Chapter 12

Slide 1:Chapter 12 Creating Reusable Components with Classes

Slide 2:Objectives Understand the concept of a component Understand the role of author and developer in creating and testing components Define a namespace Create events in a class module Understand the types of class data Create properties with Public variables and Property Procedures Create enumerations Create methods Learn about overloading

Slide 3:The Theory of Component Creation Remember that VB .NET is an object-oriented programming language Data and the processes that act on that data are encapsulated into classes supporting properties, methods, and events The encapsulated package is called a component The interface for a component is the properties, methods, and events supported by the component The implementation is that part of the component hidden from the developer

Slide 4:The Role of Author and Developer The author creates components intended for reuse by other developers The author creates the Class Library project The developer uses components created by an author The developer uses the Class Library project in a Windows Application project When developing and testing components, you switch back and forth between the roles of author and developer

Slide 5:Introduction to Component Creation Create a solution file with two project files One project file is a Class Library project Class Library project contains the components Second project is a typical Windows Application project Windows Application project is used to test the Class Library project

Slide 6:Relationship between the Solution File and its Project Files

Slide 7:Creating a Component Create a Class Library project by adding a new project to an existing solution Project is created from a template using the Add New Project dialog box Specify Class Library as the template

Slide 8:Add New Project Dialog Box

Slide 9:Solution Explorerwith Two Projects

Slide 10:Defining the Namespace for a Component Remember that all classes are organized into namespaces When you created a desktop application, VB .NET defined a default name for you By default, the Namespace name is the same as the project name Namespaces are organized into a hierarchy System is a root namespace because it appears at the top of the hierarchy System.Windows.Forms belongs to the System namespace

Slide 11:Naming Rules for Namespaces Microsoft recommends that namespace names be defined as follows: CompanyName.TechnologyName CompanyName contains the name of the company who developed the namespace TechnlogyName contains a particular technology For Example: Course.TrafficSignals

Slide 12:Defining the Root Namespace

Slide 13:Referencing Namespaces between Projects (Steps) Select the Windows Application project in the Solution Explorer Click Project, and then click Add Reference to display the Add Reference dialog box Click the Projects tab Select the Class Library project, and then click OK to add it

Slide 14:Add Reference Dialog box

Slide 15:Understanding the Imports Statement The Imports statement does not make a namespace available for use Its purpose is to allow you to reference the classes in a namespace without using a fully qualified reference Multiple Imports statements can appear to import multiple namespaces Imports statements typically appear after the Option Explicit and Option Strict statements

Slide 16:Namespace Hierarchies Namespaces are organized hierarchically You can create your own namespace hierarchies

Slide 17:Namespace Hierarchy (Illustration)

Slide 18:Declaring a Namespace Namespace statement forms a block and contains one or more classes End Namespace statement marks the end of the namespace Namespaces, in turn, contains classes

Slide 19:Class Events (Introduction) As the developer, you have written event handlers throughout this book Windows raised events as the user interacted with the form or control instances on the form As the author you must define events and raise them to the developer Event statement declares an event RaiseEvent statement raises an event Note that we use the terms "raising an event" and "firing an event" synonymously

Slide 20:Class Events (Syntax) [ Public ] Event procedurename [ (arglist ) ] Optional Public keyword indicates that the event has Public access (visibility) Event keyword declares an event Note that it does not fire the event Procedurename defines the name of the event Standard variable and procedure naming rules apply Optional arglist contains arguments passed to the event Arguments can be passed by value or by reference

Slide 21:Declaring an Event (Example) Declare an event named LightChange Public Event LightChange(ByVal _ CurrentLight As Color) Event is named LightChange Event takes one argument having a data type of color

Slide 22:Raising an Event RaiseEvent statement fires an event Syntax RaiseEvent eventname ( arglist ) RaiseEvent statement appears in a procedure and will fire an event eventname contains the event to fire Declare the event with the Event statement arglist contains zero or more arguments Arguments must match event declaration

Slide 23:Declaring and Raising Events

Slide 24:Creating a Class Instance (1) Use the same syntax as you would use to create any other class instance Use the New keyword to create a class instance Omit the New keyword to just declare an object variable that references Nothing

Slide 25:Creating a Class Instance (2) Create an instance of the StopLight class Private LightNorth As New StopLight() Create an instance of the StopLight class using two statements Private LightNorth As StopLightLightNorth = New StopLight()

Slide 26:Types of Class Data Data can be exposed or hidden from the developer Hidden data is not available to the developer and is part of the class�s implementation Exposed data is available to the developer and is part of the interface Define hidden or exposed data using the same declaration statements that you have used in the past Public, Private, Friend, Dim

Slide 27:Declaring Class Data Public variables are exposed (part of the interface) and are considered properties Friend variables are shared by the classes in a project but not outside the project Private variables are hidden in the class containing the declaration so they are part of the implementation Local variables appear in a procedure and are part of the implementation Declare local variables with the Dim keyword

