Enumeration

Int32 on the wire, always

Enumeration at i=29 is abstract; concrete enumerations are its subtypes, in namespace 0 for standard ones such as ServerState and NodeClass and in a model namespace for the rest. Whatever the declared members are, the value travels as an Int32 and a Variant reports it as Int32. Clients that read the numeric value and stop are technically correct and operationally useless - the number means nothing without the name table.

Three places the names live

EnumStrings is a Property holding a LocalizedText array, valid only when the values are contiguous and start at zero, because the array index is the value. EnumValues is a Property holding an EnumValueType array of value, display name and description, and it is the one to use for sparse or non-zero-based sets - a state machine numbered 10, 20, 30, for example. Since 1.04 the DataTypeDefinition attribute also carries an EnumDefinition with the same information, which is the form a modern client should prefer because it needs no extra Property read.

Enumerations in state models

Machine state is where enumerations earn their keep in practice: the Machinery item state, PackML's 17-state model, operation-mode enumerations and condition classification all reduce to an enumerated value that a client must render as text. Reading the name table once per session and caching it is standard - these Properties change only when the model does - and it is what turns a raw 4 into Execute on a dashboard.

OptionSets are not enumerations

A bit-field is modelled as an OptionSet, not an Enumeration: the DataType derives from OptionSet or is marked as an option set in its definition, and the names come from an OptionSetValues Property where each entry names one bit. Decoding a bit-field with EnumStrings semantics produces a valid-looking single state where several flags were set at once.

Example

Contiguous versus sparse enumerations, and where the names come from

Contiguous, zero-based -> EnumStrings works
  ns=0;i=852  ServerState
    EnumStrings (LocalizedText[]) :
      [0] Running   [1] Failed   [2] NoConfiguration
      [3] Suspended [4] Shutdown [5] Test  [6] CommunicationFault  [7] Unknown
    Wire value 0 -> "Running"   (array index == value)

Sparse or non-zero-based -> EnumStrings is WRONG, use EnumValues
  ns=4;i=3100  DowntimeReason
    EnumValues (EnumValueType[]) :
      { Value 10,  DisplayName "Tool change" }
      { Value 20,  DisplayName "Material shortage" }
      { Value 100, DisplayName "Unplanned stop" }
    Wire value 20 -> "Material shortage"
    Indexing EnumStrings with 20 -> out of range, or the wrong name

Since 1.04, the same table without a Property read:
  Read DataTypeDefinition of ns=4;i=3100
    EnumDefinition Fields: [10 Tool change] [20 Material shortage] [100 Unplanned stop]

Defined in: Part 3 - Address Space Model, Enumeration DataTypes, EnumStrings, EnumValues and EnumDefinition; Part 6 - Mappings for the Int32 encoding.

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.