Simple Classes in C#

1. Simple Classes in C# CSCI 293 September 12, 2005

2. Today's Objectives • Terminology • Scope of Variables and Methods public, private, and more • Constructors, Initializers, and Destructors • Instance Variables and Class (static) Variables

3. Terminology of C# Classes • an object is a particular instance of a class • class = elephant, object = Dumbo • Classes contain • methods - functions • fields - member variables • properties - methods that looks like fields to users of the method

4. Limiting Scope of Fields Given a class named "list" with a variable named "size" • public int size; • everyone can see size • private int size; • only list objects can see size, default is private • protected int size; • list objects and objects derived from list can see size • internal and protected internal

5. Constructors • Just like C++ public class list { private int size; // variable public list () // constructor { size = 0; } • Constructors are optional in C# • this constructor sets size=0, which is the default anyway

6. Initializers • Since constructors are optional… public class list { private int maxsize = 10; // set default public list () { maxsize = 10; // redundant } public list (int newmax) { maxsize = newmax; // override default }

7. Creating an Object • Don't forget to call new() class MainProgram { static void Main() { list mylist1 = new list(); list mylist2 = new list(50); ...

8. Passing Parameters void find_max (int size, ref int max); { search the array and set max } ... // pass 10 by value and new_max by reference find_max (10, ref new_max);

9. "this" = this object public class list { private int size; ... public set_size (int size) { this.size = size; }

10. Instance Variables vs Class Variables • Sometimes, you need a variable that belongs to the entire class, not one variable for each object. public class WarningBox { private string message; private static int boxcount; Each WarningBox instance has its own message, but there is only one boxcount for all WarningBox objects.

11. Static Constructors • Operate on the class, not an individual object. • Only called once, even with multiple instances of a class. • Not allowed to access non-static variables. public class WarningBox { private string message; private static int boxcount; static WarningBox() { boxcount = 0; }

12. Destructors • Not necessary to call free(), because garbage collection is automatic. • Example Destructor ~WarningBox() { boxcount--; }

13. Properties Properties look like variables to the class user, but they look like methods to class programmer. private int securitylevel; // object variable ... public int SecurityLevel // property { get { //search database for level return level; } set { securitylevel = value; } } --- inside main() --- mymessage.SecurityLevel = 5;

14. Q u i z • Are variables and methods public or private by default? • Why do I need a constructor if I can initialize fields when I declare them? • How is a "class" variable declared? • Given "private static int BoxCount" is this legal: "this.BoxCount = 0;" ?