ASP.NET Programming with C# and SQL Server First Edition Chapter 10 Developing Object-Oriented C# Programs

Objectives In this chapter, you will: Study object-oriented programming concepts Define custom classes Declare class fields Work with class methods ASP.NET Programming with C# and SQL Server, First Edition

Introduction The programs written so far have been self-contained Most of the code exists within a script section Object-oriented programming allows you to use and create self-contained sections of code that can be reused without having to copy them ASP.NET Programming with C# and SQL Server, First Edition

Introduction to Object-Oriented Programming Object-oriented programming (OOP): refers to the creation of reusable software objects Object: programming code and data that can be treated as an individual unit or component Code that comprises an object is organized into classes Objects may range from simple controls, such as a button, to entire programs Objects are often designed to perform specific tasks ASP.NET Programming with C# and SQL Server, First Edition

Introduction to Object-Oriented Programming (cont’d.) Popular object-oriented programming languages include C++, Java, and Visual Basic In many cases, objects written in one language can be used by programs written in another language You only need to know how to access the methods and properties of those objects to use them ASP.NET Programming with C# and SQL Server, First Edition

Understanding Encapsulation Encapsulated objects: all code and required data are contained within the object itself Usually within a single file Interface: refers to the methods and properties that are required for a source program to communicate with an object Objects can be thought of as “black boxes” Users of the object can see only the methods and properties that you allow them to see Example: handheld calculator You do not need to know its inner workings to use it ASP.NET Programming with C# and SQL Server, First Edition

Understanding Encapsulation (cont’d.) Figure 10-1 Calculator interface ASP.NET Programming with C# and SQL Server, First Edition

Using Objects in C# Programs Object-oriented techniques will help you build more extensible code that is easier to reuse, modify, and enhance Class: a template, or blueprint, that serves as the basis for creating new instances of objects from the class Instantiation: the creation of an object from a class A particular instance of an object inherits its methods and properties from the class from which it was created ASP.NET Programming with C# and SQL Server, First Edition

Using Objects in C# Programs (cont’d.) Syntax to instantiate an object from a class: ClassName objectName = new ClassName(); Class constructors are used to initialize properties when an object is first instantiated Many constructors accept arguments Use a period to access the methods and properties of an object Methods require a set of parentheses at the end of the method name, like functions Methods can accept arguments ASP.NET Programming with C# and SQL Server, First Edition

Using Objects in C# Programs (cont’d.) Sample application: Central Valley Bakery Web site Has four shopping categories: cakes, cookies, pies, and breads Goal: develop a reusable class named ShoppingCart to handle the functionality of building and updating a shopping cart for a Web site ShoppingCart class requires four fields for product information: productID, name, description, and price ASP.NET Programming with C# and SQL Server, First Edition

Figure 10-2 Central Valley Bakery home page ASP.NET Programming with C# and SQL Server, First Edition

Figure 10-3 Cakes product page after adding Web server controls ASP.NET Programming with C# and SQL Server, First Edition

Defining Custom C# Classes Data structure: a system for organizing data Field: a variable defined within a class Class members: the methods and fields defined in a class Data members (or member variables): class variables Function members (or member functions): the methods defined in a class ASP.NET Programming with C# and SQL Server, First Edition

Defining Custom C# Classes (cont’d.) After an object has been instantiated: Class data members are called properties of the object Class function members are called methods of the object Classes are also called user-defined data types or programmer-defined data types ASP.NET Programming with C# and SQL Server, First Edition

Defining Custom C# Classes (cont’d.) Why use classes? They help make complex programs easier to manage by logically grouping related methods and fields They can be used to hide information that users of the class do not need to access They provide an easy way to reuse code Instances of class objects inherit class members from the class on which they are based, allowing you to build new classes based on existing classes ASP.NET Programming with C# and SQL Server, First Edition

Working with Access Modifiers Access modifiers: control a client’s access to classes, individual fields, methods and their members There are five access modifiers in C#: public private protected internal protected internal The default value is internal ASP.NET Programming with C# and SQL Server, First Edition

Working with Access Modifiers (cont’d.) Table 10-1 C# access modifiers ASP.NET Programming with C# and SQL Server, First Edition

Creating a Class Definition Class definition: contains the class members that make up the class Syntax: accessModifier class ClassName { //Class member definitions } Class names should start with an uppercase letter partial keyword: indicates that a class definition can be split across multiple files ASP.NET Programming with C# and SQL Server, First Edition

Collecting Garbage Garbage collection: the cleaning up, or reclaiming, of a member that is reserved by a program C# knows when a program no longer needs a variable or object and automatically cleans up the memory for that program Exception: open database connections must be closed by the program ASP.NET Programming with C# and SQL Server, First Edition

Declaring Class Fields A class definition requires the declaration of class fields To design a class definition, you must understand the principle of information hiding ASP.NET Programming with C# and SQL Server, First Edition

What Is Information Hiding? Information hiding: any class members that client programs do not need to know about should be hidden Information hiding gives an encapsulated object its “black box” capabilities Users of the class see only the members of the class that you allow them to see Reduces the complexity of the code that clients see Prevents the accidental introduction of a bug into a program by other programmers ASP.NET Programming with C# and SQL Server, First Edition

