1. Advanced Object-Oriented Programming Features11
C# Programming: From Problem Analysis to Program Design
4th Edition
C# Programming: From Problem Analysis to Program Design

2. 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

3. Chapter Objectives (continued)
Distinguish the differences between sealed and abstract classes
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

4. Chapter Objectives (continued)
Investigate static versus dynamic typing and become aware of when dynamic and var types are used
Work through a programming example that illustrates the chapter’s concepts
C# Programming: From Problem Analysis to Program Design

5. Object-Oriented Language Features
Abstraction
Generalizing, identifying essential features, hiding nonessential complexities
Encapsulation
Packaging data and behaviors into a single unit, hiding implementation details
Inheritance
Extending program units to enable reuse of code
Polymorphism
Providing multiple (different) implementations of same named behaviors (Overloading)

6. Component-Based Development
Figure Component-based development

7. Component-Based Development (continued)
Component-based applications associated with multitier applications – (different layers)
Business layer(s) – classes that perform processing
Provide functionality for specific system; provide necessary processing
Data access layer - classes for accessing data from text files and databases
Presentation layer - User interface tier for user interaction
Graphical User Interface such as Windows or Web

8. Component-Based Development (continued)
Components facilitates reuse-based approach
Define and implement independent components into systems
Components are implemented through classes
Takes the form of objects or collection of objects
Components can then be packaged with different components and used for other applications
Classes can be created that extend functionality of original classes
C# Programming: From Problem Analysis to Program Design

9. Inheritance Enables you to: Associated with an "is a" relationship
Create a general class and then define specialized classes that have access to the members of the general class (Person→Student →Graduate)
Associated with an "is a" relationship
Specialized class “is a” form of the general class (Graduate is-a Student)
Classes can also have a "has a" relationship, not associated with inheritance
"has a" relationship is associated with containment or aggregation
(e.g. Student has-a Schedule)

10. Inheriting from the Object Class
Every object inherits four methods as long as reference to the System namespace included
Figure Methods inherited from an object

11. Inheriting from Other .NET FCL Classes
Experienced inheritance when you developed Windows-based programs
Form you created inherited characteristics from System.Windows.Forms.Form class
Already had title bar, close, and minimize buttons
Add functionality to your programs with minimal programming
Extend System.Windows.Forms.Form class to build GUIs (Button, Label, TextBox, ListBox, ImageBox,…)
C# Programming: From Problem Analysis to Program Design

12. Inheriting from Other .NET FCL Classes
Base class
Derived class
Figure Derived class
Class listed after the colon is the base class. It already has some functionality
Class to the left, PresentationGUI, is a derived class
Derived class is the new class, the user-defined class
C# Programming: From Problem Analysis to Program Design

13. 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
In general, the base class has
Data members defined with a private access modifier
Constructors defined with public access modifiers
Properties offering public access to data fields
C# Programming: From Problem Analysis to Program Design

14. Class Diagram: Person - Student
Base class
Sub-class
(Derived)

15. Person Class (Base) 1 of 2 1 2 class Person { //data members
private string lastName = "n.a";
private string firstName= "n.a";
private int age = 0;
private string occupation = "n.a";
//Properties
// Verbose style - Property for last name
public string LastName
get
return lastName; }
set
lastName = value;
// Properties –using C# 7.0 style- Expression Bodied Method notation
public string FirstName { get => firstName; set => firstName = value; }
public int Age { get { return age; } set => age = Math.Abs(value); }
public string Occupation { get { return occupation; } set => occupation = value; } 1 2

16. Person Class (Base) 2 of 2 3 4 5 // Constructor with zero arguments
public Person() { }
// Constructor with four arguments
public Person( string lNameValue,
string fNameValue, int ageValue) {
LastName = lNameValue;
FirstName = fNameValue;
Age = ageValue; }
//user-defined methods (new formatting feature C# 7.0)
public override string ToString()
return string.Format( $"Person [ "
+ $"First= {FirstName} Last= {LastName} "
+ $"Age= {age} "
+ $"Ocuppation= {Occupation} ]");
//a virtual method could be overridden by its derived sub-classes
public virtual int GetSleepAmount()
return 8; 3 4 5

