I was wondering here if can't I model directly in a IFC file the 3D elements like walls, windows, doors, etc by using a software (or developing it).
I'm asking it because we are always trying to export an Arch project file to IFC but it always has data loss and a lot of another bugs. In my opinion I think it happens because private company will never adopt open BIM because there are personal interests in selling their products, just like open-source VS. non open-source softwares.
Of course you can develop your own BIM software and use IFC as its data storage format. A modern BIM editing software is a pretty complex bit of programming, though...
Theoretically yes, in practice no (or at least to my knowledge).
First the practice: I simply currently don't know of a piece of software you can really use to model directly within the pure IFC schema. Maybe FreeCAD comes close.
Theoretically you can program software to manipulate data based on the IFC schema. It can be done. I've created a cloud based application which is using the IFC schema and allows manipulation and creation of properties. But it's not easy, mostly because of rules that apply to the schema.
Don't forget that the IFC file is just a representation of the schema. In the case of IFC it is most often a STEP format file, but it could just as well be a JSON based dump of the data.
I tend to tell people to see IFC files as if they were PDF files. If I were to ask: "can I open and edit my PDF in Word?" you'd laugh at me. Everybody knows that's not possible, you need the original Word (source) file. And yes, intelligence of the data is lost in the PDF, but the representation is correct. Same with IFC.
There is no such thing as "modelling in a file". I am simplifying a bit, but you could think of it like this: When you author building models, you effectively manipulate computer memory. When you save it, you write it to disk, often to a file. It becomes clear that there are two models (and their meta models) involved - the transient model in memory and the persistent model on disk. Import and export is the conversion between the two.
We can rephrase the question to ask why not use the same meta model for both, e.g. the IFC schema as the domain model of a CAD application. The answer to that is simple: CAD application data models are optimized along the criteria of the specific application, efficient display, navigation, manipulation, memory-consumption etc. While IFC has been design to cover many of the common principles and geometry kernels used in CAD software, it's purpose is data exchange.
Thus unless your application is exclusively about processing IFC files (storage, querying, analysis, comparison etc.) - and even then - you will need some other data structures to efficiently implement the task at hand. And thus you will need some mapping between different data models.
