Visual Basic .NET面向对象进阶教程：实例详解与构造器

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"《Visual Basic .NET面向对象编程进阶教程》是一本专门针对已有Visual Basic基础知识的学生设计的全英文教材。该书第三版由Jinan Darwiche编写，于2010年首次出版，后续在2015年更新。本书深入讲解了面向对象编程（Object-Oriented Programming, OOP）的基本理论和实践，以Visual Basic .NET为平台，帮助读者理解并掌握面向对象编程的核心概念。第一章“Object-Oriented Programming Theory”（面向对象编程理论）首先介绍了什么是对象，强调了OOP的优势，如封装（Encapsulation）和继承（Inheritance）。章节中详细解释了属性（Properties）和事件（Events），以及它们如何在程序中发挥作用。此外，作者还鼓励读者思考面向对象编程对自己工作的重要性，并通过实例说明如何在实际项目中运用这些概念。第二章“VB Classes and Objects”（VB类和对象）通过一个员工的例子来介绍类的概念，展示了如何创建和使用第一个对象。章节探讨了公有（Public）和私有（Private）属性的区别，以及如何定义和调用方法（Methods）。同时，还涉及只读（ReadOnly）和只写（WriteOnly）属性的使用，以及默认属性（Default Properties）的访问方式。书中提供了丰富的练习题帮助读者巩固所学。第三章“Constructors”（构造函数）着重讲解构造方法（Constructor method）的作用，包括`ReadOnly`和`Sub New`的使用。这部分内容展示了如何初始化对象并确保其正确构造。章节最后的部分是将理论知识与实践结合起来，通过实例演示如何将构造函数与其他部分紧密连接。这本教材不仅适合希望深入学习VB .NET面向对象编程的开发者，也适合那些希望通过实例理解面向对象概念的初学者。通过阅读这本书，读者可以系统地提升在VB .NET环境中设计、实现和管理复杂程序的能力，从而在软件开发项目中更高效地应用面向对象的设计原则。"

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Chapter!2! ! VB!Classes!and!Objects!

10!

Now Manager1 is available for any event handler on the form due to the declaration

statement being at the form level. But why did we need to move the New keyword,

and what’s the purpose of the assignment statement we typed at line 5 as shown

above?

By moving New to under the Button’s click handler, we are now able to get a new

object in RAM each time the button is clicked. If the New keyword was kept in the

declaration, then only one object is being created during the lifetime of the form, and

that creation happens during the form loading once the start button is clicked.

You may keep the New keyword in the form-level declaration and in the

instantiation, but then the code is not consistent in when a new Employee is

generated. This may lead to logical errors later, and should be avoided.

You can now go back to the Employee class code and add the property code for the

LastName property. Remember to add the needed controls on the form - the client

side- to set and manipulate the property. Below is the code for the DOB property.

Private Dim mDOB As Date

Property DOB() As Date

Set (ByVal Value As Date)

If DOB < Now() Then

mDOB = Value

End If

End Set

Get

Return mDOB

End Get

End Property

Chapter!2! ! VB!Classes!and!Objects!

11!

Note that the Set code is executed when the client code (e.g. the code on the form)

uses:

ObjectName.PropertyName = SomeValue

In that case, SomeValue is passed into the Value variable shown above.

Appropriately, we chose Date as the data type for the property in this case.

Now move to the form, and do the following:

1. Add a Textbox control and keep its generic name.

2. Add code under the button’s click event handler, as shown in the following:

Manager1 = New Employee

Manager1.FirstName = Textbox1.Text

Manager1.LastName = Textbox2.Text

Manager1.DOB = CDate(Textbox3.Text)

‘to check that the above code has executed properly add this line:

Messagebox.show(“New Employee created is “ & Manager1.FirstName & _

“ “ & Manager1.LastName & “ with Date of Birth “ & Manager1.DOB.ToString)

The importance of the code above is that it first uses the FirstName, LastName and

DOB properties’ Set code in the Employee class, when executing lines 2, 3 and 4

from above.

Later the Messagebox statement calls the Get codes of the properties in the

Employee class. To see this for yourself, add messagebox statements as shown

below, but remove them later as those are added only for you to understand how the

Property code works.

Private Dim mDOB As Date

‘in the above code, the variable is declared with the Private keyword

‘we will learn what that means and why later.

Property DOB() As Date

Set (ByVal Value As Date)

If DOB < Now() Then

Messagebox.Show(“You are attempting to set DOB”)

mDOB = Value

Messagebox.Show(“DOB has been set”)

End If

End Set

Get

Messagebox.Show(“You are requesting the value of DOB”)

Return mDOB ‘we do not place code after this line

‘Return above causes any code placed here not to execute

End Get

End Property

Moving on, add the code to define the Salary property as shown below. This code is

part of the Employee class.

Chapter!2! ! VB!Classes!and!Objects!

12!

Private mSalary As Single ‘note the type chosen

Public Property Salary As Single

Get

Return mSalary

End Get

Set (byVal Value As Single)

If Value > 0 Then

mSalary = Value

Else

Messagebox.show(“Salary must be more than 0”)

End If

End Set

End Property

Note in the above code, the Salary property is defined with a Single data type. This

is because we will need to manipulate the salary amount as a floating point numeric

value. Also note that we added the test Value > 0 to ensure that when a new salary

is to be assigned, then it can’t be 0 or negative. Other tests may be applied based on

the logic.

