Ok after several hours of searching I figured out that I was able to use the method by typing the whole path of the method (inside the runtime directory) :
using System.IO;
void Start()
{
//Absolute path to the method (working, no need to import the System.IO namespace) :
System.IO.Path.GetExtension("file.jpg");
//Relative path to the method (error, even if the namespace has been imported) :
Path.GetExtension("file.jpg");
}
I should add that I still don’t know why the “relative” path method call is not working, and I’d be glad to know !