C# Programming: From Problem Analysis to Program Design1 10 Advanced Object-Oriented Programming Features C# Programming: From Problem Analysis to Program

C# Programming: From Problem Analysis to Program Design 1

10Advanced Object-Oriented Programming Features

C# Programming: From Problem Analysis to Program Design 2nd Edition


Chapter Objectives• Learn the major features of object-oriented

languages

• Design and develop multitier applications using component-based development methods

• Use inheritance to extend the functionality of user-defined classes

• Create abstract classes that include abstract methods


Chapter Objectives (continued)• Become aware of partial classes

• Design and implement interfaces

• Understand why polymorphic programming is a common goal in .NET

• Explore generics and learn how to create generic classes and generic methods

• Work through a programming example that illustrates the chapter’s concepts


Object-Oriented Language Features

• Abstraction

– Abstract or identify the objects involved in the problem

• Encapsulation

– Packaging data and behaviors into a single unit

• Inheritance

– Reuse of code through extending program units

• Polymorphism

– Multiple implementations of the same behaviors


Component-Based Development

Figure 10-1 Component-based development


Component-Based Development (continued)

• Multitier applications – Data access tier for accessing data from text files

and databases

– Graphical user interface tier for user interaction• Windows

• Web

• Components implemented through classes in C# • Class library files with a dynamic link library

(DLL) extension


Inheritance

• Enables you to: – Create a general class and then define specialized

classes that have access to the members of the general class

• Associated with an "is a" relationship– Specialized class “is a” form of the general class

• Classes can also have a "has a" relationship, not associated with inheritance– "has a" relationship associated with containment or

aggregation


Inheriting from the Object Class

• Every object inherits four methods as long as reference to the System namespace included

Figure 10-2 Methods inherited from an object


Inheriting from Other .NET FCL Classes

• Add functionality to programs with minimal programming

• Extend System.Windows.Forms.Form class to build GUIs (Button, Label, TextBox, ListBox)

Base class

Derived class

Figure 10-3 Derived class


Creating Base Classes for Inheritance

• Can define your own classes from which other classes can inherit

• Base class is called the super or parent class • Data members are defined with a private access

modifier • Constructors are defined with public access

modifiers • Properties offer public access to data fields


Overriding Methods

• Replace the method defined at a higher level• Keyword override included in derived class

– Base method includes virtual, abstract, or override keyword

• Overriding differs from overloading a method – Overridden methods have exactly the same signature

– Overloaded methods each have a different signature

Figure 10-4 ToString( ) signature


Overriding Methods (continued)• Example of polymorphism

– ToString( ) method can have many different definitions

– ToString( ) uses the virtual modifier implying any class can override it

• Derived classes inherit from a base class– Also called subclasses or child classes

• Protected access modifiers– Access only to classes that derived from them

– Access to change data in the base class


Calling the Base Constructor• To call the constructor for the base class, add

keyword :base between the constructor heading for the subclass and the opening curly brace

public Student(string id, string fname, string lname,

string maj, int sId)

:base (id, lname, fname) // base constructor arguments

{ . . .

• Base constructor must have a constructor with matching signature


Using Members of the Base Class

• Scope

– Methods defined in subclass take precedence when named the same name as member of a parent class

• Can call an overridden method of the base class

– Use keyword base before the method name

return base.GetSleepAmt( ) // Calls GetSleepAmt( ) in

// parent class


Relationship Between the Person and

Student Classes

Figure 10-5 Inheritance class diagram


Making Stand-alone Components• Compile class and create an assembly

– Assemblies are units configured and deployed in .NET

• Classes can be compiled and stored as a dynamic link library (DLL) instead of into the EXE file type

• Adds a reference to the DLL

– That referenced file with the .dll extension becomes part of the application’s private assembly


Using Visual Studio to Create DLL Files

Figure 10-6 Creating a DLL component


Build Instead of Run to Create DLL

• Create the parent class first in order to use IntelliSense

• Create the subclass class in same way as usual, except Build instead of Run the project to create the DLL

Figure 10-7 Attempting to run a class library file


Add Reference to Base Class

Figure 10-8 Adding a reference to a DLL

One of the first things

to do is Add a Reference to the Parent

DLL


Add Reference to Base Class (continued)

Figure 10-9 Add Reference dialog box

Use Browse button

to locate DLL


Add Reference to Base Class (continued)

Figure 10-10 Locating the Person.dll component


Adding a New Using Statement

• In the subclass class, if you simply type the following, you receive an error message

public class Student : Person

Figure 10-11 Namespace reference error


Adding a New Using Statement (continued)

• To avoid error, could type:

public class Student : PersonNamespace.Person

• Better option is to add a using directive

using PersonNamespace; // Use whatever name you

// typed for the namespace for Person

• After typing program statements, build the DLL from the Build option under the Build menu bar

Notice fully

qualified name


Creating a Client Application To Use the DLL

• DLL components can be reused with many different applications

• Two Steps

– Add a reference to the DLL components

– Include a using statement with the namespace

• Then declare an objects of the component type(s)

• Use members of the derived, base, or referenced classes


Creating a Client Application To Use the DLL (continued)

Figure 10-12 DLLs referenced in the PresentationGUI class


Creating a Client Application To Use the DLL (continued)

Figure 10-13 PresentationGUI output referencing two DLLs


Using ILDASM to View the Assembly

• (ILDASM): Intermediate Language Disassembler tool

• Assembly shows the signatures of all methods, data fields, and properties

• One option – display the source code as a comment in the assembly

• Can be run from the command line or from within the Visual Studio IDE– Must be added as an external tool in Visual Studio


