C# - Introduction

1. C# - Introduction Language Fundamentals in Brief UCN T&B: Computer Science Programme

2. As in Java C# …but note the capital ‘M’ • All program logic must be embedded in (typically) a class. • Every executable program must contain a Main-method. The Main-method is the starting point of the application. • C# is case-sensitive • No multiple inheritance (only between interfaces) • All classes inherit from Object • Garbage-collection • C# supports operator and method overloading UCN T&B: Computer Science Programme

3. A class public class Book { private string title; private string author; public Book(string t, string a) //Constructor { title= t; author= a; } public override string ToString(){ return (title+" "+author); } } UCN T&B: Computer Science Programme

4. Driver Program (Main) public class BookMain { public static void Main() { Book b1= new Book("C#","Troelsen"); Book b2= new Book("Java","Kölling"); System.Console.WriteLine(b1.ToString()); System.Console.WriteLine(b2); } } UCN T&B: Computer Science Programme

5. C#- Namespaces and Using • Namespaces is a tool for structuring programs and systems • Makes it possible to use the same names (identifiers) in different parts of an application. • Namespaces may be nested • Visual Studio creates default a namespace with the same name as the project • using <namespace name> tells the compiler where to look for definitions that our program refers to • Namespaces are not the same as Java-packages, but they are used for the same things and there are many similarities UCN T&B: Computer Science Programme

6. C#- value- and reference-types • Objects of value-type are stack allocated – objects of reference type are allocated on the heap • Value types die, when control goes out of the scope where they were declared – reference types are removed by the garbage collector (non-deterministic) • Value types are copied with assignment – with reference types a reference (the address) to the object is copied UCN T&B: Computer Science Programme

7. C#- reference types - example • creation, assignment and comparison: Customer c1, c2, c3; string s1, s2; c1 = new Customer("Flemming Sander", 36259); c2 = new Customer(”Bjarne Riis", 55298); c3 = null; // c3 refers to nothing c3 = c1; // c3 refers to the same object as c1 if (c1 == null) ... // is c1 referring to something? if (c1 == c2) ... // compare references if (c1.Equals(c2)) ... // compares object-values UCN T&B: Computer Science Programme

8. C#- When are objects equal? public class Customer { . . . public override bool Equals(object obj) { Customer other; if ((obj == null) || (!(obj is Customer))) return false; // surely not equal other = (Customer) obj; // explicit typecast return this.id == other.id; // equal if ids are... } } • Classes ought to override the Equals-method UCN T&B: Computer Science Programme

9. C#- Boxing and Unboxing • C# converts automatically between simple values and objects • value => object = "boxing“ (the value is “wrapped in a box”) • object => value = "un boxing“ (the value is unwrapped again) int i, j; object obj; string s; i = 32; obj = i; // boxing (copy) i = 19; j = (int) obj; // unboxing! s = j.ToString(); // boxing! s = 99.ToString(); // boxing! UCN T&B: Computer Science Programme

10. C#- arrays • Arrays are reference types • Created from the Array-class in FCL • Created using the new-operator • 0-based indexing • Are initialised with default value (0 if numeric, null if reference) int[] a; a = new int[5]; a[0] = 17; a[1] = 32; int x = a[0] + a[1] + a[4]; int l = a.Length; Creation Access element 1 Number of elements UCN T&B: Computer Science Programme

11. C#- structs • In some ways like a class, but there are differences: • Can have instance variables and methods • Cannot have a default constructor • Variables of a struct-type are value types and as such stack allocated • Can only inherit from interfaces • Cannot be inherited from • Can be used to implement ADTs, but no inheritance and polymorphism UCN T&B: Computer Science Programme

12. C#- selection and iteration x = obj.foo(); if (x > 0 && x < 10) count++; else if (x == -1) ... else { ... } while (x > 0) { ... x--; } for (int k = 0; k < 10; k++) { ... } UCN T&B: Computer Science Programme

13. C#- foreach-loop • foreach loop is used to sweep over collections as arrays • Reduces the risk of indexing errors int[] data = { 1, 2, 3, 4, 5 }; int sum = 0; foreach (int xin data) { sum += x; } foreach type value collection UCN T&B: Computer Science Programme

14. C#- Methods • A class may have two kind of methods: • Instance methods • Static methods (class methods) • Instance methods need an object to be invoked • Static methods are called using the class name only UCN T&B: Computer Science Programme

15. C#- Example • The array-class in BCL (FCL) • The class is a member of namespace System (System.Array) namespace System { public class Array { public int GetLength(int dimension) { ... } public static void Sort(Array a) { ... } . . . } } instance method static method UCN T&B: Computer Science Programme

16. C#- calling the methods /* main.cs */ using System; public class App { public static void Main() { int[] data = { 11, 7, 38, 55, 3 }; Array.Sort(data); for (int i=0; i<data.GetLength(0); i++) Console.WriteLine(i + ": " + data[i]); } } Class-method Instance-method UCN T&B: Computer Science Programme

17. Exercises Use NotePad and the command prompt compiler (csc.exe) to do the following: (see HelloWorld – demo) • Create a class including a Main method that instantiates an array of int (previous slide). • Write methods (static) that do the following: • A method that returns the sum the elements in the array • A method that returns the average of the elements in the array • A method that returns the number of elements with the value 7 in the array • A method that returns true if the value 3 is contained in the array and false otherwise • Generalise your solution to exercise 2 and 3, so the value in question (7 and 3 resp.) is passed as an argument to the method. • For each method write driver code in the Main method that tests the method. UCN T&B: Computer Science Programme