If your writing code in languages that are based on a common language but their files names have an uncommon extension, for example, the 3D shading languages OSL, and HLSL, which are written in C syntax but have .osl and .hlsl, or .fx as file name extensions,
Notepad++ wont automatically recognize that the code in these files is actually C language and present their syntax properly.
To set Notepad++ to recognize specific file associations with a wanted language:
Settings > Style Configurator:
- In the Style Configurator window:
Select the wanted language in the language list on the left,
And in the User ext. field, type a list of file extensions separated by spaces:
These file extensions will be interpreted as the selected language.