17. Student Class (Derived/Sub class) 1 of 26
class Student : Person {
//data members
private string major = "not declared yet";
private double gpa = 0.0;
private int studentId = 0;
//properties (bodied expressions)
public string Major { get => major; set => major = value; }
public double Gpa { get => gpa; set => gpa = value; }
public int StudentId { get=> studentId; set=> studentId = value; }
//constructors
public Student() :base() { }
public Student(string lNameValue,string fNameValue, int ageValue,
int idValue, string majorValue, double gpaValue)
:base(lNameValue, fNameValue, ageValue)
StudentId = idValue;
Major = majorValue;
Gpa = gpaValue; } 7 8 9

18. Student Class (Derived/Sub class) 2 of 2
//user-defined methods
public override string ToString() {
return string.Format(
$"\nStudent [ ID= {StudentId} Major= {Major} GPA= {Gpa}" +
$"\n\t" + base.ToString() +
$" ]"); }
public override int GetSleepAmount()
//return base.GetSleepAmount();
return 6; 10 11

19. Access Modifiers
Class members defined with private access are restricted to members of the current class
Data members are typically defined with private access modifier
Private access enables class to protect its data and only allow access to the data through its methods or properties
Constructors use public access modifier
If they are not public, you would not be able to instantiate objects of the class in other classes
Constructors are methods named as the class and have no return type
Ancestor
private...
protected…
public…
Subclass
Only ancestor's public & protected members are visible

20. Creating Base Classes for Inheritance
class Person {
//data members
private string lastName = "n.a";
private string firstName= "n.a";
private int age = 0;
private string occupation = "n.a";
Base class
All derived classes will include these four attributes 1
All data members are customarily defined private.
Good practice! These default values could be used by zero-args constructor

21. Properties (Verbose Style)
Classic verbose style
// Property for last name
public string LastName {
get
return lastName; }
set
lastName = value;
Properties are defined with public access. They provide access to private data and enforcing of 'bussines rules'.
No need to declare value. It is used, almost like magic…
value refers to the argument sent through an assignment statement
get, set and value are contextual keywords 2
Private data member
RHS of an assignment

22. Properties (Compressed notation)
In C# Version 7 there is support for a compact notation called EXPRESSION BODIED METHOD that employs the => operator (not to be mistaken with Lambda expressions).
The new notation could be used Properties, Constructors, and Destructors (~Person() =>...)
//Properties (using Expression Bodied Method's notation)
public string Occupation { get => occupation; set => occupation = value; }
public string LastName { get=> lastName; set=> lastName = value; }
public string FirstName { get => firstName; set => firstName = value; }
public int Age { get { return age; } set => age = Math.Abs(value); } 2

23. Public Access Modifier
Properties should offer public access to data fields
Properties look like data fields, but are implemented as methods
Properties implement the getters (accessors) and setters (mutators) for the class
You can make properties be read-only by NOT defining the “set”
GOOD PRACTICE. Properties often carry the same name as their associated data member. However, properties use Pascal case (start with capital letter). For example:
LastName → lastName
Property
field/data member

24. Creating Base Classes for Inheritance: Constructors
// Constructor with zero arguments
public Person( ) { }
// Constructor with four arguments
public Person (string lnameValue,string fnameValue,
int ageValue)
LastName = lnameValue;
FirstName = fnameValue;
Age = ageValue;
All the object's
data-members will used their already declared default values 3
Notice, constructor is a method, has same name as class (Person), has no return type, and is overloaded
Properties

