Organizing code
In this part of the Visual Basic tutorial, we will show, how to organize code. We will cover modules, procedures and namespaces and the scope.Visual Basic statements are organized into blocks, modules, classes and namespaces. This helps to make the code more maintanable and robust. Correct code organization prevents from making errors in the code.
The basic building blocks of a Visual Basic program are:
- Assembly
- Namespace
- Modules
- Classes
- Procedures and functions
- Blocks
- Statements
Closely related to this topic is the scope and duration of a variable. A scope is the visibility of the declared variable.
| Scope | Description |
|---|---|
| Block scope | Available only within the code block in which it is declared |
| Procedure scope | Available within the procedure in which it is declared |
| Module scope | Available to all code within the module, class, or structure in which it is declared |
| Namespace scope | Available to all code in the namespace in which it is declared |
The basic example
First, we cover some basics.Option Strict OnIn this example, we have a Module called Example. Inside the example, we have a Main() subroutine. The statement that prints some message to the console is placed within the Main() procedure. Event the most simple Visual Basic programs must be properly organized.
Module Example
Sub Main()
Console.WriteLine("Simple example")
End Sub
End Module
Option Strict OnThe exact example, now without the module. We can put the code inside a class too. The Main() procedure must be declared
Public Class Example
Public Shared Sub Main()
Console.WriteLine("Simple example")
End Sub
End Class
Shared, because the class is not instantiated. The compiler calls the Main() method without creating an instance of the class. That is why it must be declared Shared. Java and C# work the same. Namespaces
Namespaces are used to organize code at the highest logical level. They classify and present programming elements that are exposed to other programs and applications. Within a namespace, we can declare another namespace, a class, an interface, a struct, an enum or a delegate.In the following code, we have two files that share the same namespace.
Option Strict OnWe have a ZetCode namespace. In the namespace, we have a module Example1.
NameSpace ZetCode
Module Example1
Public Dim x As Integer = 0
Sub Init()
x += 100
Console.WriteLine(x)
End Sub
End Module
End NameSpace
NameSpace ZetCodeWe declare a namespace called ZetCode.
Public Dim x As Integer = 0In the module, we declare and initialize a x variable.
Sub Init()We have an Init() method, in which we work with the x variable.
x += 100
Console.WriteLine(x)
End Sub
Option Strict OnIn the second file, we work with the Init() method from the previous file.
NameSpace ZetCode
Module Example
Sub Main()
Init()
x += 100
Console.WriteLine(x)
End Sub
End Module
End NameSpace
NameSpace ZetCodeWe work in the same namespace.
Init()We call the Init() procedure and work with the x variable. Both the procedure and the x variable are defined in a different file and different module. But they are defined in the same namespace, so we can use them.
x += 100
Console.WriteLine(x)
$ ./samenamespace.exeOutput.
100
200
The following code example has two distinct namespaces. We use the
Imports keyword to import elements from a different namespace. Option Strict OnWe have a skeleton of a Math class in a MyMath namespace. In the Basic class, we define a PI constant and a GetPi() method.
NameSpace MyMath
Public Class Basic
Public Shared PI As Double = 3.141592653589
Public Shared Function GetPi() As Double
Return Me.PI
End Function
End Class
End NameSpace
Option Strict OnIn this file, we use the elements from the MyMath namespace.
Imports MyMath
NameSpace ZetCode
Public Class Example
Public Shared Sub Main()
Console.WriteLine(Basic.PI)
Console.WriteLine(Basic.GetPi())
End Sub
End Class
End NameSpace
Imports MyMathWe import the elements from the MyMath namespace into our namespace.
Figure: Root namespace
On Visual Basic 2008 Express edition, there is a root namespace automatically created. This can be found under project properties, Application tab. Either delete the root namespace or include it into the imports path. For example, if you have Testing there, change the line to Imports Testing.MyMath.
Console.WriteLine(Basic.PI)Now we can use those elements. In our case the PI variable and the GetPi() method.