Using Access Modifiers with Fields Class fields are declared like standard variables, except that an access modifier is included at the beginning of the declaration It is good programming practice to assign an initial value to a field when it is declared To access a field as an object property, append the property name to the object with a period ASP.NET Programming with C# and SQL Server, First Edition

Serializing Objects Serialization: the process of converting an object’s field into a string that can be stored for reuse .NET Framework supports two types of serialization: Binary serialization XML serialization Binary serialization: converts object properties to a binary format Very efficient Maintains the data types of the properties Readable only by the .NET Framework ASP.NET Programming with C# and SQL Server, First Edition

Serializing Objects (cont’d.) XML serialization: converts object properties to XML Does not maintain the data types of properties Good for sharing the data with other applications Must mark a class as serializable in order to serialize an object of that class Add the Serializable attribute immediately above the class definition, surrounded by brackets [] Binary serialized objects are commonly stored in binary files on a local computer ASP.NET Programming with C# and SQL Server, First Edition

Serializing Objects (cont’d.) File stream: used for accessing a resource, such as a file Input stream: reads data from a resource Output stream: writes data to a resource Response.Write() statements send data to an output stream (the Web browser window) You must include the System.IO namespace before you can use the FileStream class Use the Serialize() method of the BinaryFormatter class to serialize objects in binary format ASP.NET Programming with C# and SQL Server, First Edition

Serializing Objects (cont’d.) Use the Deserialize() method of the BinaryFormatter class to convert serialized data back into an object Add the NonSerialized attribute in brackets before the declaration of any field in the class that does not have to be serialized You can store a serialized object in session state To deserialize, you must cast the session variable to the class from which the serialized object was created ASP.NET Programming with C# and SQL Server, First Edition

Working with Class Methods Methods perform most of the work in a class Methods are usually declared as public or private public methods can be called by anyone private methods can be called only by other methods in the same class Only methods that clients need to access should be declared as public Include an access modifier before the method’s return type ASP.NET Programming with C# and SQL Server, First Edition

Initializing with Constructor Methods Constructor method: a special method that is called automatically when an object is instantiated from a class Its name is the same as the class name It is declared without a return type Must have the public access modifier Usually used to assign initial values to fields or to perform other initialization tasks A class may have one or more constructor methods ASP.NET Programming with C# and SQL Server, First Edition

Cleaning Up with Destructor Methods Destructor method: called when the object is destroyed Cleans up any resources allocated to an object You cannot explicitly call a destructor method It is called automatically by the C# garbage collection Often used to close files or database connections Destructor name is the same as the class name, but preceded by a tilde symbol (~) No access modifier or return type is used ASP.NET Programming with C# and SQL Server, First Edition

Writing Accessors Accessor methods: public methods that a client can call to retrieve or modify the value of a field Also called set or get methods Set methods modify field values and accept parameters to pass in the new value for the field Get methods retrieve field values C# allows you to create accessors using properties Properties: special methods that can be used as public data members to set and get field values Defined using the reserved words get and set ASP.NET Programming with C# and SQL Server, First Edition

Figure 10-5 Shopping Cart page after adding several items ASP.NET Programming with C# and SQL Server, First Edition

Summary An object is programming code and data that can be treated as an individual unit or component Objects are encapsulated; all code and data are contained within the object itself An interface refers to the methods and properties that are required for a source program to communicate with an object A class is a template, or blueprint, that serves as the basis for new objects When you create an object from a class, you are said to be instantiating the object ASP.NET Programming with C# and SQL Server, First Edition

Summary (cont’d.)‏ An instance of an object inherits its methods and properties from the class on which it is based A data structure is a system for organizing data A field is a variable defined within a class Methods and fields defined in a class are called class members Classes help make complex programs easier to manage by logically grouping related methods and fields Access modifiers control a client’s access to classes, data members, and function members ASP.NET Programming with C# and SQL Server, First Edition

Summary (cont’d.)‏ Garbage collection refers to cleaning up, or reclaiming, memory that is reserved by a program The principle of information hiding states that any class members that clients do not need to access or know about should be hidden Fields are declared like standard variables, except that you must include an access modifier Serialization is the process of converting an object’s fields into a string that can be stored for reuse ASP.NET Programming with C# and SQL Server, First Edition

Summary (cont’d.)‏ A file stream is used for accessing a resource, such as a file, that you can read from and write to Public methods can be called by anyone, while private methods can be called only by other methods in the same class Only methods that clients need to access should be declared as public A constructor method is a special method that is called automatically when an object is instantiated from a class ASP.NET Programming with C# and SQL Server, First Edition

Summary (cont’d.)‏ Any resources allocated to an object are cleaned up by a destructor method after the object is destroyed Accessor methods are public methods that a client can call to retrieve or modify the value of a field C# allows you to create accessors using properties, which are special methods that can be used as public data members to set and get field values ASP.NET Programming with C# and SQL Server, First Edition