25. Overriding Methods Person class example has two additional methods
public override string ToString( ) // Defined in Person
public virtual int GetSleepAmt( ) // Defined in Person
You can replace a method originally defined at a higher level with a new definition
Use keyword override in method heading whenever the derived class wants to provide a new definition
In order to be a method that can be overridden, keywords virtual, abstract, or override must be part of the heading for the parent class
ANCESTOR Method's modifier:
virtual, abstract, override
DERIVED CLASS
override

26. Base class ToString( ) method
Overriding Methods
Base class ToString( ) method
The Person class example overrides ToString()
//using string interpolation - new C# 6.0 formatting feature
public override string ToString() {
return string.Format( $"\nPerson [ ID= {IdNumber} Age= {age} "
+ $"First= {FirstName} Last= {LastName} "
+ $"Ocuppation= {Occupation} ]"); } 4
NOTE: Remember, each class derives from the Object super-class. Therefore, although Person is the Base class in our example, it could be considered a subclass as it derives from Object.

27. Virtual Methods (continued)
A virtual method is one that could be modified by any descendant in its hierarchy. This behavior is in contrast with abstract methods that must be overridden by derived subclasses.
//a virtual method could be overridden by its derived sub-classes
public virtual int GetSleepAmount() {
return 8; } 5
Comment:
In our example, the base class sets eight hours of sleep for a Person object. A descendant of Person, say HeavyCoffeeDrinker may require less rest and establishes a different value (4 is enough!).

28. Virtual Methods (continued)
ToString( ) uses the virtual modifier in the Object super class, implying that any derived class can override it
ToString( ) method can have many different implementations
Example of polymorphism
Overriding differs from overloading a method
Overridden methods have exactly the same signature
Overloaded methods each have a different signature

29. Creating Derived Classes
Derived classes inherit from a base class
Derived classes also called subclasses or child classes
Derived classes do not have access to data members defined with private access in the parent or base class
Derived classes have access to data members defined with protected (and public) access in the parent or base class
C# Programming: From Problem Analysis to Program Design

30. Derived Classes 6 Reference to base class
class Student : Person { //data members private string major = "not declared yet"; private double gpa = 0.0; private int studentId = 0; //properties (bodied expressions) ... //constructors public Student() :base() { }
Additional subclass data members 6
Indicates which base class constructor to use
C# Programming: From Problem Analysis to Program Design

31. 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
//all-arguments constructor
public Student(string fnameValue, string lnameValue, int ageValue,
string majValue, string studentIdValue)
: base(lnameValue, fnameValue, ageValue) // calling base constructor {
Major = majValue;
StudentId = studentIdValue; }
Base constructor must have a constructor with matching signature 9
Use base constructor to create a Person object

32. Calling the Base Constructor (cont)
Student s2 = new Student("Parker", "Peter", 21,
, "CHEM", 4.0);
First three arguments, "Parker", "Peter" and 21, are sent to the constructor for Person class which requires first name, last and age.
Last three arguments ( , "CHEM" 4.0) are used by the Student class constructor to fill up the additional data memebers affiliated to the derived Student class.
Student anotherStudent = new Student();
Both, Person and Student default zero-arguments constructors are called when new Student() is invoked.

33. Calling Overridden Methods of the Base Class
Scope
Methods defined in subclass take precedence when named the same name as member of a parent class
Use keyword base before the method name to call an overridden method of the base class
return base.GetSleepAmt( ) // Calls GetSleepAmt( ) in
// parent class
C# Programming: From Problem Analysis to Program Design

34. Relationship between the Person and Student Classes
Figure Inheritance class diagram

35. Relationship between the Person and Student Classes
Figure Inheritance class diagram

36. Abstract Classes
Abstract classes are created solely for the purpose of inheritance
You need to identify and pull out common characteristics that all objects of that type possess
Provide a common definition of a base class so that multiple derived classes can share that definition
For example, Person → Student, Faculty
Person could be defined as a base abstract class
Student defined as derived concrete subclass

37. Abstract Classes Add keyword abstract on class heading
[access modifier] abstract class ClassIdentifier { } // base class
In our running example, add abstract to heading of Person base class
public abstract class Person
...