Slide 28:Creating a First Property The easiest way to create a property is to declare a Public variable in a class Create property names such that they contain full words Capitalize the first letter of each word Avoid obscure abbreviations Example to declare a property named Location having a data type of String Note that the declaration appears in the class module Public Class StopLight Public Location As String End Class

Slide 29:Referencing a Property Use the same object.property syntax with which you are familiar Note that the Intellisense technology works too Example

Slide 30:Introduction to Enumerations You have been using predefined enumerations throughout this book Message box icons Colors Etc. You can create your own enumerations with the Enum statement A class can have multiple enumerations

Slide 31:Enum Statement (Syntax 1) [ Public | Private | Friend ] Enum name membername [ = constantexpression ] membername [ = constantexpression ] . . . End Enum

Slide 32:Enum Statement (Syntax 2) Public, Private, and Friend keywords define the scope of the enumeration Meaning is the same as with other variable declarations Enum statement marks the beginning of the enumeration block name contains the name of the enumeration membername is an element of the enumeration An enumeration can contain as many elements as needed constantexpression contains the value of the enumeration member By default numbering is sequential starting at 0 End Enum statement marks the end of the enumeration

Slide 33:Enumeration (Example 1) Declare an enumeration named Color with 4 members Public Enum Color Green = 1 Yellow = 2 Red = 3 None = 4 End Enum

Slide 34:Enumeration (Example 2) Enumerations need not have unique or sequential values Example to declare an enumeration for the number of days in a particular month

Slide 35:Creating Enumerated Variables After declaring an enumeration, you can declare variables having an enumerated type Example: Public CurrentColor As Color Store a value in the enumeration: CurrentColor = Color.Green

Slide 36:Hiding Data in a Class Simply declare a Private constant or Private variable in a class Local variables declared in a procedure are also hidden Static local variables declared in a procedure are also hidden

Slide 37:Creating Procedures in a Class Function and Sub procedures in a class module have the same syntax as Function and Sub procedures declared anywhere else Private procedures are hidden and are part of the implementation Public procedures are exposed and are part of the interface Procedures can also be declared as Friend

Slide 38:Creating a Method Create a Public Function procedure to create a method that returns a value Create a Public Sub procedure to create a method that does not return a value Both Function and Sub procedures can accept 0 or more arguments Naming conventions: Use a verb in the procedure name to denote a method Use whole words avoiding obscure abbreviations

Slide 39:Property Procedures (Introduction) Property Procedures provide an alternative to creating properties with Public variables Code executes when the developer reads or writes a property created with a Property Procedure Thus, the data can be validated In addition, code can be written to store and retrieve values from other variables

Slide 40:Property Procedures (Syntax 1) [ Default | ReadOnly | WriteOnly ] Property varname ( [ parameter list ] ) [ As type ] [ Get [ block ] End Get ] [ Set ( ByVal value As typename ) [ block ] End Set ] End Property

Slide 41:Property Procedures (Syntax 2) Default keyword means that the property is read-write. It can be omitted Both Get and Set blocks must exist ReadOnly keyword indicates that property is read-only Get block must exist Set block must not exist WriteOnly keyword indicates that property is write-only Get block must not exist Set block must exist

Slide 42:Property Procedures(Syntax 3) varname contains property name Standard variable naming rules apply parameter list argument defines arguments used to set the property As type clause defines the data type of the property Required because we use Option Explicit in this book Code in the Get block executes when the developer reads the property's value Code in the Set block executes when the developer writes the property's value

Slide 43:Creating a Read-Write Property Example Public Property GreenInterval() As Integer Get Return mintGreenInterval End Get Set (ByVal Value As Integer) mintGreenInterval = Value Call ResetTimer(mOperationMode, mLight) End Set End Property

Slide 44:Property Procedures (Illustration)

Slide 45:Creating a Read-Only Property (Example) Include the ReadOnly keyword Omit the Set block Public ReadOnly Property CurrentColor() As Color Get Return mLight End Get End Property

Slide 46:Creating a Write-Only Property (Example) Include the WriteOnly keyword Omit the Get block Private WriteOnly Property SetCurrentColor() _ As Color Set (ByValue Value As Color) mLight = Value End Set End Property

Slide 47:Creating a Write-Once Property Use the same syntax as you would use to create a read-write property but Declare a Static Boolean variable in the Set block Set the value to True when the property is set the first time Use an If statement to test whether the property has been set previously and throw an exception if necessary

Slide 48:Creating a Write-Once Property (Example)

Slide 49:Parameterized Properties Parameterized properties accept multiple arguments Example using a hidden array

Slide 50:Overloading Overloading refers to the ability to create multiple procedures having the same name but varying argument lists Methods can have a different number of arguments Methods can have arguments having different data types Use the Overloads keyword to overload a method

Slide 51:Overloading (Illustration)