Classes

A class is a reference type that can be derived directly from another class and that is derived implicitly from System..::.Object. The class defines the operations that an object (which is an instance of the class) can perform (methods, events, or properties) and the data that the object contains (fields). Although a class generally includes both definition and implementation (unlike interfaces, for example, which contain only definition without implementation), it can have one or more members that have no implementation.

The following table describes some of the characteristics that a class may have. Each language that supports the runtime provides a way to indicate that a class or class member has one or more of these characteristics. However, individual programming languages that target the .NET Framework may not make all these characteristics available.

Characteristic Description
sealed Specifies that another class cannot be derived from this type.
implements Indicates that the class uses one or more interfaces by providing implementations of interface members.
abstract Indicates that the class cannot be instantiated. To use it, you must derive another class from it.
inherits Indicates that instances of the class can be used anywhere the base class is specified. A derived class that inherits from a base class can use the implementation of any public members provided by the base class, or the derived class can override the implementation of the public members with its own implementation.
exported or not exported Indicates whether a class is visible outside the assembly in which it is defined. This characteristic applies only to top-level classes and not to nested classes.

Note : A class can also be nested in a parent class or structure. Nested classes also have member characteristics. For more information, see Nested

Class members that have no implementation are abstract members. A class that has one or more abstract members is itself abstract; new instances of it cannot be created. Some languages that target the runtime let you mark a class as abstract even if none of its members are abstract. You can use an abstract class when you want to encapsulate a basic set of functionality that derived classes can inherit or override when appropriate. Classes that are not abstract are referred to as concrete classes.

A class can implement any number of interfaces, but it can inherit from only one base class in addition to Object, from which all classes inherit implicitly. All classes must have at least one constructor, which initializes new instances of the class. If you do not explicitly define a constructor, most compilers will automatically provide a default (parameterless) constructor.