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

§e.Effect = DragDropEffects.Move

§End Sub

§

§// Visual C#

§private void panel1_DragEnter(object sender,

§System.Windows.Forms.DragEventArgs e) {

§e.Effect = DragDropEffects.Move;

§}

§

§private void panel2_DragEnter(object sender,

§System.Windows.Forms.DragEventArgs e) {

§e.Effect = DragDropEffects.Move;

}

Enable dropping using the DragDrop event

§Add code to the DragDrop events of the Panel controls to move the dragged control to the new panel. Before moving the button, the code checks that the button is being moved to a different panel.

§‘ Visual Basic

§Private Sub panel1_DragDrop(ByVal sender As Object, ByVal e _

§As System.Windows.Forms.DragEventArgs) Handles panel1.Dra

gDrop

§Dim theCard As Card = CType(m_pickedUp.Tag, Card)

§If (Not m_hand1.Contains(theCard)) Then

§m_hand1.Add(theCard)

§m_hand2.Remove(theCard)

§End If

§ShowHand(panel1, m_hand1)

§ShowHand(panel2, m_hand2)

§m_pickedUp = Nothing

§End Sub

§

§Private Sub panel2_DragDrop(ByVal sender As Object, ByVal e _

§As System.Windows.Forms.DragEventArgs) Handles panel2.Dra gDrop

§Dim theCard As Card = CType(m_pickedUp.Tag, Card)

§If (Not m_hand2.Contains(theCard)) Then

§m_hand2.Add(theCard)

§m_hand1.Remove(theCard)

§End If

§ShowHand(panel1, m_hand1)

§ShowHand(panel2, m_hand2)

§m_pickedUp = Nothing

§End Sub

§

§// Visual C#

§private void panel1_DragDrop(object sender,

§System.Windows.Forms.DragEventArgs e) {

§Card theCard = (Card)m_pickedUp.Tag;

§if (!m_hand1.Contains(theCard)) {

§m_hand1.Add(theCard);

§m_hand2.Remove(theCard);

§}

§ShowHand(panel2, m_hand2);

§ShowHand(panel1, m_hand1);

§m_pickedUp = null;

§}

§

§private void panel2_DragDrop(object sender,

§System.Windows.Forms.DragEventArgs e) {

§Card theCard = (Card) m_pickedUp. Tag;

§if (!m_hand2.Contains(theCard)) {

§m_hand2.Add(theCard);

§m_hand1.Remove(theCard);

§}

§ShowHand(panel2, m_hand2);

§ShowHand(panel1, m_hand1);

§m_pickedUp = null;

}

All that remains now is to program the Remove Pairs button to remove the pairs from the hands and from the form.

Match the cards

§Add code to the Click event of the Remove Pairs button. This code simply calls the RemovePairs method for each Hand object and redisplays the Hand objects.

§‘ Visual Basic

§Private Sub removePairs_Click(ByVal sender As System.Object, ByVal e _

§As System.EventArgs) Handles removePairs.Click

§m_hand1.RemovePairs()

§m_hand2.RemovePairs()

§ShowHand(panel2, m_hand2)

§ShowHand(panel1, m_hand1)

§End Sub

§

§// Visual C#

§private void removePairs_Click(object sender, System.EventArgs

e){

§m_hand1.RemovePairs();

§m_hand2.RemovePairs();

§ShowHand(panel2, m_hand2);

§ShowHand(panel1, m_hand1);

}

Run the application

1.In the Solution Explorer window, right-click the project name and click Properties in the shortcut menu.

2.In the properties tree, expand the Common Properties folder if necessary and select General.

3.In the Startup Object list, click Form1, and click OK.

4.Press F5 to run the application. The following screen shot shows the results after the pairs have been matched. You can clear all the cards by dragging them all to one panel and clicking the Remove Pairs button.

because these terms more closely match the keywords used in Visual Basic and C# to implement inheritance.

Polymorphism describes the behavior of classes that derive from a common base class. A savings account isn’t the only type of bank account. There are also checking accounts, money market accounts, and mutual fund accounts. So Checking, Savings, MoneyMarket, and MutualFund all derive from BankAccount. Polymorphic behavior allows a developer to use a BankAccount variable to refer to any of the derived classes of BankAccount. Polymorphism allows each derived class to handle identical method names with different behavior. For example, both Savings and Checking provide a Withdraw method through inheritance, but the Checking class’s Withdraw method deducts a small service charge along with each withdrawal.

This chapter concentrates on the mechanics of inheritance. Even with rudimentary techniques, you can develop sophisticated results by inheriting from .NET Framework classes. Later chapters develop other aspects of inheritance, such as base class design, polymorphism, and component development.

BankAccount: A Simple Example

The simple bank account provides the basis for this exercise in inheritance. You will implement a BankAccount class as a base class with the following members:

Create the base class, BankAccount

1.Create a new project and name it TheBank.

2.On the Project menu, click Add Class. The Add New Item dialog box appears.

3.Name the file BankAccount.vb or BankAccount.cs, depending on the language you’re using.

Add the properties and constructor

The account ID is based on the owner’s name. In the limited world of this exercise, each person can have only one account, and all names are unique. So the ID is the same as the owner’s name.

