Org Comp Refs Typ vro Prop Deleg Attrib Version Part. Introduction Part I. Radu Nicolescu Department of Computer Science University of Auckland

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1 Introduction Part I Radu Nicolescu Department of Computer Science University of Auckland 1 August / 50

2 1 Organisation and Contents 2 Compiling C# 3 References 4 Types 5 var, ref and out 6 Properties and Indexers 7 Delegates 8 Attributes 9 Versioning explicit overriding 10 Partial classes 2 / 50

3 Course materials All course materials in this part lectures, assignments, links,... are available in one single location: compsci335s2c/lectures/radu/ Of course, also linked from Canvas / Modules: (next slide) slides are for online study only handouts are recommended for printing 4 / 50

4 Course materials 5 / 50

5 Organisation Three lectures per week: handouts, slides, samples, additional readings requires homework Three tutorials per week, repeating the same topics: optional, but recommended immediately after the lectures Assignments: 20% of total course marks Test: 20% of total course marks likely MCQ Exam: 60% of total course marks likely MCQ Office hours: by appointment or immediately after the teaching? 6 / 50

6 Contents two parts Part R (weeks 3-8): Introduction to functional programming (FP) and applications such as XML and Web Services using REST Part I uses C# primarily, but also other languages, such as F#, Java, Javascript... Assignment requires a command-line C#, F# application Part M (weeks 1-2, 9-12): Web application security,... 7 / 50

7 Why functional programming? Functional programming (FP) is a programming style (paradigm) which often: leads to more concise and bug-free programs enables a better separation of concerns allows a uniform integration of data from heterogeneous sources (e.g. objects, SQL, XML, REST/ODATA) provides better support for concurrency (parallel programming) will make you a better programmer 8 / 50

8 Why functional programming? Tzu-li and Tzu-ssu were boasting about the size of their latest programs. Two-hundred thousand lines, said Tzu-li, not counting comments! Tzu-ssu responded, Pssh, mine is almost a million lines already. Master Yuan-Ma said, My best program has five hundred lines. Hearing this, Tzu-li and Tzu-ssu were enlightened. Master Yuan-Ma: The Book of Programming (Marijn Haverbeke: Eloquent Javascript) 9 / 50

9 Why functional programming? There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, And the other way is to make it so complicated that there are no obvious deficiencies. C.A.R. Hoare, 1980 ACM Turing Award Lecture (Marijn Haverbeke: Eloquent Javascript) 10 / 50

10 What about object-oriented programming? The problem with object-oriented languages is they ve got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle. Joe Armstrong, interviewed in Coders at Work (Marijn Haverbeke: Eloquent Javascript) 11 / 50

11 Why C#? C# and most modern imperative/oo languages (C++, Java) are becoming multi-paradigm languages In this direction, C#,.NET and related tools are years ahead of other imperative languages We ll also have short introduction to F# s basics We further use F# in a grad course (734), to study advanced functional topics, including modern parallel programming 12 / 50

12 Why C#? For example, a critical ingredient of functional programming lambda expressions C#: 2007 (!) C++: 2011 (C++11 Why Lambdas Rock) lambda-closures.html Java: 2014 (Java 8 is Revolutionary, Java is back) everything-about-java-8/ 13 / 50

13 Why C#? Another critical ingredient, true generics, i.e. generic types which are kept at runtime C#: 2006 (!) Current Java: generic types are processed by the compiler and then erased thus not present at runtime Java: 201? Project Valhalla http: // 14 / 50

14 Why C#? The functional concepts and skills that we learn using C# are or will be, sooner or later, available in other languages (Java) C# is cross-platfrom, free and open source C# is supported by powerful tools Linqpad a lightweight interactive editor, excellent for learning and developing small snippets (free version) Visual Studio (2015) a complex tool for large projects (free community version; also, other tools freely available to compsci students) technical-facilities/software.html 15 / 50

15 Compiling C#.CS C# source file Hello.cs 1 using System ; 2 3 p u b l i c c l a s s H e l l o { 4 p u b l i c s t a t i c void Main ( s t r i n g [ ] a r g s ) { 5 Console. W r i t e L i n e ( H e l l o! ) ; 6 Console. ReadLine ( ) ; 7 } 8 } Line 5 is the only real business line, hmm / 50

16 Compiling C# CSC cf PATH env variable Compiler output:.dll library;.exe program; collectively known as assemblies CSC C# compiler (VS 2015) C:\Program Files (x86)\msbuild\14.0\bin CSC C# compiler (VS 2017) C:\Program Files (x86)\microsoft Visual Studio\ 2017\Enterprise\MSBuild\15.0\Bin\Roslyn 1 CSC H e l l o. cs 1 CSC / t a r g e t : exe /out : H e l l o. exe H e l l o. cs language-reference/compiler-options/ command-line-building-with-csc-exe 18 / 50

17 Compiling C# Linqpad.LINQ C# snippet for Linqpad (also: F#, VB, SQL) Linqpad automatically references most frequently used libraries Linqpad automatically wraps your code into full classes Linqpad can easily create and maintain DB and Web REST connections 19 / 50