Console.WriteLine(Basic.GetPi())
Option Strict OnNote, that we do not need the
' Imports MyMath
NameSpace ZetCode
Public Class Example
Public Shared Sub Main()
Console.WriteLine(MyMath.Basic.PI)
Console.WriteLine(MyMath.Basic.GetPi())
End Sub
End Class
End NameSpace
Imports keyword. In the example, it is commented out. We can use elements from other namespaces by using fully qualified names of the elements. Modules
A module is used to organize code and wrap up variables, properties, events, and procedures of similar use. Unlike a class, a module is not a type. A module can be created in a namespace or in a file. A module cannot be created inside another module, class, structure, interface or block. All members in a module are implicitly Shared. Modules have a Friend access. This means, that a module is accessible everywhere in an assembly.Option Strict OnWe have three modules defined. The first two modules have variables and procedures. These will be used in the third module.
Module First
Public x As Byte = 11
Public Sub FirstModule()
Console.WriteLine("First module")
End Sub
End Module
Module Second
Public y As Byte = 22
Public Sub SecondModule()
Console.WriteLine("Second module")
End Sub
End Module
Module Example
Sub Main()
Console.WriteLine(x)
Console.WriteLine(Second.y)
FirstModule()
SecondModule()
End Sub
End Module
Module FirstWe can use access specifiers inside modules too. This way we can control the accessibility of the elements in the modules.
Public x As Byte = 11
...
End Module
Console.WriteLine(x)We print the x, y variables. They are Public and are accessible from a different module. We may use the module name to fully specify the variable name.
Console.WriteLine(Second.y)
A scope is a visibility of a variable. A variable with a module scope is available within the module, where it was declared.
Option Strict OnWe have x variable inside the module. The variable is available in all three procedures.
Module Example
Private x As Integer = 0
Sub Main()
proc1()
proc2()
proc3()
End Sub
Sub proc1()
Console.WriteLine(x)
End Sub
Sub proc2()
x += 100
Console.WriteLine(x)
End Sub
Sub proc3()
x += 100
Console.WriteLine(x)
End Sub
End Module
Private x As Integer = 0This is a variable with a module scope. It is declared outside any procedure.
Sub proc2()Inside the proc2() procedure, we increase the x variable and print its contents to the console. We refere to the x variable defined in the module.
x += 100
Console.WriteLine(x)
End Sub
$ ./modulescope.exeOutput of the example.
0
100
200
Procedures
Procedures provide modularity to the code project. They should do only a specific task.Option Strict OnIn the preceding code example, we have three procedures beside the main procedure. The three procedures create a local x variable and print it to the terminal. The main procedure refers to the module x variable.
Module Example
Dim x As Integer = 0
Sub Main()
Console.WriteLine(x)
proc1()
proc2()
proc3()
End Sub
Sub proc1()
Dim x As Integer
x += 100
Console.WriteLine(x)
End Sub
Sub proc2()
Dim x As Integer
x += 100
Console.WriteLine(x)
End Sub
Sub proc3()
Dim x As Integer
x += 100
Console.WriteLine(x)
End Sub
End Module
Sub proc1()The proc1() procedure creates a local x variable. This variable shadows the one, declared at a module scope.
Dim x As Integer
x += 100
Console.WriteLine(x)
End Sub
$ ./procedurescope.exeThe main procedure prints 0. The other procedures print 100 to the terminal. They create their local x variables, initiate them to 0, increase by 100.
0
100
100
100
Block scope
It is important to understand that variables declared within a block of code like If/End If or While/End While have a limited block scope and lifetime. The next example illustrates this.Option Strict OnWe have an x variable declared Inside the If/End If block.
Module Example
Sub Main()
If True
Console.WriteLine("Inside If block")
Dim x As Integer = 0
Console.WriteLine(x)
x += 500
Console.WriteLine(x)
End If
Console.WriteLine("Outside If block")
Rem Will not compile
Rem Console.WriteLine(x)
End Sub
End Module
Rem Will not compileThe variable is not available outside the block. If we uncomment the second line, the example will not compile.
Rem Console.WriteLine(x)
This part of the Visual Basic tutorial was dedicated to organizing code. We mentioned basic organizational elements of the code like namespaces, modules or procedures. We described variable scope and duration, which is closely related to those elements.
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