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Creating Classes. Tonga Institute of Higher Education. Current Status. The form class contains all of our code Code is added to the form class Code may use variables, methods and events Code may use other objects When a program is run, the form object is used. Custom Classes and Objects.

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creating classes

Creating Classes

Tonga Institute of Higher Education

current status
Current Status
  • The form class contains all of our code
  • Code is added to the form class
    • Code may use variables, methods and events
    • Code may use other objects
  • When a program is run, the form object is used
custom classes and objects
Custom Classes and Objects
  • Sometimes, the objects provided by Microsoft do not fit our needs.
  • We can define custom objects.
  • We can use custom objects just like normal objects
how to define a class
How to Define a Class

Access Specifier

Class Name

  • Access Specifier - Covered later
  • Class Name
    • Class names should be nouns
    • One word
    • The first letter of each internal word is capitalized.
    • Keep your class names simple and descriptive.
    • Example:
      • Customer
      • SalesOrder
demonstration

Demonstration

Defining a class

how to define a variable
How to Define a Variable

Access Specifier

Variable Name

Type

  • Access Specifier – Covered later
  • Variable Name
    • One word
    • The first letter of each internal word is capitalized.
    • Keep your variable names simple and descriptive.
    • Example:
      • FirstName
      • PhoneNumber
  • The type can be a primitive or an object
    • Example: Integer, String, Boolean, Student
demonstration1

Demonstration

Defining a variable

how to define a method subroutines and functions
How to Define a Method(Subroutines and Functions)
  • Methods - Pieces of code that perform a single function
  • Subroutine – A method that does not return anything
  • Function – A method that returns something
difference between subroutines and functions
Difference Between Subroutines and Functions

Access Specifier

Parameter Type

Parameter Name

Method Name

Access Specifier

Parameter Type

Method Name

Parameter Name

Return Type

Return Data

subroutines and function names
Subroutines and Function Names
  • Verbs
  • One word
  • The first letter of each internal word is capitalized.
  • Keep your method names simple and descriptive.
  • Example:
    • RunFast
    • GetBackground
    • ShowWelcomeMessage
subroutines and function parameters
Subroutines and Function Parameters
  • Parameters are optional
  • Each parameter that your method accepts must have a name and type
    • The name should be:
      • One word
      • The first letter of each internal word is capitalized.
      • Keep your parameter names simple and descriptive.
    • The type can be a primitive or an object
      • Example: String, Integer, Customer
    • Example:
      • Private Sub ShowSummary(ByVal StudentID As Integer)
  • If multiple parameters are used, separate them with commas
    • Example: Private Sub ShowSummary(ByVal StudentID As Integer, ByVal StudentName as String)
  • Use the name inside the method parameter list to access the parameter in your code
    • StudentID
    • StudentName
function return types
Function Return Types

Access Specifier

Parameter Type

Method Name

Parameter Name

Return Type

Return Data

  • Return Type – Sets the type of data that will be returned from the function.
  • The type of data can be a primitive or an object
    • Example: String, Integer, Customer
  • If you are not returning anything, do not use a function. Use a subroutine.
  • Do not forget to use the Return keyword in your function
demonstration2

Demonstration

Defining a subroutine

demonstration3

Demonstration

Defining a function

method overloading
Method Overloading
  • Overloading - Having multiple methods with the same name but different parameters
  • To make an overloaded method, create a method with the same name but different parameters.
  • Return types for functions can be overloaded but it is not a good idea

Same Name

Different Parameters

demonstration4

Demonstration

Method Overloading

properties and code conventions
Properties and Code Conventions
  • Generally, make all class variables private
  • Use properties to access your variables
  • Properties – a specialized method used to get and set a class’ variables
  • This could be done using regular methods, but properties are used more often
property features
Property Features
  • Read-Only Properties
  • Write-Only Properties
demonstration5

Demonstration

Properties

using the custom class
Using the custom class
  • Use custom objects just like normal objects
    • Declare and instantiate them
    • Use methods and properties
  • A class cannot use itself
    • A different class must use a class
    • Example: A StudentSystem class would use a Student class
demonstration6

Demonstration

Using the custom class

dynamic link libraries
Dynamic Link Libraries
  • Until now, all of our VB.Net programs have been .exe files
  • This allows us to run a program
  • Sometimes, we need to share a class that we’ve made
  • A Dynamic Link Library is a file that contains classes for use by other programs.
  • They end with .dll
    • Example: System.dll
seeing dlls in action
Seeing DLLs in Action
  • When a form is run, many different classes are needed. Example: Form class, textbox class, and more.
  • VS.Net loads dll files when a form is run. The dll files contain the class data for the objects needed to run the program
  • The loading of dll files can be seen in the Output box
demonstration7

Demonstration

Seeing DLLs in Action

creating a dll file
Creating a DLL File
  • We can create Dynamic Link Libraries
    • Right click on the project in the Solution Explorer
    • Select Properties
    • Select the Class Library Output Type
    • Click OK button
    • Compile the project. When the project is compiled, VS.Net creates a .dll file in the Bin directory
demonstration8

Demonstration

Creating a DLL File

references
References
  • Many projects use dll files
  • Many dll files used are listed in the references folder
demonstration9

Demonstration

References

adding dll files to a project 1
Adding DLL Files to a Project - 1
  • To use a dll file in a project, right click on the References folder and select Add Reference
  • The Add Reference form is displayed
adding dll files to a project 2
Adding DLL Files to a Project - 2
  • Use the .Net tab to add a .Net dll
  • Use the COM tab to add a COM dll. (These are dll files made before VB.Net)
  • Use the Projects tab to add a reference from a project in the same solution
demonstration10

Demonstration

Adding DLL Files to a Project

using dll files
Using DLL Files
  • To use a class defined in a DLL file
    • Add a reference to the dll file to the project
    • Write the fully qualified namespace to access the object

Dim x as new ClassLibrary1.Student

Remember: Namespaces are not always the same as the file name!

