We saw in Section 10.4 that only methods declared virtual, override or abstract can be overridden in derived classes. A method declared sealed in a base class cannot be overridden in a derived class. Methods that are declared private are implicitly sealed, because it is impossible to override them in a derived class (though the derived class can declare a new method with the same signature as the private method in the base class). Methods that are declared static also are implicitly sealed, because static methods cannot be overridden either. A derived class method declared both override and sealed can override a base class method, but cannot be overridden in derived classes further down the inheritance hierarchy.
A sealed method's declaration can never change, so all derived classes use the same method implementation, and calls to sealed methods are resolved at compile timethis is known as static binding. Since the compiler knows that sealed methods cannot be overridden, it can often optimize code by removing calls to sealed methods and replacing them with the expanded code of their declarations at each method-call locationa technique known as inlining the code.
A class that is declared sealed cannot be a base class (i.e., a class cannot extend a sealed class). All methods in a sealed class are implicitly sealed. Class string is a sealed class. This class cannot be extended, so applications that use strings can rely on the functionality of string objects as specified in the FCL.
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Case Study Creating and Using Interfaces |
Preface
Index
Introduction to Computers, the Internet and Visual C#
Introduction to the Visual C# 2005 Express Edition IDE
Introduction to C# Applications
Introduction to Classes and Objects
Control Statements: Part 1
Control Statements: Part 2
Methods: A Deeper Look
Arrays
Classes and Objects: A Deeper Look
Object-Oriented Programming: Inheritance
Polymorphism, Interfaces & Operator Overloading
Exception Handling
Graphical User Interface Concepts: Part 1
Graphical User Interface Concepts: Part 2
Multithreading
Strings, Characters and Regular Expressions
Graphics and Multimedia
Files and Streams
Extensible Markup Language (XML)
Database, SQL and ADO.NET
ASP.NET 2.0, Web Forms and Web Controls
Web Services
Networking: Streams-Based Sockets and Datagrams
Searching and Sorting
Data Structures
Generics
Collections
Appendix A. Operator Precedence Chart
Appendix B. Number Systems
Appendix C. Using the Visual Studio 2005 Debugger
Appendix D. ASCII Character Set
Appendix E. Unicode®
Appendix F. Introduction to XHTML: Part 1
Appendix G. Introduction to XHTML: Part 2
Appendix H. HTML/XHTML Special Characters
Appendix I. HTML/XHTML Colors
Appendix J. ATM Case Study Code
Appendix K. UML 2: Additional Diagram Types
Appendix L. Simple Types
Index