Tài liệu Create a Class That Implements the Interface You Defined docx

9.2 Create a Class That Implements the Interface You Defined Now that you have defined the public interface of your class, you need to create a class that will implement that interface,

9.2 Create a Class That Implements the Interface You Defined

Now that you have defined the public interface of your class, you need to create a class that will implement that interface, along with all of its methods and properties

The place to start is with the properties of your class You'll also need to create some code to test your class, so you'll need to create a form that interacts with the instances of the class

Technique

This section uses a form with text boxes that mirror the properties of the CCustomer class Visual Basic NET allows you to have classes and forms within the same vb file,

so this section will have both in one file to make editing and debugging easier

After setting up the form, you need to implement the class properties If you have worked with Property statements before, this technique will be old hat (although the syntax will

be new) If you have not worked with properties and classes, you will need the following:

• A private variable for each property to store class data

• Code to modify and return that data

Steps

1 Create a Windows Form and name it frmHowTo9_2 Then place text boxes for each of the properties of the class, naming the text boxes with the property name, prefaced by txt For example, the postal code text box should be named

txtPostalCode Next add command buttons called cmdDelete, cmdSave,

cmdRetrieve, and cmdNew Finally, add a RichTextBox control called rtbToString

to contain the output of the ToString method (This enables you to see how the Form maps to the data of the class See Figure 9.1)

Figure 9.1 Arrange the controls on the form you created to look like this

form

2 Add a class declaration block to frmHowTo9_2 and name the class CCustomer After the line Public Class CCustomer, type Implements ICustomer This tells Visual Basic that the CCustomer class includes all of the properties and methods

of the ICustomer interface as shown in Listing 9.9

Listing 9.9 frmHowTo9_2.vb: The Empty Customer Class

Public Class CCustomer

Implements ICustomer

End Class

3 Copy all of the property and method declarations in the ICustomer interface, and paste them within the CCustomer class block

4 Visual Basic NET requires that you specify which property or method in your class implements which public member of your interface After each property or method declaration that you just pasted into the CCustomer class block, you must add Implements ICustomer.[Property/Method Name]

Note

Visual Basic NET allows you to have property and method names

in your class that differ from the public member names listed in your interface This feature exists because one class can

implement many interfaces, and those interfaces can have public members of the same name Although typing Implements

ICustomer.[Property/Method Name] after a property and method with the same name-as in our example-might seem frustrating, it will come in handy when you write more complex code

5 By default, all methods, properties, and module-level variables are Public unless

an access modifier, such as Private or Protected, is used (see Table 9.2) It is, however, good programming practice to use the Public access modifier, so be explicit and add Public before each member

Table 9.2 Visual Basic NET Access Modifiers Object Purpose

Private You can only access the member within the class

Protected You can only access the member from classes that are derived

from (inherit from) the member's class

Friend You can only access the member by objects within the same

project

Protected

Friend

You can only access the member by derived classes within the same project

Public You can access the member by any object

6 When you are finished, your code should look like Listing 9.10

7 Listing 9.10 frmHowTo9_2.vb: The Empty CCustomer Class

8 Public Class Customer

9 Implements ICustomer

10

11 Public ReadOnly Property CustomerID() As String _

12 Implements ICustomerData.CustomerID

13 Public Property CompanyName() As String Implements

ICustomerData.CompanyName

14 Public Property ContactName() As String Implements

ICustomerData.ContactName

15 Public Property ContactTitle() As String _

16 Implements ICustomerData.ContactTitle

17 Public Function Delete() As Boolean Implements ICustomerData.Delete

18 Note

Copying and pasting the code from the interface will result in code with an invalid syntax because of the lack of End Property and End Function/Sub lines Although this doesn't matter for the moment, it does disable Intellisense, which normally appears after typing both Implements and the period after the Interface name If you want to enable Intellisense, press Enter at the end of each

property and method declaration Doing so inserts the appropriate End tag When all the tags are in the class, Intellisense is

reenabled

19 Add private variables directly below the class declaration to store class data, as shown in Listing 9.11 You should have one variable for each property As

mentioned in Chapter 3, "Viewing Data with ADO.NET," prefacing class-level variables with "m" is a Visual Basic programming convention

Listing 9.11 frmHowTo9_2.vb: Class Variable Declarations

Public Class CCustomer

Implements ICustomer

#Region "Class Variables"

Private mstrCustomerID As String

Private mstrCompanyName As String

Private mstrContactName As String

Private mstrContactTitle As String

Private mstrAddress As String

Private mstrCity As String

Private mstrRegion As String

Private mstrCountry As String

Private mstrPostalCode As String

Private mstrPhone As String

Private mstrFax As String

#End Region

20 Write code to set and return data for the properties Place your cursor at the end of

a property declaration line, and press Enter Visual Studio NET automatically inserts code blocks for setting and getting your property If you have worked with properties before, you might notice that Microsoft has changed the syntax Listing 9.12 shows Visual Basic 6 property get and let declarations, each with its own separate block

Listing 9.12 Property Declarations in Visual Basic 6

Public Property Get PropertyName() As String

PropertyName = mstrClassVariable

End Property

Public Property Let PropertyName(ByVal strValue As String)

mstrClassVariable = strValue

End Property

In Visual Basic NET, the property declaration has been combined in the format shown in Listing 9.13

Listing 9.13 Property Declarations in Visual Basic NET

Public Property PropertyName() As String

Get

Return mstrClassVariable

End Get

Set(ByVal Value As String)

mstrClassVariable = Value

End Set

End Property

For each of your properties, add Return and the variable you declared in step 1 in the Get block In the Set block, type the name of a variable from step 1 and = Value All of your property declarations should now look like Listing 9.14

