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++ 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

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;" ?