of the virtual method. Thus in your source code you can call a method on a base class, and cause a derived class's version of the method to be executed.
Virtual methods enable you to work with groups of related objects in a uniform way. For example, suppose you have a drawing application that enables a user to create various kinds of shapes on a drawing surface. You do not know at compile time which specific types of shapes the user will create. However, the application has to keep track of all the various types of shapes that are created, and it has to update them in response to user mouse actions. You can use polymorphism to solve this problem in two basic steps:
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Create a class hierarchy in which each specific shape class derives from a common base class.
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Use a virtual method to invoke the appropriate method on any derived class through a single call to the base class method.
First, create a base class called Shape, and derived classes such as Rectangle, Circle, and Triangle. Give the Shape class a virtual method called Draw, and override it in each derived class to draw the particular shape that the class represents. Create a List<Shape> object and add a Circle, Triangle and Rectangle to it. To update the drawing surface, use a foreach loop to iterate through the list and call the Draw method on each Shape object in the list. Even though each object in the list has a declared type of Shape, it is the run-time type (the overridden version of the method in each derived class) that will be invoked.
[!code-csharpcsProgGuideInheritance#50]
In C#, every type is polymorphic because all types, including user-defined types, inherit from xref:System.Object.
When a derived class inherits from a base class, it gains all the methods, fields, properties and events of the base class. The designer of the derived class can choose whether to
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override virtual members in the base class,
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inherit the closest base class method without overriding it
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define new non-virtual implementation of those members that hide the base class implementations
A derived class can override a base class member only if the base class member is declared as virtual or abstract. The derived member must use the override keyword to explicitly indicate that the method is intended to participate in virtual invocation. The following code provides an example:
[!code-csharpcsProgGuideInheritance#20]
Fields cannot be virtual; only methods, properties, events and indexers can be virtual. 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. The following code provides an example:
[!code-csharpcsProgGuideInheritance#21]
Virtual methods and properties enable derived classes to extend a base class without needing to use the base class implementation of a method. For more information, see Versioning with the Override and New Keywords. An interface provides another way to define a method or set of methods whose implementation is left to derived classes. For more information, see Interfaces.
If you want your derived member to have the same name as a member in a base class, but you do not want it to participate in virtual invocation, you can use the new keyword. The new keyword is put before the return type of a class member that is being replaced. The following code provides an example:
[!code-csharpcsProgGuideInheritance#18]
Hidden base class members can still be accessed from client code by casting the instance of the derived class to an instance of the base class. For example:
[!code-csharpcsProgGuideInheritance#19]
Virtual members remain virtual indefinitely, regardless of how many classes have been declared between the virtual member and the class that originally declared it. If class A declares a virtual member, and class B derives from A, and class C derives from B, class C inherits the virtual member, and has the option to override it, regardless of whether class B declared an override for that member. The following code provides an example:
[!code-csharpcsProgGuideInheritance#22]
A derived class can stop virtual inheritance by declaring an override as sealed. This requires putting the sealed keyword before the override keyword in the class member declaration. The following code provides an example:
[!code-csharpcsProgGuideInheritance#24]
In the previous example, the method DoWork is no longer virtual to any class derived from C. It is still virtual for instances of C, even if they are cast to type B or type A. Sealed methods can be replaced by derived classes by using the new keyword, as the following example shows:
[!code-csharpcsProgGuideInheritance#25]
In this case, if DoWork is called on D using a variable of type D, the new DoWork is called. If a variable of type C, B, or A is used to access an instance of D, a call to DoWork will follow the rules of virtual inheritance, routing those calls to the implementation of DoWork on class C.
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. The following code provides an example:
[!code-csharpcsProgGuideInheritance#26]
For more information, see base.
Note
It is recommended that virtual members use base to call the base class implementation of that member in their own implementation. Letting the base class behavior occur enables the derived class to concentrate on implementing behavior specific to the derived class. If the base class implementation is not called, it is up to the derived class to make their behavior compatible with the behavior of the base class.
C# Programming Guide
C# Programming Guide
Inheritance
Abstract and Sealed Classes and Class Members
Methods
Events
Properties
Indexers
Types