1. Object-Based Programming in VB.NET

2. Must Understand Following: Encapsulation Information hiding Abstract Data Type Class, Instance, Reference Variable v. value variable, Object, Method (function or subroutine), Property, …

3. Create Class Add new class file with appropriate name Add appropriate methods, class-level variables, and properties Public Class Person public firstName as string public MI as char public lastName as string public function fullName() return firstname.trim() & “ “ & MI.trim() & _ lastName.trim() end function end class

4. Create Reference Variable and Class Instance Dim x as Person ‘Note first char Cap ‘pointer on stack x = new Person‘this creates instance on heap x.firstname = “fred” x.MI = “F” x.lastName = “farfinkle” msgbox(x.fullname())

5. Class Scope Class's Scope –Instance variables and methods Class’s members –Class members that are visible can be accessed only through a “handle” (ReferenceVariable.memberName) Variables within methods –Only methods can access that variable Keyword Me –A hidden instance variable can be accessed in a method by preceding its name with the keyword Me and dot operator

6. Controlling Access to Members Public versus Private –Control access to a class’s instance variables and methods Public –Serve primarily to present interfaces of a class Private –Holds clients private data safely –Get and set functions Have ability to access private data

7. Initializing Class Objects: Constructors Initializing Constructors –False for Booleans and Nothing for references If an instance variable is not initialized the compiler will assign a default value –Form of declarations Dim ObjectReference As New ClassName(arguments) Programs without default constructor are provided with one by the compiler

8. Using Overloaded Constructors Overloaded Constructors –Must have different numbers and/or types and/or orders of parameters (See example of Person2)

9. Properties Private and Public –Get accessor In Visual Basic instance variables as private does not guarantee data integrity –Set accessor Cannot return values indicating a failed attempt to assign invalid data to objects of the class Control the setting of instance variables to valid values –Get and Set accessors are not required A property with only Get accessor is called ReadOnly A property with only Set accessor is called WriteOnly

10. Composition: Objects as Instance Variables of Other Classes –Note “spouse” reference Variable pointing to second instance of Person2 contained inside first instance of Person2. DANGER!!!!!!! – NOTE WHERE OBJECT IS INSTANCIATED!!!!! – what happens if we had: –Dim spouse as new Person2 rather than dim spouse as person2

11. Using the Me Reference Me Reference –Every object can access a reference to itself using a Me reference. Me explicitly Me implicitly –The explicit use of the Me reference can increase program clarity where Me is optional

12. Garbage Collection Garbage collector –Resource leaks Objects must have an efficient way to return memory and release resources when the program no longer uses those objects –Memory leaks In Visual Basic memory is reclaimed automatically, hence it experiences rare memory leaks as compared to C and C++ –Reference counting!! –Finalization Finalizer method performs termination housekeeping on that object just before the garbage collector reclaims the object's memory. Not the same as deconstructor in C++

13. Shared Class Members Shared Class Variable –Contains only one copy of this variable in memory When a single copy of the data will suffice, use Shared class variables to save storage. Shared class variables are not the same as global variables because Shared class variables have class scope Shared method has no Me reference

14. Const and ReadOnly Members Const or ReadOnly –Const A data member must be initialized in its declaration Cannot be modified once initialized –ReadOnly A data member can be initialized either in the class structure or in its declaration Cannot be modified once initialized

15. Namespaces and Assemblies Framework Class Library –.NET Framework: Must be imported to a Visual Basic program by including a reference to those libraries –Namespaces: Namespaces help minimize naming collisions by proving a convention for unique class names