Scope in VB.NET

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That is, we do not have to explicitly indicate that these methods are in the console namespace, as we have done previously: Write() and ReadLine() are now valid substitutes for ConsoleWrite() and ConsoleReadLine() We can create a namespace by simply surrounding the required code by the namespace declaration, as shown in Listing 556
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Namespace MyNamespace 'Whatever we put in here is part of the namespace 'eg Public Class SomeClass 'Class code End Class End Namespace Listing 556: Declaring a namespace
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Now, whatever declarations we place in the namespace can only be accessed by pre xing the name with the namespace name, eg Dim C As MyNameapaceSomeClass, or by importing the namespace, using:
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Imports MyNamespace
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A namespace does not have to be continuous, which is unusual, since all other code constructs in VB NET must be This allows us to write code like that in Listing 557
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Namespace MyNamespace 'First block of declarations End Namespace Class NotInNamespace 'This class is not in the namespace, so code using it does 'not require the MyNamespace prefix End Class Namespace MyNamespace 'Next block of declarations End Namespace Listing 557: A non-contiguous namespace
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With the code in Listing 557, any declarations within either MyNamespace would have to be declared explicitly using the MyNamespace pre x Objects of the NotInNamespace class can be declared without this One useful consequence of this is that we can create a namespace that exists over several different modules of code, and so can incorporate a number of related constructs (classes, modules, type de nitions and variable declarations) within a single namespace, no matter how we have chosen to organize the code into separate les Namespaces have nothing to do with algorithms, but have a lot to do with organizing program code so that declarations using similar names in different assemblies do not clash If we had two namespaces, Namespace1 and Namespace2, and each
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5 n Controlling Program Code
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contained a MyClass class declaration, we would be able to distinguish between the two same-named classes by the namespace pre x Note, however, that if we were to import both namespaces, the declarations would become ambiguous and VB would not allow the code to be compiled if a declaration of MyClass was used without a pre x stating which namespace was to be used
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564 Block Scope
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Block scope covers just about every other structural feature of Visual Basic Recall that a block is a sequence of program statements that are to be considered as an indivisible unit Every statement in a sub or function is collectively part of a block, as are all the statements inside a IfThen structure, or a loop or WithEnd With structure Visual Basic allows you to declare a variable anywhere you wish within a sequence of executable program statements However, once you have declared a variable, its scope is limited to the block in which it is declared This has fairly profound consequences For example, if I declare a new variable inside a IfThenEnd If structure, I am not allowed to refer to it in any statements beyond the end of the structure (or before it either, although you might have expected that) In Listing 558, the intention for the If statement might have been to record the fact that X had a value of zero at that point in the program However, the variable used to record this fact (XWasZero) would go out of scope as soon as execution has passed beyond the End If statement, so the variable effectively would not exist As a result the code would not compile, except in the circumstances where a different variable with the same name had been declared before the IfThen block
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If X = 0 Then Dim XWasZero As Boolean = True 'Other statements End If 'Other statements (including a scope error) ConsoleWriteLine("X was zero: {0}", XWasZero) Listing 558: Misusing a variable declared in block scope (within the IfThen block)
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Block scope can be tricky simply because blocks can be nested to any depth In Figure 56, there are four levels of block scope Working from the inside out, the variable I, declared in block 4 (the WithEnd With block) is only accessible inside this block, although all of the other variables (X,Y,Z and Q) can also be accessed in it Code within the For loop can access Q, declared within the loop, and also X, Y and Z, ie those variables declared in the blocks than enclose the For loop The IfThen block (block 2), encloses blocks 3 and 4, so variables de ned within it are in scope to all code in this block Finally, the variable X, declared at the level of the whole subroutine, is visible throughout the subroutine
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56 Scope Sub Test() Dim X As Integer = 0 If X = 0 Then Dim Y As Integer = 5 Dim Z As Integer For Z = 1 To Y Dim Q As String = "Test" With Q Dim I As Integer I = Length() End With Next End If End Sub
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Figure 56 Block scope Block 4 (WithEnd With) Block 3 (For Loop)
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Block 2 (IfEnd if)
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Block 1 (Whole Sub)
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The general rules for Block scope are: 1 a variable declared within a block can only be accessed within it, and is invisible to code outside the block 2 a variable cannot be declared inside a block with the same identi er as one declared in an outer block The second rule is important since otherwise it would be possible to inadvertently hide a variable to an inner block of code Consider Listing 559
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Dim X As Integer = 100 Dim Y, Z As Integer For Z = 1 To Y Dim X As String = "Hello" 'Not Legal 'More code Note, if this was permitted, X the 'Integer could not be accessed here Next Listing 559: Block scope hiding a variable in an outer scope
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If this code was legal, statements inside the ForNext loop would never be able to access the integer X, because the string X would mask it Other languages, for example C and C++, permit this and as a consequence programmers must take care never to use a variable name that is declared further out in the scope hierarchy The problem would probably never occur in short code sequences as shown Listing 559, but is easily possible in long sections of code in which many variables are declared at various levels of Block scope