Conditional Compilation in Visual Basic

Članek
09/15/2021

In conditional compilation, particular blocks of code in a program are compiled selectively while others are ignored.

For example, you may want to write debugging statements that compare the speed of different approaches to the same programming task, or you may want to localize an application for multiple languages. Conditional compilation statements are designed to run during compile time, not at run time.

You denote blocks of code to be conditionally compiled with the #If...Then...#Else directive. For example, to create French- and German-language versions of the same application from the same source code, you embed platform-specific code segments in #If...Then statements using the predefined constants FrenchVersion and GermanVersion. The following example demonstrates how:

#If FrenchVersion Then
   ' <code specific to the French language version>.
#ElseIf GermanVersion Then
   ' <code specific to the German language version>.
#Else
        ' <code specific to other versions>.
#End If

If you set the value of the FrenchVersion conditional compilation constant to True at compile time, the conditional code for the French version is compiled. If you set the value of the GermanVersion constant to True, the compiler uses the German version. If neither is set to True, the code in the last Else block runs.

Note

Autocompletion will not function when editing code and using conditional compilation directives if the code is not part of the current branch.

Declaring Conditional Compilation Constants

You can set conditional compilation constants in one of three ways:

In the Project Designer
At the command line when using the command-line compiler
In your code

Conditional compilation constants have a special scope and cannot be accessed from standard code. The scope of a conditional compilation constant is dependent on the way it is set. The following table lists the scope of constants declared using each of the three ways mentioned above.

How constant is set	Scope of constant
Project Designer	Public to all files in the project
Command line	Public to all files passed to the command-line compiler
`#Const` statement in code	Private to the file in which it is declared

To set constants in the Project Designer
- Before creating your executable file, set constants in the Project Designer by following the steps provided in Managing Project and Solution Properties.

To set constants at the command line

To set constants at the command line
- Use the -d switch to enter conditional compilation constants, as in the following example: `vbc MyProj.vb /d:conFrenchVersion=–1:conANSI=0` No space is required between the -d switch and the first constant. For more information, see -define (Visual Basic). Command-line declarations override declarations entered in the Project Designer, but do not erase them. Arguments set in Project Designer remain in effect for subsequent compilations. When writing constants in the code itself, there are no strict rules as to their placement, since their scope is the entire module in which they are declared.

- Use the -d switch to enter conditional compilation constants, as in the following example:
vbc MyProj.vb /d:conFrenchVersion=–1:conANSI=0
No space is required between the -d switch and the first constant. For more information, see -define (Visual Basic).
Command-line declarations override declarations entered in the Project Designer, but do not erase them. Arguments set in Project Designer remain in effect for subsequent compilations.
When writing constants in the code itself, there are no strict rules as to their placement, since their scope is the entire module in which they are declared.