38. Abstract Classes
Abstract classes prohibit other classes from instantiating objects based on the abstract class
If Person is made abstract, then new Person() would be an invalid call.
Can create subclasses (derived classes) of the abstract class
Derived classes inherit characteristics from base abstract class
Objects can only be created using classes derived from the abstract class (new Student() would be a valid call)

39. Abstract Methods
Consider them as a contract that details what methods must be implemented be derived classes.
Abstract methods have no body
Implementation details of the method are left up to classes derived from the base abstract class
Only permitted in abstract classes
Every class that derives from the abstract class must provide implementation details for all abstract methods

40. Abstract Methods (continued)
No additional special keywords are used when a new class is defined to inherit from the abstract base class
[access modifier] abstract returnType MethodIdentifier
([parameter list]) ; // No { } included
Declaration for abstract method ends with semicolon; NO method body or curly braces
Syntax error if you use the keyword static or virtual when defining an abstract method

41. Abstract Methods (continued)
Assume Person has been declared an abstract class.
Assume Person includes the following method declaration
public abstract void SetHobby(string activityValue);
Defined in Person
In the derived Student class, you must include an overridden version of the SetHobby method. For instance,
public override void SetHobby(string activityValue) {
//assume List<string> listHobbies has been added to data members
listHobbies.Add(activityValue); }
Defined in Student

42. Sealed Classes A Sealed class cannot be a base class
Sealed classes are defined to prevent derivation
Objects can be instantiated from the class, but subclasses cannot be derived from it
public sealed class Student
In this case it would not be possible to create
public class UndergraduateStudent : Student

43. Sealed Methods
If you do not want subclasses to be able to provide new implementation details, add the keyword sealed
Helpful when a method has been defined as virtual in a base class
Don’t seal a method unless that method is itself an override of another method in some base class
Assume Person defines the method
public virtual void RaiseSalary(double incrementValue) {...}
Derived class Student could implement
public sealed override void RaiseSalary(double incrementValue) {...}
Observe that no subclass of Student could override the method.

44. Partial Classes Break class up into two or more files
Each file uses partial class designation
public abstract partial class Person
At compile time, the files are merged together
All the files must use the partial keyword
All of the partial class definitions must be defined in the same assembly (.exe or .dll file)
Class name and accessibility modifiers, such as private or public, must also match

45. Interfaces C# supports single inheritance
Classes can implement any number of interfaces
Only inherit from a single class, abstract or non-abstract
Think of an interface as a class that is totally abstract; all methods are abstract
Interfaces are contracts that mandate you what needs to be implemented
Abstract classes can have abstract and regular methods
Classes implementing interface agree to define details for all of the interface’s methods

46. Using .NET Interfaces
A useful .NET interface that you may consider using is called Icomparable.
It allows you to define how two objects could be compared.
The interface has only one method called CompareTo()
The integer result returned by CompareTo(Object other) when matching this object with the other object are numbers as follows
ZERO if this object is equal to other object
POSITIVE if this object is greater than other object
NEGATIVE if this object < other object

47. Using .NET Interfaces Assume Person is declared as follows
public class Person : IComparable<Person>
Assume Person includes the following method
public virtual int CompareTo(Person other) {
if (this.age == other.age) return 0;
else if (this.age > other.age) return 1;
else return -1; }
Observe that the criteria for comparison is arbitrary. In this case we have chosen age to compare two Person objects.

48. Using .NET Interfaces
Assume a test class has already defined two instances of Person identified as p1 and p2 respectively.
Person p1 = new Person("Prince", "Diane", 22);
Person p2 = new Person();
To compare both objects, one may say
if (p1.CompareTo(p2)!=0) {
Console.WriteLine("Different people!"); }
In this case, the output would be "Different people!"

