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If at a later date it is decided that an account going into overdraft requires a different type of action, there will be no need to change the coding of the BankAccount class, only the event handler We now have a full implementation for a class that models a simple bank account, providing methods that allow users of the class to deposit and withdraw money and determine how much is currently in the account So that you can see the complete picture in one go, the full code for this is reproduced in Listing 413
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Module Bank Public Class BankAccount Public AccountName As String Private Balance As Decimal Public Event Overdrawn(ByVal Amount As Decimal) Public Sub Deposit(ByVal Amount As Decimal) Balance += Amount End Sub Public Function Withdraw(ByVal Amount As Decimal) mvarBalance -= Amount If Balance < 0 Then RaiseEvent Overdrawn(mvarBalance) End If End Function Public ReadOnly Property CurrentBalance() _ As Decimal Get Return Balance End Get End Property End Class Private WithEvents Account As BankAccount Sub DealWithOverdraft(ByVal Amount As Decimal) _ Handles AccountOverdrawn ConsoleWriteLine("Account is overdrawn by : {0}", _ Amount) End Sub Sub Main() Dim Account As BankAccount = New BankAccount() AccountAccountName = "John Smith" ConsoleWriteLine("Account name: {0}", _ AccountAccountName) ConsoleWriteLine("Account balance: {0}", _ AccountGetBalance) AccountDeposit(100) ConsoleWriteLine("Account balance: {0}", _ AccountGetBalance) End Sub End Module Listing 413: The full BankAccount class and code to test it
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Unlike in earlier versions of Visual Basic, or in languages such as C++ which are hybrid languages that support object-oriented programming, Visual Basic NET and the other NET languages are fully object-oriented As such, their core principle is that everything in a program is an object or part of an object Recall that according to the de nitions given in 1, an object is an assembly of data and operations properties and methods Whenever you create a variable from the Common Type System, you can expect it to provide properties and methods and use these to manipulate the object and derive information from it For example, in Sub Main() of a VB NET console program you can create an Integer variable and use it as shown in Listing 414
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Sub Main() Dim i As Integer = 42 ConsoleWriteLine("Min value = {0}, Max value = {1}", _ iMinValue, iMaxValue) End Sub Listing 414: Using an Integer variable s methods
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Run the program and you will see that the Integer variable i has two values associated with it because of the type of variable it is The Integer type comes along with the two properties, MinValue and MaxValue, along with a number of properties that allow us to manipulate it in ways speci c to all integers We can now see that these properties are a result of some code inside the object that has been given Public scope so that they form part of the object s interface These speci c properties tell us the minimum and maximum values we can expect any integer to take on The properties (and methods) come along as part of the type In fact, you will get a strong hint of this relationship between variables and their properties and methods if you run Visual Studio, create a Console application and enter the lines shown into Sub Main() As soon as you enter the (dot) between the name of the variable (i) and either of the properties, VS NET will pop up a list of object properties and methods as a way of assisting you to write the correct code (see Figure 43) All of the types in the Common Type System come with a set of properties and methods, each appropriate to the type it belongs to Integers have minimum and maximum values; Strings have a length (number of characters) and methods that make it easy to convert them to upper or lower case, extract sub-strings, etc; Dates have properties to indicate their Day, Month and Year, etc and to add a time interval to them to form new dates Although everything in VB NET is an object, you will notice that the objects you work with in programs fall into two broad categories With some variables, as soon as you de ne them, they will automatically take on a value, which will be zero, or the equivalent for a non-numeric type (False for a Boolean, and 01/01/0001 for a
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