Dear Ian,

Following your answer as well as with informations from Ankit and ChatGPT I could have something working! Thanks for your help.

I share what I did below :

I worked with 2 elementary "add" and "subtract" functions.

example.c code :

example.h code :

Then to compile I used the x64 native tool command prompt of Visual studio 2022 with the following command :

cl /LD example.c /Fe:example.dll /DDLL_EXPORT

Modelica function code :

For some reason that I don't understand I had to write the full path for the .h file. It didn't work with the "modelica://" shortcut...