49. Polymorphism
Ability for classes to provide different implementations details for methods with same name
Determines which method to call or invoke at run time based on which object calls the method (Dynamic binding)
Example…ToString( ) method
Through inheritance, polymorphism is made possible by allowing classes to override base class members

50. 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
public class Stack<T> {
private T[] items;
private int stackPointer = 0;
public Stack(int size)
items = new T[size]; } …
Generic Stack class

51. Generic Classes (continued)
public T Pop( ) { return items[--stackPointer]; } public void Push(T anItem) items[stackPointer] = anItem; stackPointer++;

52. Generic Methods Similar to defining a generic class
Can define generic methods that are not part of generic classes
Insert identifier between left and right brackets on the method definition line to indicate it is a generic method
Then place that identifier either in the parameter list or as a return type or in both places

53. Generic Methods (continued)
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);

54. Dynamic
C# was originally characterized as being a strongly typed language
Compiler checks to ensure that only compatible values are attempting to be stored
Variables can still be defined as objects and then cast as different data types during run time
Additional boxing/unboxing is needed
C# Programming: From Problem Analysis to Program Design

55. Dynamic Data Type
An object defined using the dynamic keyword can store anything
No unboxing or casting is necessary prior to their use
With dynamic data types, the type checking occurs at run time
Once defined as dynamic, the memory location can hold any value
Dynamic types can be used for data types, method parameters or method return types

56. Dynamic Data Type (continued)
dynamic intValue = 1000;
dynamic stringValue = "C#";
dynamic decimalValue = 23.45m;
dynamic aDate = System.DateTime.Today;
Console.WriteLine("{0} {1} {2} {3}" ,
intValue, stringValue, decimalValue, aDate);
C# /16/ :00:00 AM
Single dynamic memory location can hold values of different data types

57. var Data Type
Variables declared inside a method can be declared using the var keyword
Compiler determines the type
One primary difference between dynamic and var is that var data items must be initialized when they are declared
Commonly used by the LINQ technology

58. var Data Type (continued)
Example
var someValue = 1000;
Implicitly typed by the compiler
Compiler determines the type (integer in this case)
Compiler infers the type of the variable from the expression on the right side of the initialization statement
C# Programming: From Problem Analysis to Program Design

59. StudentGov Application Example
Figure Problem specification for StudentGov example
C# Programming: From Problem Analysis to Program Design

60. StudentGov Application Example (continued)
Table Data fields organized by class
C# Programming: From Problem Analysis to Program Design

61. StudentGov Example (continued)
Figure Prototype for StudentGov example
C# Programming: From Problem Analysis to Program Design

62. StudentGov Example (continued)
Figure Class diagrams for StudentGov example
C# Programming: From Problem Analysis to Program Design

63. StudentGov Example (continued)
Figure References added to StudentGov example
C# Programming: From Problem Analysis to Program Design

64. StudentGov Example (continued)
Figure References added to StudentGov example (continued)
C# Programming: From Problem Analysis to Program Design

65. StudentGov Example (continued)
Table PresentationGUI property values
C# Programming: From Problem Analysis to Program Design

66. StudentGov Example (continued)
Table PresentationGUI property values (continued)
C# Programming: From Problem Analysis to Program Design

67. StudentGov Example (continued)
Table PresentationGUI property values (continued)
C# Programming: From Problem Analysis to Program Design

68. StudentGov Example (continued)
Table PresentationGUI property values (continued)
C# Programming: From Problem Analysis to Program Design

69. StudentGov Example (continued)
Figure Setting the StartUp Project
C# Programming: From Problem Analysis to Program Design

70. StudentGov Application Example (continued)
Figure Part of the PresentationGUI assembly
C# Programming: From Problem Analysis to Program Design

71. StudentGov Application Example (continued)
Figure Output from StudentGov example
C# Programming: From Problem Analysis to Program Design

72. Coding Standards
Declare members of a class of the same security level together
When declaring methods that have too many arguments to fit on the same line, the leading parenthesis and the first argument should be written on the same line
Additional arguments are written on the following line and indented
C# Programming: From Problem Analysis to Program Design

