@Bastien I build with MinGW on Qt Creator. And when I use DLL, I've the error. I didn't ask you that. I know that. That's why I asked you: And when you build it in Visual Studio you are using the MinGW compiler, not the MSVC one, just like you show you are using from Qt Creator, right? So I asked you what compiler you are using from Visual Studio, not Qt Creator. Having to make a guess and assuming you use MinGW from Qt Creator but MSVC from VS, I imagine that is precisely the answer to your: This code works fine in Visual Studio but not in QtCreator... Why ?