1.Add this code for the read-only ID property:

2.‘ Visual Basic

3.Private m_owner As String

4.Public ReadOnly Property ID() As String

5.Get

6.Return m_owner

7.End Get

8.End Property

9.

10.// Visual C#

11.private string m_owner;

12.public string ID {

13.get {

14.return m_owner;

15.}

16.}

The next property, Balance, will also be read-only. In the real world, you can’t just tell the bank you have a certain amount of money. To change your balance, you have to make a deposit or a withdrawal. So it will be with this example: the Balance is read-only and can be changed only by means of the

Deposit and Withdraw methods.

The balance is stored in a decimal field. The decimal data type (System.Decimal) is used to store numbers with a particular precision. Thus 2.37 is stored as 2.37, not something extremely close to 2.37, as might happen in using a System.Double variable. The advantage of using the System.Decimal data type is that rounding doesn’t occur, so the System.Decimal type is appropriate for representing currency.

17.Add this code for the read-only Balance property:

18.‘ Visual Basic

19.Private m_balance As Decimal

20.Public ReadOnly Property Balance() As Decimal

21.Get

22.Return m_balance

23.End Get

24.End Property

25.

26.// Visual C#

27.private decimal m_balance;

28.public decimal Balance {

29.get {

30.return m_balance;

31.}

}

32.Add the following code to the BankAccount class to create a constructor. All accounts need to have an owner, so the only constructor provided has one string parameter for the owner’s name. In Visual C#, replace the default constructor with the one below.

33.‘ Visual Basic

34.Public Sub New(ByVal owner As String)

35.m_owner = owner

36.m_balance = 0D

37.End Sub

38.

39.// Visual C#

40.public BankAccount(string owner) {

41.m_owner = owner;

42.m_balance = 0M;

43.}

Add the methods

1.Add the following code to the BankAccount class for the Deposit method. This method adds the indicated amount to the balance and returns the new balance.

2.‘ Visual Basic

3.Public Function Deposit(ByVal amount As Decimal) As Decimal

4.m_balance += amount

5.Return m_balance

6.End Function

7.

8.// Visual C#

9.public decimal Deposit(decimal amount) {

10.m_balance += amount;

11.return m_balance;

}

12.Add the following code to the BankAccount class for the Withdraw method. This method subtracts the indicated amount from the balance and returns the new balance.

13.‘ Visual Basic

14.Public Function Withdraw(ByVal amount As Decimal) As Decimal

15.m_balance -= amount

16.Return m_balance

17.End Function

18.

19.// Visual C#

20.public decimal Withdraw(decimal amount) {

21.// since an assignment returns the assigned value,

22.// only need one line

23.return (m_balance -= amount);

}

Test the BankAccount interface

The base class, BankAccount, is now complete. Before moving on to the first derived class, SavingsAccount, take a look at the public interface of the BankAccount class.

1.Open Form1 in the designer.

2.Double-click on the form to create the Form1_Load method in the code editor.

3.Add the following code to the method, and note the members displayed in IntelliSense, shown in the subsequent graphic.

4.‘ Visual Basic

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

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

7.Dim account As BankAccount = New BankAccount("Robin")

8.account.Deposit(25D)

9.MessageBox.Show(String.Format("{0:C}", account.Balance))

10.End Sub

11.

12.// Visual C#

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

14.BankAccount account = new BankAccount("Robin");

15.account.Deposit(25M);

16.MessageBox.Show(String.Format("{0:C}", account.Balance));

}

String.Format

The String.Format method gives you a way to create strings based on variable values without using long string concatenation statements. The String.Format method has several overloads. In each case, the first parameter is a format statement, which is a string that’s interspersed with formatting specifications. For example, {0:C} specifies that a value should be converted to a string using a currency format. The rest of the parameters of the method are used to replace the formatting specifications. Thus the following statements are equivalent and produce You have $1.23 in the bank.

String.Format("You have {0:C} in the bank.", balance)

"You have " + balance.ToString("C") + " in the bank."

The class members contain the public properties and methods of the BankAccount class: Balance, ID, Deposit, and Withdraw. The members list also contains those members inherited from the Object class: GetType, Equals (C# only), GetHashCode (C# only), and ToString (C# only). In the next section, you’ll create a derived class from BankAccount, and it will contain the public interface of Object and BankAccount.

12.Press F5 to run the application. The result is shown in the following screen. You have created the completely functional base class, BankAccount.

Creating the SavingsAccount Derived Class

The first derived class you create will be the SavingsAccount class. In this example SavingsAccount is identical to BankAccount except in the following respects:

§A savings account offers interest. The SavingsAccount class will have an additional property, Interest.

§A savings account can accrue interest over time. The SavingsAccount class will have an additional method, AddInterest.

§One person can have both a savings account and a checking account, an expansion of your banking world. To distinguish between the two accounts, the ID property will indicate the type of account.

Create the SavingsAccount class

1.On the Project menu, click Add Class. The Add New Item dialog box appears.

2.Name the file SavingsAccount.vb or SavingsAccount.cs, depending on the language you’re using.

3.Modify the class declaration to indicate that BankAccount is the base class as shown here:

4.‘ Visual Basic

5.Public Class SavingsAccount

6.Inherits BankAccount

7.End Class