C# ПІДРУЧНИКИ / c# / MS Press - Oop With MS Vb Net And C# Step By Step (2002)

9.private CheckingAccount checking = new CheckingAccount("Your Name");

10.private SavingsAccount savings = new SavingsAccount("Your Name");

11.Add this code to the Form1_Load method to initialize the accounts:

‘Visual Basic

Private Sub Form1_Load(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles MyBase.Load

Me.account.Items.Add(checking)

Me.account.Items.Add(savings)

Me.account.SelectedIndex = 0

Me.action.SelectedIndex = 0

Me.amount.Text = "100"

End Sub

// Visual C#

private void Form1_Load(object sender, System.EventArgs e) {

this.account.Items.Add(checking);

this.account.Items.Add(savings);

this.account.SelectedIndex = 0;

this.action.SelectedIndex = 0;

this.amount.Text = "100";

}

In the designer, you used the String Collection Editor dialog box to add strings to the ComboBox control. You can add any item to the ComboBox using the Items.Add method. At run time, the name of the class will be displayed in the ComboBox control, as shown in the following screen. If you have defined a ToString method for the class, the ToString method would be called and displayed in the ComboBox. For example, if you had defined a ToString method in the SavingsAccount object that returned the ID property, the ComboBox would display Your Name-S instead of TheBank.SavingsAccount.

Submit the transaction

1.In the designer, double-click the Submit button to create the Click event method in the code editor.

2.Add this code to the code editor to submit the transaction and report the new account balance:

3.‘ Visual Basic

4.Private Sub submit_Click(ByVal sender As System.Object, _

5.ByVal e As System.EventArgs) Handles submit.Click

6.Dim selectedAccount As BankAccount

7.Dim item As Object = Me.account.SelectedItem

8.selectedAccount = CType(item, BankAccount)

9.Select Case action.Text

10.Case "Deposit"

11.selectedAccount.Deposit(Decimal.Parse(amount.Text))

12.Case "Withdraw"

13.selectedAccount.Withdraw(Decimal.Parse(amount.Text))

14.End Select

15.MessageBox.Show(String.Format("{0}: {1:C}", _

16.selectedAccount.ID, selectedAccount.Balance))

17.End Sub

18.

19.// Visual C#

20.private void submit_Click(object sender, System.EventArgs e) {

21.BankAccount selectedAccount;

22.object item = this.account.SelectedItem;

23.selectedAccount = (BankAccount)item;

24.switch (action.Text) {

25.case "Deposit" :

26.selectedAccount.Deposit(decimal.Parse(amount.Text));

27.break;

28.case "Withdraw" :

29.selectedAccount.Withdraw(decimal.Parse(amount.Text));

30.break;

31.}

32.MessageBox.Show(String.Format("{0}: {1:C}",

33.selectedAccount.ID, selectedAccount.Balance));

}

As with the SortedList and ArrayList classes, you can add any type of object to the ComboBox, but the ComboBox treats them all as being of type System.Object. (Remember that all classes implicitly derive from System.Object.) That means that what is returned from

ComboBox.SelectedItem can be directly assigned only to a System.Object reference.

Because your code has control over the Account combo box, you have restricted the items of the combo box to be of type CheckingAccount or SavingsAccount. Therefore you can cast the System.Object item in the submit_Click method to a BankAccount object. Once you have a BankAccount object, you can call any of its properties and methods.

When you cast an object from one type to another, no changes occur in the instance itself. The only thing that changes is the view of the object. The preceding code doesn’t convert account.SelectedItem from a System.Object into a SavingsAccount object. The cast only directs the compiler to treat the instance as a SavingsAccount instead of a System.Object. There is, after all, only one instance of SavingsAccount. It’s just that the account combo box is storing a System.Object reference to it, and the Form is storing a SavingsAccount reference to it.

When you have a base class reference to an instance, you can access only the properties and methods defined on the base class. In this example, you

couldn’t call the AddInterest method using the selectedAccount variable. You’ll see how to do that in the next section.

34.Press F5 to run the application. Make some deposits and withdrawals with the accounts, and you’ll see the common and specialized behaviors of the CheckingAccount and SavingsAccount classes.

Find the type of the object

1.Open Form1 in the designer.

