May 13, 2021 vb.net
Vb.
Net compiler instructions are given to compilers to preprocess information before the actual compilation begins.
All of these instructions begin with a . . . and only space characters appear before an instruction on a line.
These instructions are not statements.
Vb. N et compilers do not have separate preprocessors; H owever, the instruction is processed as if there were one. I n VB.Net, compiler instructions are used to help the condition compile. Unlike the C and C instructions, they are not used to create macros.
Vb. Net provides the following set of compiler instructions:
The #Const directive
The #ExternalSource directive
The #If... Then... #Else instructions
The #Region directive
The instruction defines the conditional compilation constant. T he syntax instruction is:
#Const constname = expression
constname: Specifies the name of the constant. N ecessary.
expression : It is a text or other conditional compiler constant, or contains any or all arithmetic or logical operators (except Is).
For example
#Const state = "WEST BENGAL"
Example
The following code demonstrates the use of pseudo-instructions:
Module mydirectives #Const age = True Sub Main() #If age Then Console.WriteLine("You are welcome to the Robotics Club") #End If Console.ReadKey() End Sub End Module
When the above code is compiled and executed, it produces the following results:
You are welcome to the Robotics Club
#ExternalSource( StringLiteral , IntLiteral ) [ LogicalLine ] #End ExternalSource
#Externalsource directive parameters are the path to the external file, The line number of the first line and the line where the error occurred.
Example
The following code demonstrates the use of pseudo-instructions:
Module mydirectives Public Class ExternalSourceTester Sub TestExternalSource() #ExternalSource("c:vbprogsdirectives.vb", 5) Console.WriteLine("This is External Code. ") #End ExternalSource End Sub End Class Sub Main() Dim t As New ExternalSourceTester() t.TestExternalSource() Console.WriteLine("In Main.") Console.ReadKey() End Sub
When the above code is compiled and executed, it produces the following results:
This is External Code. In Main.
This pseudo-instruction conditionally compiles the selected Visual Basic block of code.
The syntax of this pseudo-instruction is:
#If expression Then statements [ #ElseIf expression Then [ statements ] ... #ElseIf expression Then [ statements ] ] [ #Else [ statements ] ] #End If
For example
#Const TargetOS = "Linux" #If TargetOS = "Windows 7" Then ' Windows 7 specific code #ElseIf TargetOS = "WinXP" Then ' Windows XP specific code #Else ' Code for other OS #End if
Example
The following code demonstrates a hypothetical instruction:
Module mydirectives #Const classCode = 8 Sub Main() #If classCode = 7 Then Console.WriteLine("Exam Questions for Class VII") #ElseIf classCode = 8 Then Console.WriteLine("Exam Questions for Class VIII") #Else Console.WriteLine("Exam Questions for Higher Classes") #End If Console.ReadKey() End Sub End Module
When the above code is compiled and executed, it produces the following results:
Exam Questions for Class VIII
This pseudo-instruction helps to collapse and hide snippppy code in the Visual Basic file.
The syntax of this pseudo-instruction is:
#Region "identifier_string" #End Region
For example
#Region "StatsFunctions" ' Insert code for the Statistical functions here. #End Region