#### 8.2.3.4 IfcReference

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This entity is used to refer to a value of an attribute on an instance. It may refer to the value of a scalar attribute or a value within a collection-based attribute. Referenced attributes may be direct values, object references, collections, inverse object references, and inverse collections. References may be chained to form a path of object-attribute references.

XSD Specification:

<xs:element name="IfcReference" type="ifc:IfcReference" substitutionGroup="ifc:Entity" nillable="true"/>
<xs:complexType name="IfcReference">
<xs:complexContent>
<xs:extension base="ifc:Entity"/>
</xs:complexContent>
</xs:complexType>

EXPRESS Specification:

ENTITY IfcReference;

TypeIdentifier | : | OPTIONAL IfcStrippedOptional;
AttributeIdentifier | : | OPTIONAL IfcStrippedOptional;
InstanceName | : | OPTIONAL IfcStrippedOptional;
ListPositions | : | OPTIONAL LIST [1:?] OF IfcStrippedOptional;
InnerReference | : | OPTIONAL IfcStrippedOptional;

END_ENTITY;

Attribute Definitions:

TypeIdentifier | : | Optional identifier of the entity or type such as 'IfcMaterialLayerSet'. For entity, type, or select-based references within a collection, this resolves the reference to such type. If omitted, the type is assumed to be the same as the declared referencing attribute. EXAMPLE IfcRelAssociatesMaterial.RelatingMaterial may be resolved to IfcMaterialLayerSet.
AttributeIdentifier | : | Optionally identifies a direct or inverse attribute within an entity such as 'MaterialLayers'. If TypeIdentifier is specified and refers to an entity, the attribute must exist within the referenced entity. A null value indicates a reference to the type or entity itself, such as for indicating that the type of a value must match a specified constraint.
InstanceName | : | Optionally identifies an instance within a collection according to name. If the instance has an attribute called 'Name', such attribute is used for comparison; otherwise the first STRING-based attribute of the entity is used. EXAMPLE IfcRoot-based entities such as IfcPropertySet use the Name attribute; IfcRepresentation entities use the RepresentationIdentifier attribute.
ListPositions | : | Optionally identifies an instance within a collection according to position starting at 1. For referencing single-level collections, this attribute contains a single member; for referencing multi-level collections, then this LIST attribute contains multiple members starting from the outer-most index.
InnerReference | : | Optional reference to an inner value for ENTITY, SELECT, SET, or LIST attributes. A path may be formed by linking IfcReference instances together. EXAMPLE A material layer thickness may be referenced using several instances: #1=IFCREFERENCE($,'IfcSlab','HasAssociations',#2); #2=IFCREFERENCE($,'IfcMaterialLayerSet','MaterialLayers',#3); #3=IFCREFERENCE('Core','IfcMaterialLayer','LayerThickness',$);

Inheritance Graph:

ENTITY IfcReference

| ENTITY IfcReference

TypeIdentifier | : | OPTIONAL IfcStrippedOptional;
AttributeIdentifier | : | OPTIONAL IfcStrippedOptional;
InstanceName | : | OPTIONAL IfcStrippedOptional;
ListPositions | : | OPTIONAL LIST [1:?] OF IfcStrippedOptional;
InnerReference | : | OPTIONAL IfcStrippedOptional;

END_ENTITY;

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