2.Add another button to Form1. Set its Name property to addInterest, its Text property to Add interest, and its Visible property to False.

3.Double-click the button to create the Click event method in the code editor.

4.In the form designer, double-click the account ComboBox control to create the account_SelectedIndexChanged method in the code editor.

5.Add the following code to the account_SelectedIndexChanged method to display the Add Interest button if the selected account is the savings account.

6.‘ Visual Basic

7.Private Sub account_SelectedIndexChanged(ByVal sender As _

8.System.Object, ByVal e As System.EventArgs) _

9.Handles account.SelectedIndexChanged

10.If TypeOf (account.SelectedItem) Is SavingsAccount Then

11.addInterest.Visible = True

12.Else

13.addInterest.Visible = False

14.End If

15.End Sub

16.

17.// Visual C#

18.private void account_SelectedIndexChanged(object sender,

19.System.EventArgs e) {

20.if (account.SelectedItem is SavingsAccount) {

21.addInterest.Visible = true;

22.}

23.else {

24.addInterest.Visible = false;

25.}

26.}

27.Add the following code to the Click event method of the Add Interest button to call the AddInterest method of SavingsAccount.

28.‘ Visual Basic

29.Private Sub addInterest_Click(ByVal sender As System.Object, _

30.ByVal e As System.EventArgs) Handles addInterest.Click

31.If TypeOf (account.SelectedItem) Is SavingsAccount Then

32.Dim theSavings As SavingsAccount = _

33.CType(account.SelectedItem, SavingsAccount)

34.theSavings.AddInterest()

35.MessageBox.Show(String.Format("{0}: {1:C}", _

36.theSavings.ID, theSavings.Balance))

37.End If

38.End Sub

39.

40.// Visual C#

41.private void addInterest_Click(object sender, System.EventArgs

e){

42.SavingsAccount theSavings = account.SelectedItem as SavingsAccount;

43.if (theSavings != null) {

44.theSavings.AddInterest();

45.MessageBox.Show(String.Format("{0}: {1:C}", theSavings.ID,

46.theSavings.Balance));

47.}

}

This method checks the type of the selected item before casting it to a SavingsAccount object. C# has a keyword, as, that tests and casts the reference in one step. The as keyword is an operator that returns null if the object cannot be cast as the selected type.

48. Press F5 to run the application. The results are shown here:

Inherit from a Control: The RoundButton Class

You can use inheritance to create new classes from .NET Framework classes, as well as from classes you have written yourself. In this next section, you will create a new class by deriving from a class that you didn’t develop.

You create a round button control by using System.Windows.Forms.Button as the base class. This task requires only that you override the OnPaint method of the Button class.

Creating the RoundButton Class

To create a derived class from a .NET class, you declare the class and indicate the base class, just as you did with the bank account classes.

Create the project

1.Create a new Windows Control Library project and name it ARoundButton.

2.Open Form1 in the code editor.

Add the RoundButton class

§If you’re using Visual Basic, add the class declaration at the end of the source file. If you’re using Visual C#, add the class declaration at the end of the file, but before the closing brace of the RoundButton namespace.

§‘ Visual Basic

§Public Class RoundButton

§Inherits Button

§

§End Class

§// Visual C#

§public class RoundButton : Button {

}

Overriding the OnPaint Method

By overriding the OnPaint method, you direct the runtime to draw a round button, instead of the usual rectangular button.

Create the OnPaint method

1.If you’re using Visual Basic, click RoundButton (Overrides) in the Class List, and then click OnPaint in the Method Name list. The following empty method definition is added to the class. You can also simply type this method into the editor.

2.‘ Visual Basic

3.Protected Overrides Sub OnPaint(ByVal pevent As _

4.System.Windows.Forms.PaintEventArgs)

5.

6.End Sub

If you’re using Visual C#, in the Class View expand the Bases And Interfaces nodes of the RoundButton class button until you find the ButtonBase class. Expand the ButtonBase node, right-click the OnPaint method, point to Add and click Override on the shortcut menu. The following empty method definition is added to the class. You can also simply type this method into the editor.

// Visual C#

protected override void OnPaint( System.Windows.Forms.PaintEventArgs pevent) {

}

The OnPaint method is called each time the control is drawn on the form. The base class draws the familiar rectangle. By overriding the OnPaint method, you can determine the appearance of the button.

