Module Scope in VB.NET

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562 Module Scope
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Module scope is similar to Class scope, in that members declared inside a module can be accessed by any code within it, but only members declared as Public can be accessed outwith it While a class behaves as a template for creating many individual objects, a module is a one-off construction When we declare a Public or Private variable inside a module, we get exactly one set of the variables declared, rather than one per object as you would expect with a class The picture also gets a bit more complex since we are able to create whole class de nitions inside a module Within the module, the scope rules still apply to the class private variables are visible only within the class, public ones are available outside the class However, the scope rules apply so that scope is nested, as shown in Listing 553 which summarizes the scope rules for code in two or more modules
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'This first module declares two classes, one Public, one 'Private We can test the visibility of these to code in 'this module and in other modules Module ScopeModule Private Class MyPrivateClass 'Any public member variables, properties or methods 'declared here will be visible to any code in the 'module Private declarations are visible to the 'class only Objects of this class can only be 'declared inside the module, since the class is 'Private to this module Public ANumber As Integer Private AString As String End Class Public Class MyPublicClass 'Any public member variables, properties or methods 'declared here will be visible to any code where an 'object is visible Private declarations are 'visible to the class only Objects of this class 'can be declared anywhere in a program which 'includes this module, since the class is Public Public ANumber As Integer Private AString As String End Class 'This sub shows which elements of the above classes are 'visible (ie in scope) Public Sub Test() Dim Pri As New MyPrivateClass() Dim Pub As New MyPublicClass() PriANumber = 2 'OK ANumber is Public PriAString = "Hello" 'Error since AString is 'Private PubANumber = 4 'Also OK PubAString = "Hello Again" ' Error Not possible End Sub End Module 'This module shows which elements of the above module are 'in 'project' scope Module ScopeTest Sub Main() 'Declare a member of the Public class Dim TPub As New ScopeModuleMyPublicClass() TPubANumber = 2 TPubAString = "Hello" 'ERROR, Member not in scope, 'even though the class is 'Try declaring a member of the Private class
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Dim TPri As New ScopeModuleMyPrivateClass() ' ERROR this Class is not in scope End Sub End Module Listing 553: Scope rules for Module and Class scope
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563 Namespace Scope
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A namespace is a unit of organization in a NET program Every NET project is automatically organized as a namespace with the same name as the project, so, for example, when you create a new Visual Basic NET project and leave it with its default name, it will occupy a namespace called Project1 The entire assembly, which is all of the les that are component parts of the project, are within the project s namespace You can also declare other namespaces within a project, which are nested within the overall project namespace By declaring a namespace in a project, you indicate that all of the contents (modules, classes, variables, etc) are placed within a scope that can be explicitly imported so that they can be used easily outside of the namespace To code outside the namespace, identi ers declared within it must pre xed with the namespace s name to be valid, unless the whole namespace is subscribed to using an Imports statement The Imports statement effectively says I want to incorporate the declarations within the speci ed namespace into my program By importing a namespace, any public declarations within it are accessible without quali cation to the code that does the importing, eg Listing 554
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Imports SystemConsole Module ScopeModule Private Class MyPrivateClass 'etc Listing 554: Importing a namespace
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The statement immediately before the start of ScopeModule in Listing 554 indicates that the declarations within the SystemConsole namespace are to be made directly available to the module Using this statement, all of the declarations within the console namespace can be accessed without the Console pre x For example, within a sub or function in ScopeModule, it is now possible to write code like Listing 555
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Public Sub Test() Dim n As String Write("Enter your name:") n = ReadLine() End Sub Listing 555: Using an imported scope (note, no need for Console )
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