Define a Class in Visual Basic .NET

Listing 9.2 CustomerInterface.vb: The ICustomer Interface with Properties Public Interface ICustomer #Region "Properties" 'Define properties for this interface 'that match the table s

9.1 Define a Class in Visual Basic NET

Before writing code, you should take a moment to decide what data the class needs to contain and what actions a developer (whether it is you or a teammate) would want to perform on that data For the moment, we'll keep it fairly simple by limiting the class to properties representing the columns in the Customers table and to methods that insert, delete, and update data

Technique

Visual Basic NET has a specific kind of code file that you can use to describe the public members of a class: an interface An interface simply lists all the public variables,

methods, and properties that any class implementing that interface must expose

Interfaces are an enormously powerful and flexible feature of object-oriented programs, and this chapter just scratches the surface Because of the fact that an interface has no code, you can focus on the design of your application without the clutter of hundreds of lines of source code

Steps

1 First, create a new Windows Application project and name it CustomerClasses

2 To create a new interface, add a new class to your project by right-clicking on the Solution Explorer window and selecting Add Class Name the new file

CustomerInterface9_1.vb

3 Change the class block declaration so that it reads interface instead of class, and name the class ICustomer You should have a code file like that shown in Listing 9.1

Listing 9.1 CustomerInterface9_1.vb: An Empty Interface

Public Interface ICustomer

End Interface

Add Properties to the Interface

The first items you need to add to your interface are properties to represent the columns

in the Customers table

1 Using the Server Explorer, drill down to the Customers table in the Northwind database so that you have an easy reference to the Customers table

2 To make your code more readable, add a #Region/#End Region block with the name Properties so that you can expand and collapse your property declarations as needed

3 For each column in the table, add a property declaration to your interface that matches the datatype of the column and its name(see Listing 9.2)

Listing 9.2 CustomerInterface.vb: The ICustomer Interface with Properties

Public Interface ICustomer

#Region "Properties"

'Define properties for this interface

'that match the table structure in name and data type

Property CustomerID() As String

Property CompanyName() As String

Property ContactName() As String

Property ContactTitle() As String

Property Address() As String

Property City() As String

Property Region() As String

Property PostalCode() As String

Property Country() As String

Property Phone() As String

Property Fax() As String

#End Region

End Interface

More About Properties

ICustomer is a simple interface It has only one kind of datatype-a string-and all of the properties are read/write When you begin to write more complex classes, you might need more than just read/write properties Visual Basic NET also has read-only and write-only properties, parameterized properties, and default properties

Read-Only and Write-Only Properties

If you have written classes in Visual Basic 6, you might be scratching your head,

wondering how the code sample in Listing 9.2 declares your properties read/write In Visual Basic 6, all that mattered was whether you wrote the appropriate Property Get and Property Let/Set blocks in your class If you did not have a Public or Friend Property Let/Set, the property was read-only If you did not have a Public or Friend Property Get, the property was write-only

In Visual Basic NET, you must explicitly declare a property as read-only or write-only using the ReadOnly and WriteOnly keywords (see Table 9.1)

Table 9.1 Visual Basic NET Property Modifiers Object Purpose

ReadOnly The property cannot be modified outside of the class

WriteOnly The property can only be modified, but only methods that are internal to the

class can read the property

In our example, a developer who is using this class could change the CustomerID

Without the original CustomerID, the class won't be able to find that record in the

database when updating the table, so perhaps you should make the CustomerID property read-only To do this, simply add the ReadOnly keyword to the beginning of the property declaration as in Listing 9.3

Listing 9.3 CustomerInterface9_1.vb: Using the ReadOnly Keyword

'Adding the ReadOnly keyword before the property

'declaration makes the property read-only

ReadOnly Property CustomerID() As String

Parameterized Properties

You can also create properties that accept a parameter For example, in the Northwind database, a customer might have many orders A parameterized property would be a perfect way to access Orders information from the Customer class

A parameterized property (see Listing 9.4) takes one or more parameters that can be used

to qualify the data that the property returns

Listing 9.4 A Parameterized Property

'The nIndex parameter would be used to determine which

'item from a collection of orders should be returned

Property CustomerOrders(ByVal nIndex As Integer) As COrder

Tip

One of the advantages to interfaces is the ability to rapidly define all of the classes you plan to use in your application For the previous code example, you could create an interface called IOrder that was similar to the ICustomer interface in Listing 9.2 Instead of returning an actual class from the CustomerOrders property, you could return the IOrder interface The interface would act as a placeholder for any object that implemented the interface This way, you could program the Customer class and put off writing code for the Order class until later