7.Add the following code to draw the round button. Not only will the button be round when it’s painted on the form, but the clickable area of the button will be round as well.

8.‘ Visual Basic

9.Protected Overrides Sub OnPaint(ByVal pevent As _

10.System.Windows.Forms.PaintEventArgs)

11.Me.Size = New Size(50, 50)

12.Dim aCircle As System.Drawing.Drawing2D.GraphicsPath = _

13.New System.Drawing.Drawing2D.GraphicsPath()

14.aCircle.AddEllipse(New System.Drawing.RectangleF(0, 0, 50, 50))

15.Me.Region = New Region(aCircle)

16.End Sub

17.

18.// Visual C#

19.protected override void OnPaint(

20.System.Windows.Forms.PaintEventArgs pevent) {

21.this.Size = new Size(50,50);

22.System.Drawing.Drawing2D.GraphicsPath aCircle =

23.new System.Drawing.Drawing2D.GraphicsPath();

24.aCircle.AddEllipse(new System.Drawing.RectangleF(0, 0, 50, 50));

25.this.Region = new Region(aCircle);

}

To make a control assume a particular shape, in this case round, you must define its Region property so that it achieves that shape. You can create a shape using the GraphicsPath object. The GraphicsPath object allows you to create a shape by drawing. In this example, you create a drawing by adding a circle to GraphicsPath. The size of the button is constrained to 50 by 50 pixels so that the full circle is visible.

Using the Class

The RoundButton class has been defined in the source file and doesn’t appear in the Toolbox for dragging on the form. To add a RoundButton instance to Form1, you can use the same methods you used to create buttons in Chapter 4.

Add a RoundButton control to the form

1.Add the following code to the Form1 class to respond to the Click event of the RoundButton object.

2.‘ Visual Basic

3.Private Sub roundButton_Click(ByVal sender As System.Object,

_

4.ByVal e As System.EventArgs)

5.MessageBox.Show("Hello")

6.End Sub

7.

8.// Visual C#

9.private void roundButton_Click(object sender, System.EventArgs

e){

10.MessageBox.Show("Hello");

}

11.In the form designer, double-click Form1 to add the Form1_Load method to the code editor.

12.Add the following code to create a RoundButton object and add it to the form.

13.‘ Visual Basic

14.Private Sub Form1_Load(ByVal sender As System.Object, _

15.ByVal e As System.EventArgs) Handles MyBase.Load

16.Dim rb As New RoundButton()

17.Me.Controls.Add(rb)

18.AddHandler rb.Click, AddressOf roundButton_Click

19.End Sub

20.

21.// Visual C#

22.private void Form1_Load(object sender, System.EventArgs e) {

23.RoundButton rb = new RoundButton();

24.rb.Click += new

System.EventHandler(this.roundButton_Click);

25.this.Controls.Add(rb);

26.}

Run the application

§Press F5 to run the application. Here are the results:

Design Considerations

Inheritance is a powerful tool in object-oriented programming and is used extensively in the .NET Framework. The following points will help you write classes that work better together and are less error-prone.

The is-a relationship Remember that inheritance models the is-a relationship between objects. The derived classes should represent objects that truly are special cases of the base object. If you find yourself trying to eliminate properties or methods of the base class, then you don’t have an is-a relationship. For example, if you’re creating a class that derives from the Button class but you’re trying to eliminate the Click event, the new class isn’t really a Button. It might be something like a button, but it isn’t a button.

Polymorphism If you aren’t going to use the classes polymorphically, consider whether you need to use inheritance at all. It might be that your class only needs to contain an instance of the class rather than serve as a base class.

Type-checking In general, type-checking (using the type of operator) is a clue that you are using inheritance incorrectly. Analyze your code to determine whether the objects truly represent an is-a relationship. Determine whether you’re defining the proper properties and methods so that type-checking is unnecessary.

Select or switch statements If your code is full of select or switch statements, consider whether using inheritance would simplify the code. Consider the following code snippet where you have defined a Shape class with a type property that will be set to Rectangle or Circle. To draw the Shape object, you might write some code like this:

If aShape.Type = "Rectangle" then

DrawARectangle()

Else

DrawACircle()

End If