Preprocessor directives in C#

With preprocessor directives you can mark code to only be compiled in an assembly within a specified configuration mode of the assembly.

If you want to debug a program it can come in handy to test certain functionality only within debugging. you could write a generated html file on disc, but you don’t want that to happen in the release version of your program.

With a preprocessor directive that call to write the html file can be capsulated to only be compiled in the assembly in the selected debug configuration in Visual Studio:

VS Configuration

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#if DEBUG
    // Code here only in Debug Mode
#endif

To use and recognize this configuration, you have to define the DEBUG constant. There are two ways to get the definition active. You can either set it up in the csproj-file or activate it in the project settings GUI.

.NET Framework

Define the constants with the project settings GUI:

Settings GUI

Define the constants with the csproj-file:

csproj

.NET

In .NET 6 and newer the constants get defined with the project initialization. You can find them in the new project settings GUI aswell:

Settings GUI