VB2008从入门到精通(PDF格式英文版)-第12章
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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（）
Dim message As String = 〃hello； world〃
Console。WriteLine（message）
End Sub
End Class
Here； we’ve defined a variable called message of type String （a String is a length of text）。
We can then refer to the message variable later when we want to place its contents into the code。
In the example； we place its contents into the Console。WriteLine（） call； which works as before。
This time； however； we have set the message to display in a separate statement。
This is very useful for us； but there is more to variables。 They have something that is called
scope。 The message variable has method…level scope； which means it is available only in the
method in which it is defined。 Consider this code：
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18 CH AP T E R 1 ■ R E A DY ； ST E A DY ； G O ！
Public Shared Sub HelloWorld（）
Dim message As String = 〃hello； world〃
Console。WriteLine（message）
End Sub
Public Shared Sub DisplayeMessageText（）
Console。WriteLine（〃hello； world〃）
Console。WriteLine（message）
End Sub
The DisplayMessageText（） method prints two lines of text to tell us what the message text
should be。 However； this doesn’t pile； because the piler knows that the variable message is
not available to the DisplayMessageText（） method because of its method…level scope。
To fix this； we need to give message class…level scope by moving it to the beginning of the
class definition （as it is used by methods marked Shared； it must also be Shared）：
Public Class Class1
Shared Dim message As String = 〃hello； world〃
Public Shared Sub HelloWorld（）
Console。WriteLine（message）
End Sub
Public Shared Sub DisplayeMessageText（）
Console。WriteLine（〃hello； world〃）
Console。WriteLine（message）
End Sub
End Class
Now the variable message is shared by all the methods of Class1。 You’ll learn much more
about method…level and class…level scopes； as well as the Public and Shared keywords； throughout
this book。
Sharing a variable among methods of a class can be useful； but it’s sometimes not wise to
do this。 That’s because methods can change variables as they carry out their tasks； which can
produce unpredictable results further down the line。 We can lock the value by using a constant
instead of a variable。 The Const keyword denotes the constant：
Public Class Class1
Const MESSAGE As String = 〃hello； world〃
Public Shared Sub HelloWorld（）
Console。WriteLine（MESSAGE）
End Sub
Public Shared Sub DisplayeMessageText（）
Console。WriteLine（〃hello； world〃）
Console。WriteLine（MESSAGE）
End Sub
End Class
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C H AP TE R 1 ■ R E AD Y ； ST E AD Y ； G O！ 19
Constant names are usually all uppercase。 The contents of a constant cannot be changed
at any point。 The following would not pile； for instance。
Public Class Class1
Const MESSAGE As String = 〃hello； world〃
Public Shared Sub DisplayeMessageText（）
MESSAGE = 〃another text that cannot be assigned〃
Console。WriteLine（MESSAGE）
End Sub
End Class
Now that you’ve worked through this chapter’s examples； let’s talk a bit about how your
Visual Basic code in Visual Basic Express actually turns into a program that can run on an oper
ating system like Windows。
Understanding How the Framework Works
When you write Visual Basic source code； you are creating instructions for the program to
follow。 The instructions are defined using the Visual Basic programming language； which is
useful for you； but not useful for the puter。 The puter does not understand pieces of
text； it understands ones and zeros。 To feed instructions to the puter； developers have
created a higher…level instruction mechanism that converts your instructions into something
that the puter can understand。 The conversion utility is called a piler。
The twist with ； in contrast to traditional programming languages such as C＋＋ and C；
is that the piler generates a binary…based intermediate language called mon Interme
diate Language （CIL）。 The Framework then converts the CIL into the binary instructions
required by the puter’s processor。
You might think converting the source code into an intermediate language is inefficient；
but it is a good approach。 Let’s use an analogy。 There are dogs that learn quickly and those that
take a while to learn。 For example； German shepherds tend to learn quickly and don’t require
much repetition of lessons。 On the other hand； bullmastiffs need quite a bit of patience； as they
tend to be stubborn。 Now imagine being a trainer who has created instructions on how to teach
things specifically geared toward the bullmastiff。 If those same instructions are used on the
German shepherd； you end up boring the German shepherd and possibly failing to teach that
dog what you wanted him to learn。
The problem with the instructions is that they are specifically tuned for a single dog。 If you
want to teach both dogs； you need two sets of instructions。 To solve this problem； the instruc
tions should be general； with added interpretation notes saying things like； “If dog is stu