ILDASM to View the Assembly

Figure 10-14 Student.dll assembly from ILDASM

Data fields

.ctors are constructors

Properties converted to methods

IL code for the method


Abstract Classes• Used to prohibit other classes from instantiating

objects of the base class

• Still inherit characteristics from base class in subclasses

• Base class can have data and method members

[access modifier] abstract class ClassIdentifier { } // Base class


Abstract Methods • Only permitted in abstract classes

• Method has no body

– Implementation details of the method are left up to classes derived from the base abstract class

[access modifier] abstract returnType

MethodIdentifier([parameter list]) ; // No { } included

• Declaration for abstract method ends with semicolon; NO method body or curly braces


Abstract Methods (continued)• Every class that derives from the abstract class

must provide implementation details

– Sign a contract that details how to implement its abstract methods

– Syntax error if you use the keyword static or virtual when defining an abstract method

• No additional special keywords are used when a new class is defined to inherit from the abstract base class


Partial Classes

• Break class up into two or more files– Each file uses partial class designation

• New features of C# 2.0• Used by Visual Studio for Windows applications

– Code to initialize controls and set properties is placed in a somewhat hidden file in a region labeled “Windows Form Designer generated code”

– File is created following a naming convention of “FormName.Designer.cs” or “xxx.Designer.cs”

– Second file stores programmer code


Interfaces• All .NET languages only support single

inheritance

• Think of an interface as a class that is totally abstract; all methods are abstract– Abstract classes can have abstract and regular methods

– Implementing interface agrees to define details for all of the interface’s methods

• Classes can implement any number of interfaces– Only inherit from one class, abstract or nonabstract


Interfaces (continued)• General form

[modifier] interface InterfaceIdentifier

{

// members - no access modifiers are used

}

• Members can be methods, properties, or events

– No implementations details are provided for any of its members


Defining an Interface• Can be defined as members of a namespace or

class or by compiling to a DLL

• Easy approach is to put the interface in a separate project

– Use the Class Library template

• Unlike abstract classes, it is not necessary to use the abstract keyword with methods

– Because all methods are abstract


Defining an Interface (continued)

Build the

interface DLL using Build option from Build menu bar


Implement the Interface• Follow the same steps as with the Person and

Student DLLs

– Add a reference to the file ending in .dll

– Type a using statement

• Heading for the class definition specifies base class and one or more interfaces following the colon (:)

[modifier] class ClassIdentifier : identifier [, identifier]

• Base class comes first


Implement the Interface: PresentationGUI Application

Figure 10-16 PresentationGUI output using interface methods


.NET Framework Interfaces

• Play an important role in the .NET Framework

– Collection classes such as Array class and HashTable class implement a number of interfaces


.NET Framework Interfaces (continued)

• .NET Array class is an abstract class – Implements several interfaces (ICloneable; IList;

ICollection; and IEnumerable)

– Includes methods for manipulating arrays, such as:

• Iterating through the elements

• Searching by adding elements to the array

• Copying, cloning, clearing, and removing elements from the array

• Reversing elements

• Sorting


NET Framework Interfaces (continued)

• HashTable is not abstract

– Implements a number of interfaces

public class Hashtable : IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, ICloneable

• Implements the IDeserializationCallback interface

• Explore the documentation for these classes and interfaces


Polymorphism

• Ability for classes to provide different implementations of methods called by the same name

– ToString( ) method

• Dynamic binding

– Determines which method to call at run time based on which object calls the method


Polymorphic Programming in .NET

• Multiple classes can implement the same interface, each providing different implementation details for its abstract methods

– “Black box” concept

• Abstract classes, classes that derive from them, are forced to include implementation details for any abstract method


Generics

• Reduce the need to rewrite algorithms for each data type

• Create generic classes, delegates, interfaces, and methods

• Identify where data will change in the code segment by putting a placeholder in the code for the type parameters


Generic Classes• Defined by inserting an identifier between left and

right brackets on the class definition line• Example

public class GenericClass <T>

{

public T dataMember;

}– //To instantiate an object, replace the T with data type

GenericClass <string> anIdentifer = new GenericClass <string>( );


Generic Methods• Similar to defining a generic class• Insert identifier between left and right brackets on the

method definition line to indicate it is a generic method• Example

public void SwapData <T> (ref T first, ref T second){

T temp;temp = first;first = second;second = temp;

} //To call the method, specify the type following method name

SwapData <string> (ref firstValue, ref secondValue);


StudentGov Application

Example

Figure 10-18 Problem specification for StudentGov example


StudentGov Application Example (continued)



Figure 10-19 Prototype for StudentGov example



Figure 10-20 Class diagrams for StudentGov example



Example (continued)

Figure 10-21 References added to StudentGov example


Properties: StudentGov Application


Properties: StudentGov Application (continued)



Example (continued)

Figure 10-22 Setting the StartUp Project



Figure 10-23 Part of the PresentationGUI assembly



Figure 10-24 Output from StudentGov example


Chapter Summary• Major features of object-oriented languages

– Abstraction

– Encapsulation

– Inheritance

– Polymorphism

• Multitier applications using component-based development methods


Chapter Summary (continued)

• Use inheritance to extend the functionality of user-defined classes

• Abstract classes

– Abstract methods

• Partial classes

• Interfaces


Chapter Summary (continued)

• Why polymorphic programming?• Generics

– Generic Classes

– Generic Methods

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C# Programming: From Problem Analysis to Program Design1 10 Advanced Object-Oriented Programming Features C# Programming: From Problem Analysis to Program