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VB2008从入门到精通(PDF格式英文版)-第9部分

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that does something is like a folder with stuff in it。 When you are creating the filing system; you  

don’t care about the contents of the folder。 And when you fill the folder; you generally don’t  

care about the filing system。 

     Modules; classes; namespaces; and methods are all concepts used to organize source  

code。 A method is filled with source code and does something like add numbers or create a  

textual string。 

     One of the most mon things that you will do when filling a method with source code is  

reference other pieces of organized source code。 Think of referencing as putting a sticky note  

in a folder with the text; “Please also look in folder B。”  

     Following is a piece of source code that is 100% organizational and does nothing。 



Namespace MyMainTypes  

   Public Class AType  

       Public Shared Sub DoSomething() 

       End Sub 

    End Class 

End Namespace 



Module AnotherType  

    Public Function DoSomething() As Integer 

    End Function 

End Module 



     The source code has three levels of organization。 A namespace (MyMainTypes in the example)  

encapsulates types like classes (AType in the example)。 Classes and modules (like AnotherType  

in the example) encapsulate methods (DoSomething() in the example) or properties。 Within a  

namespace; all types must be unique。 You can have two types with the same identifier in different  

namespaces。 Within a type; you cannot have identical identifiers with identical parameters。  

(This will be clearer as you learn more about Visual Basic in the uping chapters。) 

     Visual Basic has the additional rule that classes and modules (like AType and AnotherType)  

are in the namespace of the Visual Basic project。 That namespace is not explicitly defined; but  

it is identified by the project; as shown in Figure 2…6。 

     In Figure 2…6; the Root Namespace text box shows the root namespace of all types in a  

Visual Basic project。 So if both AType and AnotherType were part of the project; the full…length  

identifiers would be Calculator。MyMainTypes。AType and Calculator。AnotherType。 


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          Figure 2…6。 Default root namespace of project 



               Following is the same organizational code with some source code added to do something  

          (shown in boldface)。 



          Namespace MyMainTypes  

             Public Class AType  

                 Public Shared Sub DoSomething() 

                 End Sub 

              End Class 

          End Namespace 



          Module AnotherType  

              Public Function DoSomething() as Integer 

                  MyMainTypes。AType。DoSomething() 

              End Function 

          End Module 



               In the bolded code; there is a reference to another namespace; type; and method with a  

          pair of parentheses。 This makes a method call on a shared class and shared method。 It says that  

          the implementation of the method is the calling of another method。 


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     Notice how the other method is referenced using both namespace and type identifiers。 Also  

notice how there is no reference to the Calculator namespace。 This is not necessary because;  

from the perspective of AnotherType; Calculator is implied。 This is how all types and methods  

are always referenced。 A namespace identifier is necessary only if the type (for example; class)  

is not defined in the current namespace。 

     If you have namespaces with long names; this referencing can get tedious。 As an alterna

tive; you can add an Imports statement to reference the namespace; similar to the following。 



Imports Calculator。MyMainTypes 

Module AnotherType  

    Public Function DoSomething() as Integer 

        AType。DoSomething() 

    End Function 

End Module 



     The Imports statement says that if the code references any types that are not defined locally;  

look in this namespace (Calculator。MyMainTypes in the example) to find the type。 When using  

the Imports statement; you need to specify the full namespace of Calculator。MyMainTypes。  

Note that if you use two namespaces that have identically named types; you will get a piler  

failure; because the piler won’t know which type to reference。 

     This covers the absolute basics of writing some code; and we are ready to use Visual Basic  

to do something。 



Writing the Add() Method 



We’ll write the code to add two numbers。 To begin; create a new project in Visual Basic: 



     1。  Open Visual Basic Express。 (If Visual Basic Express is open; choose File  Close Project  

         to ensure you have a clean slate。) 



     2。  Click File  New Project or choose Create: Project from the Start Page tab。 



     3。  Choose Class Library; name it Calculator; and click OK。 



     4。  Rename Class1。vb to Operations。vb。 



     5。  Save the solution。 



     We can now write the Add() method。 Add the bolded code to the Operations。vb file。 



Public Class Operations 

    Public Shared Function Add(ByVal number1 As Integer; ByVal number2 As _ 

Integer) As Integer 

        Return number1 + number2 

    End Function 

End Class 



     This simple code actually has many different pieces that fit together。 The type Operations  

is implied to be in the namespace Calculator because the default root namespace is the same  

as the project。 Defined within Operations is a method that is both Public and Shared。 Using the  


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           Function declaration implies that the caller expects to get a value back from the method。 The  

           data type of the returned value is specified by the As Integer keywords—this particular func

           tion will return an Integer value。 (If you want to define a method that does not return a value;  

           use Sub rather than  Function。) 

                 Methods and parameters must be associated with a type; as Visual Basic is a type…safe  

           programming language。  Type…safe means that when you write code; you know what you are  

           manipulating。 

                 Suppose that you are writing code and are confronted with the numbers 1; 1。0; and 〃1。0〃。  

           To you; these three numbers are identical。 But in the context of the source code; they are not  

           identical。 The 1 is an integer; the  1。0 is a double; and the  〃1。0〃 is a string。 When you want to  

           add; subtract; or otherwise manipulate pieces of data; they should be the same types; other

           wise; you might run into consistency errors。 Type…safe programming languages help avoid  

           such problems。 The  number types are discussed in more detail in the “Understanding the  