Now add code to set and get the Salary property for Manager1 on the form side as

shown below.

1. Add a Textbox control and keep its generic name.

2. Add the following code to the existing code of the click event handler of the

‘Create Employee’ button:

Manager1.Salary = CSng(Textbox4.Text)

‘and to test the Get code of the property use

Messagebox.show(“The new salary is “ & CStr(Manager1.Salary))

Test the code above by typing negative numbers in the textbox and you will see that

those illegal values will not be assigned as the new salary.

Important Note: We are violating one of the main design characteristics we

mentioned before in section 2.1, and that is the class code should not make any

assumptions about the client code. We have already assumed that the client code is

a Windows application, and that is not recommended. Do you recognize which

statement in the class code makes that assumption?

It’s the Messagebox statement, since it is only available for Windows applications. At

this point, we will accept this violation since we are using these messages to help us

understand how the code works. Later we will learn alternatives to issuing those

error messages without having the class code use Messagebox statements.

2.3 Going Public or Staying Private

The variables declared with the prefix m in the class are preceded with the Private

keyword. This is done for a good reason. To see why it is important to use the

Private keyword, let us try the Public keyword instead. Use the following code:

Public mSalary As Single

Chapter!2! ! VB!Classes!and!Objects!

13!

Run the project, and try to assign the salary a value by typing a positive number in

the salary textbox, then click the button.

Now, type a negative number in Textbox4 and click the button, and you will get an

error message stating that “Salary must be more than 0” and the salary stays at the

old positive value. All of these results are as expected.

Since the mSalary variable in the Employee class was declared Public, then that

means that variable is accessible from anywhere and any file in the project. In this

case we can change the code under the ‘Create Employee’ button, so that it assigns

mSalary to the value in the textbox, instead of using the Property Salary. So we will

change the code under the click event handler of the button from:

Manager1.Salary = CSng(Textbox4.Text)

‘and to test the Get code of the property use

Messagebox.show(“The new salary is “ & CStr(Manager1.Salary))

Manager1.mSalary = CSng(Textbox4.Text)

‘above code does not call Set code-below code does not call Get code

Messagebox.show(“The new salary is “ & CStr(Manager1.mSalary))

Now run the project and type a negative number in the textbox. You will notice the

new illegal value is accepted and the messagebox now shows the new salary at the

negative number. As you can see, making those member variables public has two

disadvantages:

1. Making a member variable Public allows the form code (client) direct access.

The form code is free to assign any illegal value and therefore erasing

previously legally stored values. Keep in mind that the form programmer may

not be you! The class designer has no control over what the programmer of

the client, unless the class members that are meant to be hidden or

encapsulated- are declared private.

2. The Property Set code is bypassed through the direct access to its member

variable, and therefore the logical checks performed to ensure a legal value

assignment are now ignored.

Hence, to ensure that the Employee class only stores legal information and maintains

its integrity, keep Private all the member variables that the property definitions use

to store their data, and use Public access only for the property definitions. This

enforces another OOP principle, which is the encapsulation principle; keeping the

data itself hidden from the end user, and only allowing access through the property

code.

Make sure you go back to the Employee class code and change the mSalary to

Private, and change the code under the form’s button to use the property name

instead. Note that once you change mSalary to Private but still try to change it in the

form code, you will get a compile error.

Incidentally, the member variable mSalary uses the prefix m in its name, which is

short for member. This is only a naming convention.

Chapter!2! ! VB!Classes!and!Objects!

14!

2.4 My First Method

Assuming that on the end-user side, we need to give someone a raise of 7%.

Without any changes to the Employee class, the end user code must calculate what

the new salary should be and then assign it to the Salary property. That means the

end-user programmer must do the work, and it is up to them to decide the rules

governing the raises.

However, to make the Employee class logically correct, we must write the code

inside the Employee class that governs how the raises are given.

In that case we need to write a method (a method is a Function or a Sub procedure)

that applies those raises, and changes the Salary to the new value. You may be

thinking that the end-user programmer can assign that Salary to any positive value

they want without the Employee class control; we will get to that imperfection later.

A method in a class code may be a Function or a Sub procedure. It is used to allow

objects of that class to “do” things. In our example, we need the Promote method to

calculate what the new salary should be based on a raise of 7%. Here is the code:

Sub Promote() ‘a method may be defined to have arguments

Salary = Salary * 1.07 ‘this line calls Get code then Set code

End Sub

The code above assigns a new value to the Salary property. Note that in the

assignment statement above, the right side of the = sign activates the Salary

property’s Get code to return the current value of the salary. That value will be

multiplied by 1.07, and the result will be stored in the Salary property, hence the

left-hand side of the assignment statement above activates the Set code of the

Salary property.

Recall the Set code of the Salary property checks for the legality of the new value

before it is assigned to the mSalary variable.

Now let’s try it on the client code. Add the following code to the form code view:

1. Add a button control, and keep its generic name of button2.

2. Add code for button2’s Click event handler as shown below:

Manager1.Promote()

‘and to check what the new Salary is:

Messagebox.show(“The new Salary is “ & Manager1.Salary)

Test the code and use a calculator to ensure that the new salary is indeed a 7% raise

from the old salary.

At this point, the Promote method does not affect the current rank or days in this

position as we intended the logic to be. To make the Promote method comply with

the logic, let’s complete the class code and define all the other properties of the

class. The code below is the Employee class definition, phase 1.

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