Listing 9.14 frmHowTo9_2.vb: Some Property Declarations for the

CCustomer Class

Public ReadOnly Property CustomerID() As Customers.CCustomerID

Implements ICustomer.CustomerID

Get

Return mCustomerID

End Get

End Property

Public Property CompanyName() As String Implements

ICustomer.CompanyName

Get

Return mstrCompanyName

End Get

Set(ByVal Value As String)

mstrCompanyName = Value

End Set

End Property

21 To test your properties, you need to write a bit more code First, you might want to implement the ToString method that returns all the object's properties as a string Also, you will want a method that clears the form for use with a new object

Listing 9.15 shows one way to implement ToString, with each property printing on

a separate line, as well as a ClearAllTextBoxes method

Listing 9.15 frmHowTo9_2.vb: The ToString Method Outputting Property Information

Public Overrides Function toString() As String Implements ICustomer.ToString Dim strReturn As String

strReturn = Me.CustomerID & vbCrLf

strReturn = strReturn & "Company: " & Me.CompanyName & vbCrLf strReturn = strReturn & "Contact: " & Me.ContactName & vbCrLf

strReturn = strReturn & "Title: " & Me.ContactTitle & vbCrLf

strReturn = strReturn & "Address: " & Me.Address & vbCrLf

strReturn = strReturn & "City: " & Me.City & vbCrLf

strReturn = strReturn & "Postal Code: " & Me.PostalCode & vbCrLf

strReturn = strReturn & "Country: " & Me.Country & vbCrLf

strReturn = strReturn & "Phone: " & Me.Phone & vbCrLf

strReturn = strReturn & "Fax: " & Me.Fax & vbCrLf

strReturn = strReturn & "Region " & Me.Region & vbCrLf

Return strReturn

End Function

Protected Sub ClearAllTextBoxes()

Me.txtAddress.Text = ""

Me.txtCity.Text = ""

Me.txtCompanyName.Text = ""

Me.txtContactName.Text = ""

Me.txtContactTitle.Text = ""

Me.txtCountry.Text = ""

Me.txtCustomerID.Text = ""

Me.txtFax.Text = ""

Me.txtPhone.Text = ""

Me.txtRegion.Text = ""

Me.txtPostalCode.Text = ""

Me.rtbToString.Text = ""

End Sub

The Overrides keyword is necessary because your class already has a ToString method You didn't write that method, but it's there This is because every class inherits from the Object class, which defines a ToString method Don't worry about this: You'll learn about inheritance and overriding methods later in this chapter

Now you need to add three pieces of code to the form that will instantiate the CCustomer class, and you need to allow its properties to be modified through the text boxes

22 First, add a form-level variable called mCustomer of type CCustomer to the form Use the new keyword in the declaration to create a new instance of CCustomer

23 Next, add code to each TextBox control's TextChanged event that updates

property values in mCustomer Note that Listing 9.16 does not actually refer to mCustomer Instead, Listing 9.16 calls an as-yet-undefined method called

TextBoxChange that handles changes to mCustomer This method is defined and explained in step 11

Listing 9.16 frmHowTo9_2.vb: The TextChanged Event of the txtPostalCode Text Box That Calls the TextBoxChange Method

Protected Sub txtPostalCode_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtPostalCode.TextChanged

TextBoxChange("PostalCode", txtPostalCode.Text)

End Sub

24 Finally, add the TextBoxChange method that takes the value passed from each text box's TextChanged event and writes it to the appropriate property of the

mCustomer object Listing 9.17 shows a sample of this code, although it does not list a case for each text box Centralizing all access to the mCustomer object in one method allows you to have one exception-handling area

Listing 9.17 frmHowTo9_2.vb: An Excerpt of the TextBoxChange Method That Writes Values in Text Boxes to Object Properties

Protected Sub TextBoxChange(ByVal pstrProperty As String, _

ByVal pstrValue As String)

Try

Select Case pstrProperty

Case "Address"

mCustomer.Address = pstrValue

Case "CompanyName"

mCustomer.CompanyName = pstrValue

Case "PostalCode"

mCustomer.PostalCode = pstrValue

End Select

' Write the class' properties to the RichTextBox

' to aid debugging

Me.rtbToString.Text = mCustomer.ToString()

Catch ex As Exception

MsgBox(ex.Message)

End Try

End Sub

How It Works

An instance of the CCustomer class is created when frmHowTo9_2 is created When the user types in a text box, the TextChanged event fires, calling the TextBoxChange

method The TextBoxChange method sets the corresponding property of the class handles any exceptions, and then writes the output of the CCustomer.ToString method to the RichTextBox

Note

In the samples for this chapter, I want to use the same form for each section's code samples without having to re-create the form each time To this end, I have used frmHowTo9xBase as a base form class, and all the frmHowTo9_x forms inherit from this class Some common methods, including the TextChanged events, are defined in frmHowTo9xBase rather than in each child form

Comments

Microsoft has made several changes to property declarations, and it's worth enumerating them here

First, the Get and Set code blocks are wrapped up in one Property declaration, making neater and easier-to-read code

Second, the Return keyword is now used instead of the Property name in the Get block Remember that no code is executed after the Return keyword

Third, there is no longer a distinction between Set and Let In Visual Basic 6, Set was used for Object properties, and Let was used for base datatypes, such as Strings and Integers Microsoft has eliminated this confusing distinction, and now Set is used in all cases

Just like Visual Basic 6, however, read-only properties will have only a Get block, whereas write-only properties will have only a Set block