           CLR Numeric Types” section later in this chapter。 

                 The declaration of Add() says that we need to pass in two integer…based numeric values;  

           and the method returns an integer…based numeric value。 The bination of parameters and  

           a return type is a method signature。 The method signature bees important when another  

           piece of code calls the Add() method。 The other piece of code must use the same types as the  

           declaration。 Figure 2…7 shows a piece of code that calls the Add() method; which we’ll do from  

           another application in the next section。 



                                                                     Namespace           Class             Method 



                 Dim total As Integer = Calculator。Operations。Add(1; 2) 



                         total is an integer declared                                           1 and 2 are integer values; 

                        variable that stores the result                                          which represent the two 

                               of the addition                                               numbers that are added together 



           Figure 2…7。  The Add() method is called by referencing the namespace and class containing the  

           method。 A period is used to separate the identifiers。 



                 The caller must do two things:  



                o  Reference the correct bination of namespace; class; and method identifiers。 



                o  Pass the correct types into the method; as specified by the method signature。 



                 In the example; the addition of 1 and 2 results in 3; and therefore the variable total should  

           contain the value 3 (the equal sign assigns the value returned from the method to the variable  

           on its left)。 I say “should contain the value;” because when writing code; you are not always  

           sure。 Sometimes the code you write will be wrong because you overlooked something or forgot  

           to reference something。  

                 Look at the calling code; and ask yourself if you are guaranteed that calling Add() with  1  

           and 2 will result in 3。 The answer is that; as a caller; you cannot be 100% sure that the total variable  

           will contain 3。 Just because a box has the label “Dishes” does not necessarily mean that dishes  


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are in the box。 You think you know the contents; but you cannot be 100% sure until you open  

the box。 Likewise; in code; you need to look at how the Add() method is implemented to be sure  

of the contents of the total variable。  

      In a production coding session; looking at the implementation code to verify it is doing  

what you expect is not a feasible solution; because that would take too much time and be  

pletely unreliable。 The only real solution is to write test code。  



Writing Code to Test the Add() Method 



Test code is caller code that passes parameters with targeted values and expects a targeted  

answer。 If the caller does not get the targeted answer; then the implementation of the tested  

method is wrong。 Figure 2…8 shows sample caller code that tests the Add() operation (we’ll add  

this to a project next)。  



                                                            Targeted caller code that adds 

                                                            1 and 2 and assigns the result 

                                                                 to the variable total 



                    Dim total As Integer = Operations。Add(1; 2) 



                    If (total  3) Then 

                     Console。WriteLine(〃Oops 1 add 2 does not equal 3〃) 

                    End If 



   Targeted testing              If targeted testing fails; the 

    of the variable              text “Oops…” is generated; 

   total; paring it               indicating an error 

     to the value 3 



Figure 2…8。  Testing the Add() method 



      The calling code of the test bears an uncanny resemblance to the code you saw in the previous  

section。 The difference is that the test code uses targeted variables and values; whereas the  

other code could contain any variables and values。 Another requirement of test code is to verify  

the answers returned by the method with targeted responses。 The If statement is used to check  

whether the value of the variable total is equal to  3。 

      When writing test code; the way the Add() method is used must be the same way the  

console or Windows application uses the method。 Otherwise; it would be like testing a winter  

tire in the middle of the Sahara—fun to do but irrelevant。 

      Another question related to testing has to do with the timing of tests。 Do you create the  

tests before or after implementing the Add() method? To get a clear understanding of the problem;  

imagine the development of a tire。 Do you define the tests for the tire before or after the tire has  

been developed? Most likely; the answer is before; during; and after development。 This is an  

important consideration when developing software。 Tests are written before; during; and after  

implementation; as follows: 


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                o  You develop tests before implementing the Add() method to get an idea of what name

                   spaces; classes; and methods you will be defining。 The definition of the different items  

                   gives the developer an idea of how the items will be used。 



                o  You develop tests during the implementation of the Add() method to verify that your  

                   source code implementation is on the right track。 



                o  You develop tests after the implementation of the Add() method as an exhaustive measure  

                   to make sure you’ve dotted the i’s and crossed the t’s in the implementation。 



           Adding a Test Project to Your Solution 



           When writing test routines; you will need to organize the source code; and that means figuring  

           out to which project the tests are added。 For the calculator application; you could place the test  

           routines within the Calculator class library。 However; doing that is not the proper approach  

           due to distribution of the class library and correct testing context。 Remember that the test  

           routines must be identical to how the code will be used。 Thus; the appropriate place for the test  

           routines is in their own application。  

                The ideal approach is to create another application that represents the tests。 Figure 2…5  

           illustrated how a Windows and console application could use the Calculator class library。  

           Figure 2…9 adds the testing console application that also uses the class library。 



           Figure 2…9。 Adding the testing console application 



                The testing console application is like the console application created in Chapter 1; and it  

           references the Calculator class library。 Both projects should be part of the Calculator solution。  

                 Go ahead and add the TestCalculator project to the Calculator soluti
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