73. Resources Comparison of Unified Modeling Language Tools –
Generic Classes (C# Programming Guide) –
C# Station Tutorial: Interfaces –
C# Programming: From Problem Analysis to Program Design

74. Chapter Summary Major features of object-oriented languages
Abstraction
Encapsulation
Inheritance
Polymorphism
Multitier applications using component-based development methods

75. Chapter Summary (continued)
Use inheritance to extend the functionality of user- defined classes
Abstract classes
Abstract methods
Sealed classes
Partial classes

76. Chapter Summary (continued)
Interfaces
Why polymorphic programming?
Generics
Generic classes
Generic methods
Dynamic data types
var data types
C# Programming: From Problem Analysis to Program Design

77. APPENDIX Making Stand-Alone Components
Can take Person and Student classes (or any user-defined class) and store in library for future use.
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

78. APPENDIX Dynamic Link Library (DLL)
Several options for creating components
One option – compile source code into .dll file type
Once you have a DLL, any application that wants to use it just adds a reference to the DLL
That referenced file with the .dll extension becomes part of the application’s private assembly
Can use the command line to create .DLL
C# Programming: From Problem Analysis to Program Design

79. Using Visual Studio to Create DLL Files
Figure Creating a DLL component

80. Build Instead of Run to Create DLL
Create the parent class first in order to use IntelliSense (for sub classes)
To illustrate creating stand-along components, separate project created for Person and Student
Because Visual Studio assigns the namespace name as the project name (Person), you should change the namespace identifier
If you don’t change it, when you start adding a reference to a created DLL, it can become confusing
Changed to PersonNamespace for the example
C# Programming: From Problem Analysis to Program Design

81. Build Instead of Run to Create DLL
After typing class, do not run it – select BUILD, Build Solution
Figure Attempting to run a class library file
Create new project for subclasses (Student)
Again, select Class Library template from Start page
Change namespace identifier (StudentNamespace)

82. Build Instead of Run to Create DLL (continued)
Add a new using statement to subclass
In Student, add using PersonNamespace;
To gain access to base class members in subclass, Add Reference in subclass to base class
Use Solution Explorer window
DLL is stored in Debug directory under the bin folder wherever you create the project
C# Programming: From Problem Analysis to Program Design

83. Add Reference to Base Class
One of the first things to do is Add a Reference to the Parent DLL
Figure Adding a reference to a DLL

84. Add Reference to Base Class (continued)
Use Browse button to locate DLL
Figure Add Reference dialog box

85. Add Reference to Base Class (continued)
Figure Locating the Person.dll component

86. 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 Namespace reference error

87. Adding a New Using Statement (continued)
Notice fully qualified name
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

88. Creating a Client Application to Use the DLL
DLL components can be reused with many different applications
Once the .DLL is in the library, any number of applications can reference it
Instantiate objects of the referenced DLL
Two steps Required
Add a reference to the DLL components
Include a using statement with the namespace name
C# Programming: From Problem Analysis to Program Design

89. Creating a Client Application to Use the DLL (continued)
Figure DLLs referenced in the PresentationGUI class

90. Declaring an Object of the Component Type
Declare object of the subclass (Student)…not the base class
public class PresentationGUI : System.Windows.Forms.Form {
private Student aStudent;
Instantiate the object
aStudent = new Student(" ", "Maria", "Woo", "CS", "1111");
Use members of the derived, base, or referenced classes

91. Creating a Client Application to Use the DLL (continued)
Figure PresentationGUI output referencing two DLLs

92. 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

93. Using ILDASM to View the Assembly (continued)
In order to use ILDASM in Visual Student, must be added as an external tool in Visual Studio (TOOLS, External Tools…)
DLL files cannot be modified or viewed as source code
Can use ILDASM to view DLL assemblies
C# Programming: From Problem Analysis to Program Design

94. ILDASM to View the Assembly
.ctors are constructors
IL code for the method
Data fields
Properties converted to methods
Figure Student.dll assembly from ILDASM