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.
Note :
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.
