VB.NET

The VB.Net compiler directives give instructions to the compiler to preprocess the information before the actual compilation starts.

All these directives begin with #, and only white-space characters may appear before a directive on a line.
These directives are not statements.
VB.Net compiler does not have a separate preprocessor, but the directives work like a preprocessor.
It also helps in conditional compilation.

In VB.Net, the following types of directives are available.

#Const Directive
#ExternalSource Directive
#If...Then...#Else Directive
#Region Directive

`#Const` Directive

The #Const directive defines conditional compiler constants, and these constants are always private to the file in which they appear.

You cannot create public compiler constants using the #Const directive.
You can create them only in the user interface or with the /define compiler option.
You can use only conditional compiler constants and literals in expression.

The following code uses the #Const directive.

#Const MyLocation = "USA"
#Const Version = 10.0

    Sub ConstDirective()

#If Version > 9.0 Then
        Console.WriteLine("Latest version installed")
#Else
        Console.WriteLine("Latest version not installed")
#End If

    End Sub

You can use constants defined with the #Const keyword only for conditional compilation. Constants can also be undefined, in which case they have a value of Nothing.

`#ExternalSource` Directive

The #ExternalSource` Directive indicates a mapping between specific lines of source code and text external to the source.

It is used only by the compiler and the debugger.
A source file may include external source directives, which indicate a mapping between specific lines of code in the source file and text external to the source file.
If errors are encountered in the designated source code during compilation, they are identified as coming from the external source.

The following code uses the #ExternalSource directive.

    Sub ExternalSourceDirective()

#ExternalSource ("c:\vbprogs\directives.vb", 5)
        Console.WriteLine("This is External Code. ")
#End ExternalSource

    End Sub

External source directives do not affect a compilation and cannot be nested. They are intended for internal use by the application only.

`#If...Then...#Else` Directive

The #If...Then...#Else directive conditionally compiles selected blocks of Visual Basic code. The behavior of the #If...Then...#Else directives appears the same as that of the If...Then...Else statements. However, the #If...Then...#Else directives evaluate at compile-time, whereas the If...Then...Else statements evaluate conditions at run time.

Conditional compilation is typically used to compile the same program for different platforms.
It is also used to prevent debugging code from appearing in an executable file.
Code excluded during conditional compilation is completely omitted from the final executable file, so it does not affect size or performance.

The following code uses the #If...Then...#Else directive.

#Const MyLocation = "USA"
#Const Version = 10.0

    Sub ConstDirective()

#If Version > 9.0 Then
        Console.WriteLine("Latest version installed")
#Else
        Console.WriteLine("Latest version not installed")
#End If

    End Sub

`#Region` Directive

The #Region directive collapses and hides sections of code in Visual Basic files.

Use the #Region directive to specify a block of code to expand or collapse when using the Visual Studio Code Editor's outlining feature.
You can place, or nest, regions within other regions to group similar regions together.

The following code uses the #Region directive.

#Region "MathFunctions" ' Insert code for the Math functions here. #End Region

Fastest Entity Framework Extensions

`#Const` Directive

`#ExternalSource` Directive

`#If...Then...#Else` Directive

`#Region` Directive

Got any VB.NET Question?

VB.NET

VB.NET Directives

Fastest Entity Framework Extensions

#Const Directive

#ExternalSource Directive

#If...Then...#Else Directive

#Region Directive

Got any VB.NET Question?

`#Const` Directive

`#ExternalSource` Directive

`#If...Then...#Else` Directive

`#Region` Directive