Class Name

Fully Qualified Namespace

demonstration11

Demonstration

Using DLL Files

import statements
Import Statements
  • Import Statements allow you to not always type the fully qualified namespace

Imports ClassLibrary1.Student

  • There are import statements defined at the project level. To see these, go to Project Properties -> Common Properties -> Imports
demonstration12

Demonstration

Import Statements

dll and exe file tips
.DLL and .EXE File Tips
  • A .Dll file cannot be run. Rather, it is built
  • Do not use the same name for a .dll and exe file
  • If the location of a .dll file reference is changed, VS .Net will show a yellow error message. To fix this, remove the incorrect reference and add a new reference
  • When you make a change in a .dll file, the project must be rebuilt.
  • When a .dll file is changed in a separate solution from the .exe file, the .exe solution must be reloaded
  • An .exe project that uses a .dll file will have a copy of the .dll file saved to the bin directory. When giving the program to other people, both the .dll and .exe files must be given to run properly.
demonstration13

Demonstration

.DLL and .EXE File Tips

what is scope
What is Scope?
  • The scope of an element (variable, class, method, etc.) defines what parts of a program can see it

Using Public instead of

Private changes the scope

of this variable.

levels of scope
Levels of Scope
  • Block
  • Procedure
  • Class
  • And others
block scope
Block Scope
  • A block is a set of statements terminated by an End, Else, Loop, or Next statement
    • Example: The code within a For...Next or If...Then...Else...End.
  • An element declared within a block can be used only inside that block
  • Use Dim to declare elements with block scope
  • The Static keyword may also be used (Covered later)

Block

demonstration14

Demonstration

Block Scope

procedure scope
Procedure Scope

Procedure

  • A procedure is a subroutine or function
  • An element declared within a procedure is only usable in the procedure.
  • Elements at this level are also known as local elements.
  • Use Dim to declare elements with procedure scope
  • The Static keyword may also be used (Covered later)
  • Works like block scope but you must also consider the parameters

Procedure

demonstration15

Demonstration

Procedure Scope

class scope
Class Scope
  • Declare elements at this level by placing the declaration statement outside of any procedure or block within the class.
  • These access specifiers can be used
    • Public
    • Private
    • Dim
    • Friend
    • Protected
    • Protected Friend
access specifiers
Access Specifiers

Access Specifier

  • Public
    • Can be used by everything
  • Private
    • Can only be used by code inside the same class
  • Dim
    • Same as Private
  • Friend
    • Can be used by code inside the same project

Name

Dim FirstName as String

Type

  • Protected
    • Can be used by code that inherits from this class
  • Protected Friend
    • Combination of Protected and Friend
demonstration16

Demonstration

Access Specifiers

scope global variables vs local variables
Scope:Global Variables vs. Local Variables
  • Global Variables
    • Variables defined outside of a procedure or block but inside of a class
    • Also called member variables
    • Can be readonly by using the ReadOnly key word
  • Local Variables
    • Variables defined inside procedures or blocks
    • They are only visible by code inside the procedure or block
  • When looking for a value, start local. Go global if you can’t find the variable
demonstration17

Demonstration

Global vs. Local Variables

element lifetimes
Element Lifetimes
  • The lifetime of a declared element is the period of time during which it is available for use.
  • Local variables declared with Dim exist only while the procedure in which they are declared is executing.
  • A variable declared in a class exists as a separate copy for each instance of the class in which it is declared; each such variable has the same lifetime as its instance.
demonstration18

Demonstration

Element Lifetimes

controlling element lifetimes
Controlling Element Lifetimes
  • We can additionally control the lifetime of a declared element
    • Code
      • Setting variables to nothing
    • Keywords
      • Shared
      • Static
demonstration19

Demonstration

Setting variables to nothing

shared lifetime 1
Shared Lifetime 1
  • Instance variable - A variable where each instance can have a different value
shared lifetime 2
Shared Lifetime 2
  • Global Variables may be shared
  • Shared elements are not associated with a specific instance of a class. All instances use the same value.
  • Access them 2 ways
    • Use the class name
    • Use the variable name of a specific instance of the class
demonstration20

Demonstration

Shared Lifetime

static lifetime
Static Lifetime
  • Local variables may be static
  • Static variables remain in existence and retain their latest values after termination of the procedure in which they are declared.
demonstration21

Demonstration

Static Lifetime

constructors
Constructors
  • Constructor – A method that is automatically executed when an object is created.
    • This allows you to set initial values for the object.
    • Many objects have multiple constructors. (They are overloaded)
default constructors
Default Constructors
  • All classes have a default constructor
    • The default constructor is a constructor that accepts no parameters
    • Automatically called when the object is created
    • Default constructors go away when you define your own constructors
creating constructors
Creating Constructors
  • Constructors do not return anything.
  • To overload constructors
    • Define multiple methods with the same name as the class with different parameters

New indicates

that this is

a constructor

Overloaded

demonstration22

Demonstration

Creating Constructors

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