Extension Objects

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Extension Objects

Path: Objects > TestServer > ExtensionObjects

Custom structured data types with binary-encoded ExtensionObject values. Unlike the Structures module (which uses plain OPC UA Object nodes with child variables), extension objects use proper OPC UA ExtensionObject encoding -- the structured value is serialized as a single typed binary blob in the OPC UA binary protocol.

Custom Types Namespace

  • Namespace Index: ns=3
  • Namespace URI: urn:opcua:test-server:custom-types
  • Encoding: Binary (Default Binary Encoding)

The extension object types and their encoding nodes are defined in namespace 3. This is separate from the main test node namespace (ns=1).

TestPointXYZ

A 3D point with three Double fields.

Field DataType Description
X Double X coordinate
Y Double Y coordinate
Z Double Z coordinate

Type definition: DataType node with Default Binary Encoding at ns=3;i=3010 (TypeId).

TestRangeStruct

A range with min, max, and current value.

Field DataType Description
Min Double Minimum value
Max Double Maximum value
Value Double Current value

Type definition: DataType node with Default Binary Encoding at ns=3;i=3011 (TypeId).

Variables

BrowseName DataType Access Initial Value
PointValue TestPointXYZ RW {X: 1.5, Y: 2.5, Z: 3.5}
RangeValue TestRangeStruct R {Min: 0.0, Max: 100.0, Value: 42.5}

Binary Encoding Details

Both extension object variables use binary-encoded ExtensionObject values. The binary body for each type consists of 3 consecutive IEEE 754 double-precision (8 bytes each) values:

PointValue binary body (24 bytes):

X = 1.5 (8 bytes) | Y = 2.5 (8 bytes) | Z = 3.5 (8 bytes)
TypeId: ns=3;i=3010

RangeValue binary body (24 bytes):

Min = 0.0 (8 bytes) | Max = 100.0 (8 bytes) | Value = 42.5 (8 bytes)
TypeId: ns=3;i=3011

Testing Notes

  • Read PointValue: Returns an ExtensionObject with TypeId pointing to ns=3;i=3010. Your client must decode the binary encoding to extract the X, Y, Z fields.
  • Write PointValue: Encode a TestPointXYZ as an ExtensionObject with the correct TypeId and binary body, then write it. Read it back to verify round-trip correctness.
  • Read RangeValue: Returns a read-only TestRangeStruct with TypeId ns=3;i=3011. Writing should return BadNotWritable.
  • Type discovery: Browse the DataType nodes in namespace 3 to discover the field structure.
  • Binary encoding: Both types have Default Binary encoding nodes in namespace 3. Clients that support automatic type dictionaries should be able to decode them without manual configuration.

Difference from Structures Module

Feature Structures Extension Objects
Encoding Object + child variables Single ExtensionObject value (binary)
Read One read per field One read returns all fields
Write One write per field One write sets all fields
Browse Browse children to find fields Browse DataType for Definition
Namespace ns=1 Types in ns=3, variables in ns=1
Use case Hierarchical browsing tests Structured type encoding tests