Create a Class That Implements the Interface You Defined

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9,2 Tạo một lớp mà thực hiện các giao diện Bạn Defined Bây giờ bạn đã định nghĩa giao diện nào của lớp học của bạn, bạn cần phải tạo ra một lớp mà sẽ thực hiện mà giao diện, cùng với tất cả các phương pháp và tính chất của nó.

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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 You can only access the member by derived classes within the Friend 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.