DataType

Built-in types

Identifiers 1 to 25 in namespace 0 are the built-in types: Boolean, SByte, Byte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Float, Double, String, DateTime, Guid, ByteString, XmlElement, NodeId, ExpandedNodeId, StatusCode, QualifiedName, LocalizedText, Structure, DataValue, BaseDataType and DiagnosticInfo. Everything a server can send is ultimately one of these, encoded per Part 6. A Variant is the container that carries any of them, plus arrays and matrices.

The abstract spine

BaseDataType at i=24 is abstract, as are Number at i=26, Integer at i=27, UInteger at i=28, Structure at i=22 and Enumeration at i=29. A Variable whose DataType attribute names an abstract type is telling you the concrete type may vary per value - you must inspect the Variant, not assume. That is common on generic gateway nodes and it is legal; it is also why blind casts fail.

Server-defined types

Structures, enumerations and unions defined by a companion specification or a vendor are DataType nodes in a non-zero namespace, subtypes of Structure, Enumeration or Union. Since 1.04 each carries a DataTypeDefinition attribute describing its fields, so a client can decode it at runtime without a compiled stub or a legacy type dictionary. Without that decode, values arrive as opaque ExtensionObjects.

Example

The DataType hierarchy as it matters for decoding

BaseDataType                     (i=24, abstract)
 +- Number                       (i=26, abstract)
 |   +- Integer                  (i=27, abstract)
 |   |   +- SByte i=2, Int16 i=4, Int32 i=6, Int64 i=8
 |   |   +- UInteger             (i=28, abstract)
 |   |       +- Byte i=3, UInt16 i=5, UInt32 i=7, UInt64 i=9
 |   +- Float i=10, Double i=11
 +- String i=12, DateTime i=13, Guid i=14, ByteString i=15
 +- NodeId i=17, StatusCode i=19, QualifiedName i=20, LocalizedText i=21
 +- Enumeration                  (i=29, abstract)
 |   +- ServerState, NodeClass, MachineryItemState_Enum, ...
 +- Structure                    (i=22, abstract)
     +- Range, EUInformation, Argument, ...
     +- ns=4;i=3002 MyRecipeType   <- vendor structure, needs DataTypeDefinition

Defined in: Part 3 - Address Space Model, DataType NodeClass and the built-in types; Part 6 - Mappings for their encodings.

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.