Default Properties

In Visual Basic 6, any property in a class could be defined as the default property for that class For example, if the CustomerID property were declared the default property, both

of the examples in Listing 9.5 would return the customer ID

Listing 9.5 Default Properties in Visual Basic 6

Dim oCustomer As CustomerData

Dim strCustomerID As String

'Example 1: This code places the value of the customer ID property

'into a variable

strCustomerID = oCustomer.CustomerID

'Example 2:This code does the same thing

strCustomerID = oCustomer

In Visual Basic NET, the only kind of property that you can declare as the default

property for a class is a parameterized property

To declare a property as the default property, simply add the Default keyword prior to the property declaration, as shown in Listing 9.6

Listing 9.6 Declaring a Default Property

'Adding the Default keyword before the property declaration

'makes this parameterized property the default property for

'the class

Default Property CustomerOrders(ByVal nIndex As Integer) As COrder

Add Methods to the Interface

Now that you have added all the properties you need, you need to add the various

methods that a developer uses to retrieve, insert, update, and delete data from the

Customers table

1 To make your code more readable, add a #Region/#End Region block with the name Methods so that you can expand and collapse your method declarations as needed

2 The point in wrapping a table in a class is to make access to this table as simple as possible You could have one method to insert new rows to the database and one

to update existing rows, leaving the task of determining the state of the object to the code that instantiates it The actions of the class, however, should be as

transparent to other developers as possible To this end, the interface should only define two methods: save and delete (The retrieve method will be defined in a subsequent section.) Add the save and delete methods to your interface, as in Listing 9.7

Listing 9.7 CustomerInterface9_1.vb: Declaring the Delete and Save Methods

#Region "Methods"

'To update or delete a record, a user would have needed to

'retrieve data before modifying any of the properties

Function Save() As Boolean

Function Delete() As Boolean

#End Region

As I mentioned in the introduction to this chapter, one of the great advantages to wrapping up all access to a database table within a class is that you only have to write complex error-handling code once To keep things simple, we are returning a Boolean value from each of these methods, letting other developers know whether the action failed without requiring them to write complex error-handling code

3 Finally, add a ToString method to output all the data of the class in a string This is extraordinarily useful for debugging Your finished interface should look like Listing 9.8

Listing 9.8 CustomerInterface9_1.vb: The Final ICustomer Interface

Public Interface ICustomer

#Region "Properties"

'Define properties for this interface

'that match the table structure in name and data type

ReadOnly Property CustomerID() As String

Property CompanyName() As String

Property ContactName() As String

Property ContactTitle() As String

Property Address() As String

Property City() As String

Property Region() As String

Property PostalCode() As String

Property Country() As String

Property Phone() As String

Property Fax() As String

#End Region

#Region "Methods"

'To update record, a user would have needed to

'retrieve data before modifying any of the properties

Function Save() As Boolean

Function Delete() As Boolean

Function ToString() As String

#End Region

End Interface

How It Works

You might have noticed that an interface has no actual code An interface is designed for one purpose: to define a set of public methods and interfaces that are related to a specific entity To put this in object-oriented terminology, an interface represents an abstraction of

an entity-in this case, a customer Interfaces can't function by themselves; a class must implement all of its properties and methods, which is what you will do in the next

sections

Comments

Of course, you don't really need to write an interface before writing the class, and in our current example, it might be overkill The power of an interface really comes into play when you have multiple classes that might implement the same interface in different ways

An interface cannot exist as an object: It has no code and no place to store object data A class that implements an interface has an explicit contract with the interface: The class must implement all the properties and methods of the interface If you declare a variable typed to an interface, that variable could represent any object of any class that

implements the interface

You've already seen several examples of interfaces in this book, such as the DataAdapter, DataReader, Connection, Command, and other ADO.NET objects Take, for example, the Command interface At present, two types of Command classes exist: the

OleDbCommand and the SqlCommand Both of these classes implement the

IDbCommand interface The OleDbCommand is responsible for handling data access to any database provider with an OleDB driver, and the SqlCommand connects directly to SQL Server If you wrote a function that processed Command objects-such as a

debugging function that wrote all the properties of a Command to an error log-you might need to handle both kinds Instead of writing two functions, one for each Command type,

Public Sub WriteCommandErrorToLog(ByVal pCommand as SqlCommand)

Public Sub WriteCommandErrorToLog(ByVal pCommand as OleDbCommand)

you could simply declare a function that takes a variable of type IDbCommand, which would accept an instance of either command type:

Public Sub WriteCommandErrorToLog(ByVal pCommand as IDbCommand)

The second solution is far more elegant because, before long, you could end up with Commands for Oracle, Sybase, MySql, SqlAnywhere, DB2, and so on, all of which would implement the IDbCommand interface

It might look like you're missing a few items in ICustomer What method do you call to add new records or retrieve data from the database? If you're working with a new

customer row, how does a developer set the CustomerID? A new type of method called a constructor is executed when a class is instantiated Because constructors can't be

declared in an interface, we will cover this subject in section 9.4