18 Compiling C# VS.SLN VS solution files (i.e. a group of related projects).csproj VS project file (i.e. a group of sources complied together) Many other files and folders... Has great tools and is excellent for large team projects Hides too much of the complexity, so it is not so great for small learning compsci projects VS is a good way to INSTALL a compatible complete set of compilers, tools and libraries But DO NOT USE VS IN 335, unless specifically advised! 20 / 50

19 C# references Required: familiarity with Chapter 1, Introduction, of C# Language Specification, version 5.0 for VS language-reference/language-specification/ Complete reference and programming guide https: //docs.microsoft.com/en-us/dotnet/csharp/csharp In the sequel, we briefly overview a few of the C# basic features that we use and are different from Java 22 / 50

20 Type system unification Java: (1) reference types (objects) and (2) primitive types (e.g. int) C#: (1) reference types one balloon two handles semantics C#: (2) value types (e.g. int or structures) copy semantics C#: conceptually, both types derive from the root class object and can be fully treated as objects! 1 s t r i n g s = 7. ToString ( ) ; 2 3 s t a c k. Push ( 1 2 ) ; 24 / 50

21 Classes vs Structures Let s contrast class and struct semantics First, consider class PointC reference semantics (one balloon, two handles) 1 c l a s s PointC { 2 p u b l i c i n t X { get ; set ; } 3 p u b l i c i n t Y { get ; set ; } 4 } 1 var pc = new PointC { X = 10, Y = 20, } ; 2 v ar qc = pc ; // ref semantics 3 qc. X += 1 ; // qc. X == pc. X == 11 pc and qc are two handles to the same object, which was changed! 25 / 50

22 Classes vs Structures Next, consider struct PointS copy semantics 1 s t r u c t PointS { 2 p u b l i c i n t X { get ; set ; } 3 p u b l i c i n t Y { get ; set ; } 4 } 1 var ps = new PointS { X = 10, Y = 20, } ; 2 v ar qs = ps ; // copy semantics 3 qs. X += 1 ; // qc. X == 11 && pc. X == 10 ps and qs are two distinct objects, initially identical, but then only one was changed! 26 / 50

23 Type-safe casting This casting raises an exception: 1 object o = abc ; 2 3 Point p = ( Point ) o ; This type-safe casting returns null (no exception) 1 object o = abc ; 2 3 Point p = o as Point ; // p = n u l l 27 / 50

24 The implicit type var The type of local variables can be statically determined by the compiler, if they are immediately initialised Instead of this redundant line 1 D i c t i o n a r y <s t r i n g, s t r i n g > t r a n s l a t e = 2 new D i c t i o n a r y <s t r i n g, s t r i n g >(); You can write a crisper one 1 var t r a n s l a t e = new D i c t i o n a r y <s t r i n g, s t r i n g >(); The compiler replaces this var by the expected type Dictionary <string, string> 29 / 50

25 The implicit type var This is still static typing; do not confuse it with more dynamic features var keyword dotnet/csharp/language-reference/keywords/var Implicitly Typed Local Variables programming-guide/classes-and-structs/ implicitly-typed-local-variables 30 / 50

26 Ref parameters Parameters that may change back (even value types): 1 s t a t i c void Swap ( r e f i n t a, r e f i n t b ) { 2 i n t t = a ; 3 a = b ; 4 b = t ; 5 } 1 i n t x = 1 ; i n t y = 2 ; 2 Console. W r i t e L i n e ( p re : x ={0}, y={1}, x, y ) ; 3 // pre : x=1, y=2 4 5 Swap ( r e f x, r e f y ) ; 6 7 Console. W r i t e L i n e ( p o s t : x ={0}, y={1}, x, y ) ; 8 // p o s t : x=2, y=1 31 / 50

27 The out parameters Ref parameters must be initialised before calling the method (recall Swap) Out parameters need not they get values from the called method In fact, out parameters are additional return values (sometimes we need more than one) programming-guide/arrays/ passing-arrays-using-ref-and-out 32 / 50

28 The out parameters Example a method with two out parameters 1 // divisor, dividend, quotient, remainder 2 void D i v i d e ( i n t a, i n t b, out i n t q, out i n t r ) { 3 q = a / b ; 4 r = a % b ; 5 } Example usage 1 i n t q, r ; 2 D i v i d e ( 7, 3, out q, out r ) ; 3 Console. W r i t e L i n e ( q={0}, r ={1}., q, r ) ; 4 // q=2, r =1. 33 / 50

29 Properties A handy structured way to define Java like accessors (i.e. a getter and setter pair) 1 p u b l i c c l a s s Button { // simplified 2 p r i v a t e s t r i n g c a p t i o n ; 1 p u b l i c s t r i n g Caption { 2 get { return c a p t i o n ; } // getter 3 set { // setter 4 c a p t i o n = v a l u e ; // v a l u e is setter param 5 Console. W r i t e L i n e ( r e p a i n t :{0}, c a p t i o n ) ; 6 } 7 } 8 } 35 / 50

