Why I could not use the function as TopoDS::Face() As it could not recognize TopoDS

Hi everyone,
I just begin to learn OCCT this week! so there were some problems that I want to ask you guys for help:
My goal is to load a step file as get some geometric features and data. But when I want to deal with the faces, I meet a problem.
I have import the related header file like <TopoDS_Shape.hxx> and <TopoDS_Face.hxx>, but vs said that "error: name followed by '::' must be a class or namespace name" so I could not use TopoDS:: , Could you provide me some solutions? thank you guys!

here is my code:

reader.TransferRoots();
TopoDS_Shape aShape = reader.OneShape();

for (TopExp_Explorer exp(aShape, TopAbs_FACE); exp.More(); exp.Next()) {
TopoDS_Face face = TopoDS::Face(exp.Current());
ProcessFace(face);
}

Thank you for your help and love