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Object-oriented

Programming in C#

for C and Java programmers

February 2010

Kurt Nørmark ©

Department of Computer Science,

Aalborg University,

Denmark.

WEB version:

ii iii

Abstract

This is a teaching material about object-oriented programming, illustrated with use of the programming language C#. The initial version was written i 2006. It is assumed, as a prerequisite, that the readers have some knowledge about imperative programming, preferably knowledge about C. I believe that it is best to learn object-oriented programming after having learned and worked with imperative programming. Thus, we adhere to an "object later approach". The starting point of of this teaching material is a number of slides. The slides are annotated with explanations and additional resources. The most comprehensive version of the material has the form of a traditional textbook. You can think of the textbook as grown on top of the slide material. Almost all programs in this material have been developed together with the material. However, a few programs come from other sources. If the programs are not original, the source programmers are mentioned and acknowledged in an initial comment line of the program. The current version of the material is complete up to (and including) the last chapter in lecture about Contracts (chapter 53). The final chapters - corresponding to the lectures about test and documentation - only contain slide material. The teaching material is an online material represented in HTML. A PDF version of the textbook is also available. In order to limit the size of the PDF files some less important programs have been left out from the PDF edition. In the web edition (in HTML files) the full content is available. We have used colors to emphasize aspects of the source programs in this material. It is therefore recommeded that you read the material from a color medium. We would like to point out a couple of qualities of the web edition of the material. First, we have provided for extensive cross linking of the material. Whenever relevant, we have provided links from one part of the material to another. We have also provided a comprehensive subject index. Finally, there are separate indexes of source programs and exercises. The source program index provides links to complete, textual versions of the C# programs of the material, ready for you to compile and use. In the spring of 2008 the material has been used in a course where the students have a background in imperative Visual Basic programming. We have therefore added a chapter that compares the fundamental (non-objected) parts of Visual Basic with the similar parts of C#. The chapter about Visual Basic is only available in the web-version. Prior to the fall semester of 2008, selected aspects of C# 3.0 have been included in the material. This includes automatic properties, object initializers, extension methods, and lambda expressions. iv The January 2009 edition includes a number of bug-fixes (as collected during the fall of

2008) and some new exercises targeted at the Aalborg University OOPA spring course 2009

at the BAIT education. The February 2010 edition is a minor revision compared with the August 2009 edition. The February 2010 edition is the last, and final, version of the teaching material.

Kurt Nørmark

normark@cs.aau.dk

Department of Computer Science

Alborg University

Denmark

February 5, 2010

Colophon: This material has been made with the LENO system. LENO is an XML language which is defined by an XML DTD. LENO is used together with LAML. LAML is a software package that makes

XML available in the Scheme programming language. For this version of the material, LAML version 35.0

(development version) has been used. This setup allows us to write LENO material in the programming language Scheme. Thus, the source of this material has been written in Scheme with use of the mirror functions of LENO XML language. Most illustrations have been written in an extension of SVG which

allows for high-level representations of graphs. Batik 1.6 has been used to rasterize the SVG images in some

versions of the material. The primary target format of the teaching material is HTML. LENO can be used to

produce different views of the material. In the concrete material we make use of the slide view and the

thematic view (the text book view). The aggregated slide view is also available. The PDF version of the

thematic view (the text book view) has been made by PDF Creator (version 0.9.5 ). Text breaking and page

enumeration has been made in Microsoft Word 2003 via "Edit with Microsoft Word" in Internet Explorer version 7.0. v

Contents

1. From structured programming to object-oriented programming 1

2. Towards Object-oriented Programming 7

3. Phenomena and Concepts 13

4. Towards Object-oriented Programs 19

5. The C# Language and System 23

6. C# in relation to C 25

7. C# in relation to Java 53

8. C# in relation to Visual Basic 57

9. C# Tools and IDEs 59

10. Classes: An Initial Example 61

11. Classes 67

12. Creating and Deleting Objects 89

13. Reference Types 97

14. Value Types 103

15. Organization of C# Programs 117

16. Patterns and Techniques 123

17. Accessing Data in Objects 133

18. Properties 135

19. Indexers 147

20. Methods 151

21. Overloaded Operators 165

22. Delegates 173

23. Events 183

24. Patterns and Techniques 189

25. Specialization of Classes 195

26. Extension of Classes 203

27. Inheritance in General 209

28. Inheritance in C# 215

vi

29. Method Combination 241

30. Abstract Classes - Sealed Classes 245

31. Interfaces 253

32. Patterns and Techniques 271

33. Fundamental Questions about Exception Handling 301

34. Conventional Exception Handling 307

35. Object-oriented Exception Handling 311

36. Exceptions and Exception Handling in C# 313

37. Streams 333

38. Directories and Files 355

39. Serialization 359

40. Patterns and Techniques 369

41. Motivation for Generic Types 373

42. Generic Types 381

43. Generic Methods 401

44. Collections - History and Overview 407

45. Generic Collections in C# 409

46. Generic Dictionaries in C# 437

47. Non-generic Collections in C# 445

48. Patterns and Techniques 447

49. Correctness 453

50. Specification with preconditions and postconditions 457

51. Responsibilities and Contracts 463

52. Class Invariants 471

1

1. From structured programming to object-oriented

programming

We will assume that the reader of this material has some knowledge of imperative programming, and that the

reader already has been exposed to the ideas of structured programming. More specifically, we will assume

that the reader has some background in C programming. In Chapter 6 (corresponding to the second lecture of

the course) we summarize the relationships between C and C#.