30 Properties Properties can used in an uniform manner from outside look like fields! 1 var b = new Button ( ) ; 2 b. Caption = ABC ; // set; causes repaint 3 s t r i n g s = b. Caption ; // get 1 b. Caption += DEF ; // get & set; causes repaint The last line in Java style: 1 b. s e t C a p t i o n ( b. g e t C a p t i o n ( ) + DEF ) ; 36 / 50

31 Indexers associative arrays In a nutshell, an indexer is anonymous property with additional parameters, which formally look like array indices parameters/indices can have any type (int, string, etc) 1 var t r a n s l a t e = 2 new D i c t i o n a r y <s t r i n g, s t r i n g >(); 3 4 t r a n s l a t e [ one ] = un ; 5 // t r a n s l a t e. s e t ( one, un ) 6 t r a n s l a t e [ two ] = deux ; 7 t r a n s l a t e [ t h r e e ] = t r o i s ; 8 9 Console. W r i t e L i n e ( t r a n s l a t e [ two ] ) ; 10 // t r a n s l a t e. g e t ( two ) 11 // deux 37 / 50

32 Delegates type-safe pointers to methods Consider the following class defining two methods with excepting the names identical signatures (int string void) 1 c l a s s Test { 2 p u b l i c s t a t i c void S ( i n t a, s t r i n g b ) {... } 3 p u b l i c void I ( i n t x, s t r i n g y ) {... } 4 } Their common signature can be abstracted by the following delegate type (pointer-to-function type): 1 delegate void D( i n t p, s t r i n g q ) ; D matches any method with the same parameter and return types (here int string void) 39 / 50

33 Delegates type-safe pointers to methods The two methods can now be processed in an uniform way 1 D d = Test. S ; 2 d ( 3, a l p h a ) ; // Test. S ( 3, a l p h a ) 3 4 Test t = new Test ( ) ; 5 d = t. I ; 6 d ( 7, beta ) ; // t. I ( 7, beta ) In most practical cases, we do not really need to define our own delegate types There are many predefined delegate types (generic, to fit as many types as possible) 40 / 50

34 Delegates type-safe pointers to methods Action<...> (0 to 16 parameters) 1 T 1 T 2 void 2 3 p u b l i c delegate void 4 Action <i n T1, i n T2> ( 5 T1 arg1, 6 T2 arg2 7 ) Instead of our previous delegate type D, we could have used the predefined Action<int, string> 1 Action <int, s t r i n g > d = Test. S ; / 50

35 Delegates type-safe pointers to methods Func<...> (0 to 16 parameters) 1 T 1 T 2 TResult 2 3 p u b l i c delegate TResult 4 Func<i n T1, i n T2, out TResult> ( 5 T1 arg1, 6 T2 arg2 7 ) 42 / 50

36 Delegates type-safe pointers to methods Predicate<T> 1 T bool 2 3 p u b l i c delegate bool 4 P r e d i c a t e <i n T> ( 5 T o b j 6 ) Predicate<T> is a handy alternative to Func<T, bool> 43 / 50

37 Attributes similar to Java annotations Here, the WebMethod attribute specifies a cache duration 1 p u b l i c c l a s s S e r v i c e 1 : WebService { 2 [ WebMethod ( CacheDuration =60)] 3 p u b l i c double ConvertTemperature ( double f a h r ) { 4 return ( ( f a h r 32) 5) / 9 ; 5 } 6 } Line 2 decorates method ConvertTemperature with an instance of class WebMethodAttribute and sets its property CacheDuration The semantics of attributes is not defined by language itself, but by various tools and frameworks that run attribute decorated code 45 / 50

38 Versioning similar to annotations In C#, versioning is strictly enforced at the language level, while in Java annotations are recommended but still optional programming-guide/classes-and-structs/ versioning-with-the-override-and-new-keywords 1 p u b l i c c l a s s Base { 2 p u b l i c v i r t u a l void f ( ) {... } 3 } 1 p u b l i c c l a s s D e r i v e d : Base { 2 p u b l i c o v e r r i d e void f ( ) {... } 3 } This helps to avoid the (in)famous fragile base problem 47 / 50

39 Partial classes The same class can defined by separate chunks, even in different files programming-guide/classes-and-structs/ partial-classes-and-methods Very helpful if one part is automatically created by a tool which could be automatically regenerated (thus overwritten) while the other is developed by a programmer (and must be controlled) 49 / 50

40 Partial classes Example 1 p a r t i a l c l a s s X { 2 p u b l i c void f ( ) {... } 3 } 1 p a r t i a l c l a s s X { 2 p u b l i c void g ( ) {... } 3 } It works! 1 var x = new X ( ) ; 2 x. f ( ) ; 3 x. g ( ) ; 50 / 50

Object Oriented Paradigm

Object Oriented Paradigm History Simula 67 A Simulation Language 1967 (Algol 60 based) Smalltalk OO Language 1972 (1 st version) 1980 (standard) Background Ideas Record + code OBJECT (attributes + methods)