VB2008从入门到精通(PDF格式英文版)-第9部分
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The Solution Explorer changes to show the additional project and now also shows the
solution。 The work area displays the source code。
Notice the simplicity of the console application。 It contains a single； plain…vanilla source
code file； called Module1。vb。 Console applications typically do not have any specialized group
ings and do not have any events。
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C H AP TE R 1 ■ R E AD Y ； ST E AD Y ； G O！ 13
Making the Console Application Say Hello
To make the console application do something； you need to add some source code to the
Main（） method； as follows：
Module Module1
Sub Main（）
Console。WriteLine（〃hello； world〃）
Console。ReadKey（）
End Sub
End Module
The bolded line writes the text “hello； world” to the console。
If you tried to run the console application by pressing Ctrl＋F5； you would instead cause
the Windows application； WindowsApplication； to run。 Let’s change that next。
Setting the Startup Project
To execute the console application； you need to set the console application as the startup project。
Did you notice how the WindowsApplication project is in bold in the Solution Explorer？ That
means WindowsApplication is the startup project。 Whenever you run or debug an application；
the startup project is executed or debugged。
To switch the startup project to ConsoleApplication； right…click the ConsoleApplication
project and select Set As StartUp Project。 ConsoleApplication will now be in bold； meaning it is
the startup project of the ThreeExamples solution。
Running the Console Project
With ConsoleApplication set as the startup project； you can now press Ctrl＋F5 to run the
console application。 The output is as follows：
hello； world
Executing the console application does not generate a window； as you saw with the Windows
application。 Instead； a mand prompt is started with ConsoleApplication as the application
to execute。 Executing that application generates the text “hello； world。” You will also see that
you can press any key to close the mand prompt window。 Visual Basic Express automati
cally generated the code to show this output and execute this action。
In general； the console application is limited； but it’s an easy way to run specific tasks。
Now let’s move on to the next example。
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14 CH AP T E R 1 ■ R E A DY ； ST E A DY ； G O ！
Creating the Class Library
The third example in this chapter is not a application； rather； it is a shareable piece of
functionality； typically called a class library。 Windows applications and console applications
are programs that you can execute from a mand prompt or Windows Explorer。 A class
library cannot be executed by the user； but needs to be accessed by a Windows application or
console application。 It is a convenient place to put code that can be used by more than one
application。
Adding a Class Library Project to the Solution
We will now create a class library for the Windows application and console application to
share。 Follow these steps to add the new project to the ThreeExamples solution：
1。 Right…click the solution name； ThreeExamples； in the Solution Explorer。
2。 Select Add New Project。
3。 Select Class Library and change the name to ClassLibrary。
The resulting solution project should look like Figure 1…8。
Figure 1…8。 Updated solution structure that contains three projects
The added ClassLibrary project has a single file called Class1。vb； which is a plain…vanilla
source code file。
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C H AP TE R 1 ■ R E AD Y ； ST E AD Y ； G O！ 15
Moving Functionality
Now we will move the code used to say “hello； world” from ConsoleApplication to
ClassLibrary。 Add the code to Class1。vb as follows （the bolded code）：
Public Class Class1
Public Shared Sub HelloWorld（）
Console。WriteLine（〃hello； world〃）
End Sub
End Class
The modified code contains a method called HelloWorld（）。 When called； this method will
output the text “hello； world。” As mentioned earlier in the chapter； a method is a set of instruc
tions that carry out a task。 Methods are discussed in more detail in Chapter 2。
In order for applications to actually share the code that’s in a class library； you must make
the projects aware of each other’s existence。 You do that through references。
Defining References
To make one project aware of definitions in another project； you need to define a reference。 The
idea behind a reference is to indicate that a project knows about another piece of functionality。
■Note The project only knows about the functionality that has been declared as being public。 Public function
ality； or what Visual Basic programmers call public scope； is when you declare a type with the Public keyword。
You will learn about public and other scopes throughout this book。
To make ConsoleApplication aware of the functionality in the ClassLibrary project； you
need to set a physical reference； as follows：
1。 In the Solution Explorer； click ConsoleApplication。
2。 Right…click and select Add Reference。
3。 Click the Projects tab。
4。 Select ClassLibrary； and then click OK。 ClassLibrary will be added to
ConsoleApplication’s references。
Once the reference has been assigned； ConsoleApplication can call the functionality in
ClassLibrary。
To know which references your application or class library has； you need to look in the
project settings。 To do so； right…click the project name； ConsoleApplication； in the Solution
Explorer and select Properties。 In the Properties window； select the References tab； as shown
in Figure 1…9。
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16 CH AP T E R 1 ■ R E A DY ； ST E A DY ； G O ！
Figure 1…9。 References used by the Visual Basic project
Calling Class Library Functionality
Now we need to change ConsoleApplication so that it calls the function in ClassLibrary。 Modify
the Module1。vb file in ConsoleApplication as follows：
Module Module1
Sub Main（）
Console。WriteLine（〃hello； world〃）
ClassLibrary。Class1。HelloWorld（）
Console。ReadKey（）
End Sub
End Module
Run ConsoleApplication by pressing Ctrl＋F5。 A mand prompt window should appear
and generate the “hello； world” text twice。 The first “hello； world” is generated by the code
Console。WriteLine（）。 Calling the function ClassLibrary。Class1。HelloWorld（） generates the
second “hello； world。”
USING REFERENCE SHORTHAND
ClassLibrary。Class1。HelloWorld（） is the longhand way to use a reference。 If we were to use long
hand for the Console。WriteLine（） call； we would write System。Console。WriteLine（）； because the
Console。WriteLine（） method is defined in the System reference。 However； Visual Basic Express includes
the System reference by default； so we don’t need to do it this way。
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C H AP TE R 1 ■ R E AD Y ； ST E AD Y ； G O！ 17
To use shorthand for the ClassLibrary call； we would include an Imports line at the beginning of
Module1。vb in ConsoleApplication and change the call to Class1’s HelloWorld（） method：
Imports ClassLibrary
Module Module1
Sub Main（）
Console。WriteLine（〃hello； world〃）
Class1。HelloWorld（）
Console。ReadKey（）
End Sub
End Module
But shorthand like this has a downside。 What if we had many references； each containing a class called
Class1？ In this case； Visual Basic Express wouldn’t know which Class1 to use without the help of longhand。
Granted； you are not likely to name multiple classes Class1； but even sensible names can be duplicated in a
collection of references。 And if you are using someone else’s code as a reference； the possibility of duplicate
names bees higher。 Therefore； you’re better off using longhand in this case。
Using Variables and Constants
One of the core concepts in a Visual Basic program is to use variables。 Think of a variable as a
block of memory where you can store data for later use。 This allows you to pass data around
within your program very easily。
In our ClassLibrary project； it would make life easier if we could define the message to
display at the beginning of the method。 That way； if we decide to change the message； we can get at
it much more easily。 As it stands； if we were to add more code before the Console。WriteLine（） call；
we would need to scroll through the text to find the message to change。 A variable is perfect for
this； as we can define some data （the message to print）； and then use it later in our program。
Public Class Class1
Public Shared Sub HelloWorld（）