1.1. Structured Programming

Lecture 1 - slide 2

We approach object-oriented programming by reviewing the dominating programming approach prior to object-oriented programming. It is called structured programming. A brief background on structured

programming, imperative programming, and - more generally - different schools of programming is provided

in Focus box 1.1. I will recommend that you read the Wikipedia article about structured programming [wiki-

str-pro]. It captures, very nicely, the essence of the ideas. Structured programming relies on use of high-level control structures instead of low-level jumping Structured programming is also loosely coupled with top-down programming and program development by stepwise refinement

Structured programming covers several, loosely coupled ideas. As summarized above, one of these is the use

of control structures (such as if, switch/case, while and for) instead of gotos. Use of relatively small procedures is another idea. A well-structured program should devote a single

procedure to the solution of a single problem. The splitting of problems in subproblems should be reflected

by breaking down a single procedure into a number of procedures. The idea of program development by

stepwise refinement [Wirth71] advocates that this is done in a top-down fashion. The items below summarize

the way it is done.

· Start by writing the main program

· Use selective and iterative control structures

· Postulate and call procedures P1, ...,Pn

· Implement P1, ... Pn, and in turn the procedures they make use of · Eventually, the procedures become so simple that they can be implemented without introducing additional procedures

Only few programmers are radical with respect to top-down structured programming. In the practical world

it is probably much more typical to start somewhere in the middle, and then both work towards the top and

towards the bottom. 2 Imperative programming, Structured programming, and Programming paradigms. Imperative programming is one of the four main programming paradigms. The others are functional programming, object-oriented programming, and logic programming. Imperative programming is closely related to the way low-level machine languages work: Commands are

used to change the values of locations in the memory of the computer. In high-level languages, this is

achieved by use of assignment statements, which is used to change the values of variables. The assignment

statement is therefore the archetypical command in imperative programming. Control structures (sequence,

selection, and iteration) come on top of that together with procedural abstractions.

Programming done in the early years of the computing era (before the introduction of Algol) is often thought

of as "unstructured programming". Unstructured programming is largely characterized by use of "jumping

around" by means of goto commands. The introduction of if and while control structures together with

procedures eliminated the need for gotos. This can be shown theoretically, but - more important - it also

holds true in the practical world of imperative programming. Armed with the common control structures (if

and while, for instance) and procedural abstraction, very few programmers are tempted to use a goto

statement in the programs they write. Such programming, without use of goto statements, is often called

structured programming.

1.2. A structured program: Hangman

Lecture 1 - slide 3

In order to be concrete we will look at parts of a C program. The program implements a simple and rudimentary version of the well-known Hangman game. We will pretend that the program has been developed according to the structured programming ideas described in Section 1.1.

The main Hangman program,

main, is shown in Program 1.1. The fragments shown in purple are postulated

(in the sense discussed in Section 1.1). I.e., they are called, but not yet defined at the calling time. The

postulated procedures are meant to be defined later in the program development process. Some of them are

shown below. 1 2 3 4 5 6 7 8 9 10 11 int main(void){ char *playerName; answer again; playerName = getPlayerName(); initHangman(); do{ playHangman(playerName); again = askUser("Do you want to play again"); } while (again == yes);

Program 1.1 The main function of the Hangman

program.

The function

getPlayerName is intended to prompt the Hangman player for his or her name. As it appears in

Program 1.2 this function only uses functions from the C standard library. Therefore there are no emphasized

parts in getPlayerName. 3 1 2 3 4 5 6 7 8 char *getPlayerName(){ char *playerName = (char*)malloc(NAME_MAX); printf("What is your name? "); fgets(playerName, NAME_MAX, stdin); playerName[strlen(playerName)-1] = '\0'; return playerName; Program 1.2 The function getPlayerName of main.

The function

initHangman calls an additional initialization function called initPuzzles, which reads a

puzzle from a text file. We will here assume that this function does not give rise to additional refinement. We

do not show the implementation of initPuzzles. 1 2 3 4 void initHangman (void){ srand(time(NULL)); initPuzzles("puzzles.txt");

Program 1.3 The function initHangman of main.

askUser is a general purpose function, which was called in main in Program 1.1. We show it in Program 1.4

(only on web) and we see that it does not rely on additional functions.

The function

playHangman, seen in Program 1.5, is called by main in the outer loop in Program 1.1. playHangman contains an inner loop which is related to a single round of playing. As it appears from

Program 1.5

playHangman calls a lot of additional functions (all emphasized, but not all of them included here). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
22
23
24
25
void playHangman (char playerName[]){ int aPuzzleNumber, wonGame; puzzle secretPuzzle; hangmanGameState gameState; char playersGuess; initGame(playerName, &gameState); aPuzzleNumber = rand() % numberOfPuzzles(); secretPuzzle = getPuzzle(aPuzzleNumber); while ((gameState.numberOfWrongGuesses < N) && (gameState.numberOfCorrectGuesses < secretPuzzle.numberOfCharsToGuess)){ gameStatistics(gameState, secretPuzzle); presentPuzzleOutline(secretPuzzle,gameState); printf("\n"); presentRemainingAlphabet(gameState); printf("\n"); if (CHEATING) presentSecretPuzzle(secretPuzzle); printf("\n"); playersGuess = getUsersGuess(); clrconsole(); updateGameState(&gameState, secretPuzzle, playersGuess); gameStatistics(gameState, secretPuzzle); wonGame = wonOrLost(gameState,secretPuzzle); handleHighscore(gameState, secretPuzzle, wonGame);

Program 1.5 The function playHangman of main.

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