Please address IFC 4x3 in the documentation — do I need to handle IFC 4x3 and IFC 4.0 separately?

[wlinna]

Right now the examples in documentation say nothing about IFC 4x3. For example, when writing data to an existing IFC file, do I now need to handle all IFC 2x3, IFC4 and IFC 4x3 separately?

Let's take modifying IfcSite as a case study. Below is how I replace the values in an existing IFCSITE entity that. It handles those three versions separately, but I am not sure yet if it is actually necessary for Ifc 4x3 :

var ifcSite = model.Instances.OfType<IIfcSite>().FirstOrDefault()!;

if (model.Header.XbimSchemaVersion == XbimSchemaVersion.Ifc2X3) {
	var ifcSite2x3 = (Xbim.Ifc2x3.ProductExtension.IfcSite) ifcSite;
	if (eastings != null && northings != null) {
		ifcSite2x3.RefLatitude = Xbim.Ifc2x3.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(northings.Value);
		ifcSite2x3.RefLongitude = Xbim.Ifc2x3.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(eastings.Value);
	}
	if (elevation != null) {
		var projectLengthUnit = ((Xbim.Ifc2x3.MeasureResource.IfcUnitAssignment)ifcProject.UnitsInContext).LengthUnit as IIfcSIUnit;
		ifcSite2x3.RefElevation = new Xbim.Ifc2x3.MeasureResource.IfcLengthMeasure(elevation.Value / projectLengthUnit.Power);
	}
} else if (model.Header.XbimSchemaVersion == XbimSchemaVersion.Ifc4) {
	if (eastings != null && northings != null) {
		ifcSite.RefLatitude = Xbim.Ifc4.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(northings.Value);
		ifcSite.RefLongitude = Xbim.Ifc4.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(eastings.Value);
	}
	if (elevation != null) {
		var projectLengthUnit = ((IfcUnitAssignment)ifcProject.UnitsInContext).LengthUnit as IIfcSIUnit;
		ifcSite.RefElevation = new Xbim.Ifc4.MeasureResource.IfcLengthMeasure(elevation.Value / projectLengthUnit!.Power);
	}
} else if (model.Header.XbimSchemaVersion == XbimSchemaVersion.Ifc4x3) {
	var ifcSite4x3 = (Xbim.Ifc4x3.ProductExtension.IfcSite) ifcSite;
	if (eastings != null && northings != null) {
		var latitude = IfcCompoundPlaneAngleMeasure.FromDouble(northings.Value);
		var longitude = IfcCompoundPlaneAngleMeasure.FromDouble(eastings.Value);
		ifcSite4x3.RefLatitude = new Xbim.Ifc4x3.MeasureResource.IfcCompoundPlaneAngleMeasure(latitude);
		ifcSite4x3.RefLongitude = new Xbim.Ifc4x3.MeasureResource.IfcCompoundPlaneAngleMeasure(longitude);
	}
	if (elevation != null) {
		var projectLengthUnit = ((Xbim.Ifc4x3.MeasureResource.IfcUnitAssignment)ifcProject.UnitsInContext).LengthUnit as IIfcSIUnit;
		ifcSite4x3.RefElevation = new Xbim.Ifc4x3.MeasureResource.IfcLengthMeasure(elevation.Value / projectLengthUnit!.Power);
	}
}

(If you are wondering why the code for Ifc4x3 is more complex, it's because Xbim.Ifc4x3.MeasureResource.IfcCompoundPlaneAngleMeasure doesn' have FromDouble method)

[andyward]

Yes we need to update the docs with some 4x3 examples. On the list. But actually cross-schema support works the same way it does with Ifc4-Ifc2x3. The important thing is that the Ifc4 interfaces are 'special' and work across schemas (Note: in future we'll make this more explicit with a 'Xbim.CrossSchema' that targets the latest schema)

But for now if you use the Ifc4 interfaces all the other schemas implement this contract and handle the cross schema mapping. So setting ifcSite.RefLatitude to an Ifc4.MeasureResource.IfcCompoundPlaneAngleMeasure in the IFC4x3 example below actually assigns an Ifc4x3.MeasureResource.IfcCompoundPlaneAngleMeasure .

Note I've also shown how to use the EntityCreator to create entities agnostic of the schema

// using Xbim.Ifc;
// using Xbim.Ifc4.Interfaces;


double? northings = 50.123d;
double? eastings = -1d;
double? elevation  = 123.4d;

using var model = IfcStore.Open("ViadottoAcerno4x3.ifc");
using var transaction = model.BeginTransaction("Edit");
var ifcSite = model.Instances.OfType<IIfcSite>().FirstOrDefault()!;
var ifcProject = model.Instances.OfType<IIfcProject>().FirstOrDefault()!;

if (eastings != null && northings != null)
{
    ifcSite.RefLatitude = Xbim.Ifc4.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(northings.Value);
    ifcSite.RefLongitude = Xbim.Ifc4.MeasureResource.IfcCompoundPlaneAngleMeasure.FromDouble(eastings.Value);
}

if (elevation != null)
{
    var projectLengthUnit = ifcProject.UnitsInContext.Units.OfType<IIfcSIUnit>().FirstOrDefault(u => u.UnitType == IfcUnitEnum.LENGTHUNIT);
    ifcSite.RefElevation = new Xbim.Ifc4.MeasureResource.IfcLengthMeasure(elevation.Value / (projectLengthUnit?.Power ?? 1d));
}
// Create a Beam in the current schema
var factory = new EntityCreator(model);

var beam = factory.Beam(c => c.PredefinedType = IfcBeamTypeEnum.JOIST);

transaction.Commit();

There are a few items I'll add to the list to follow up on:

1. We need to complete the IFC4x3 partials = e.g. IfcCompoundPlaneAngleMeasurePartial's AsDouble()
2. We should consider making some of these partials extension methods on the Cross-schema interfaces. E.g. IfcUnitAssignmentPartial
3. We need to extend EntityCreator with the new Ifc4x3 entities.

[wlinna]

Thank you for your quick and enlightening comment :)