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if-then-else-directives.md

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title #If...Then...#Else Directives
ms.date 07/20/2015
ms.prod .net
ms.suite
ms.technology
devlang-visual-basic
ms.topic article
f1_keywords
vb.#EndIf
#End If
#Then
#ElseIf
vb.#ElseIf
vb.#Else
vb.#If
helpviewer_keywords
Visual Basic code, compiling
#If directive [Visual Basic]
conditional compilation [Visual Basic], directives
#End if directive [Visual Basic]
selective compiling
else directive (#else)
#Else directive [Visual Basic]
ms.assetid 10bba104-e3fd-451b-b672-faa472530502
caps.latest.revision 14
author dotnet-bot
ms.author dotnetcontent

#If...Then...#Else Directives

Conditionally compiles selected blocks of Visual Basic code.

Syntax

#If expression Then  
   statements  
[ #ElseIf expression Then  
   [ statements ]  
...  
#ElseIf expression Then  
   [ statements ] ]  
[ #Else  
   [ statements ] ]  
#End If

Parts

expression
Required for #If and #ElseIf statements, optional elsewhere. Any expression, consisting exclusively of one or more conditional compiler constants, literals, and operators, that evaluates to True or False.

statements
Required for #If statement block, optional elsewhere. Visual Basic program lines or compiler directives that are compiled if the associated expression evaluates to True.

#End If
Terminates the #If statement block.

Remarks

On the surface, 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 what is compiled by the compiler, 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 has no effect on size or performance.

Regardless of the outcome of any evaluation, all expressions are evaluated using Option Compare Binary. The Option Compare statement does not affect expressions in #If and #ElseIf statements.

Note

No single-line form of the #If, #Else, #ElseIf, and #End If directives exists. No other code can appear on the same line as any of the directives.

Example

This example uses the #If...Then...#Else construct to determine whether to compile certain statements.

[!code-vbVbVbalrConditionalComp#1]

See Also

#Const Directive
If...Then...Else Statement
Conditional Compilation