Simple Classes in C# CSCI 293 September 12, 2005

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

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

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

Constructors Just like C++ Constructors are optional in 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

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

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

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);

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

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.

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; }

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

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;

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;" ?