Unionsare encoded as a switch field preceding one of the possible fields. The encoding for the selected field is determined by the data type for the field.

The switch field is encoded as a UInt32.

The switch field is the index of the available union fields starting with 1. If the switch field is 0 then no field is present. For any value greater than the number of defined union fields the encoders or decoders shall report an error.

A Unionwith no fields present has the same meaning as a NULL value. A Unionwith any field present is not a NULL value even if the value of the field itself is NULL.

The following is an example of a union using C/C++ syntax:

struct Type2

{

Int32 A;

Int32 B;

};

struct Type1

{

Byte Selector;

union

{

Int32Field1;

Type2Field2;

}

Value;

};

In the C/C++ example above, the Selector, Field1 and Field2 are semantically coupled to form a union.

An instance of Type1would be encoded as byte sequence. The length of the byte sequence depends on the selected field.

An instance of Type1 where field Field1 is available would be encoded as 8-byte sequence. If the instance of Type 1 was encoded in an ExtensionObjectit would have the encoded form shown in Table 22and it would have a total length of 17 bytes. The TypeId, Encodingand the Lengthare fields defined by the ExtensionObject.

Table 22– Sample OPC UA Binary Encoded Structure

Field

Bytes

Value

Type Id

4

The identifier for Type1

Encoding

1

0x1 for ByteString

Length

4

8

SwitchValue

4

1 for Field1

Field1

4

The value of Field1

The Valueof the DataTypeDefinition Attributefor a DataType Nodedescribing Type1 is:

Name

Type

Description

defaultEncodingId

NodeId

NodeId of the “Type1_Encoding_DefaultBinary” Node.

baseDataType

NodeId

“i=22” [Union]

structureType

StructureType

Union_2 [Union]

fields [0]

StructureField

name

String

“Field1”

description

LocalizedText

Description of Field1

dataType

NodeId

“i=6” [Int32]

valueRank

Int32

-1 (Scalar)

arrayDimensions

UInt32[]

null

maxStringLength

UInt32

0

isOptional

Boolean

true

fields [1]

StructureField

name

String

“Field2“

description

LocalizedText

Description of Field2

dataType

NodeId

NodeId of the Type2 DataType Node (e.g. “ns=3; s=MyType2”)

valueRank

Int32

-1 (Scalar)

arrayDimensions

UInt32[]

null

maxStringLength

UInt32

0

isOptional

Boolean

true

The Valueof the DataTypeDefinition Attributefor a DataType Nodedescribing Type2 is:

Name

Type

Description

defaultEncodingId

NodeId

NodeId of the “Type2_Encoding_DefaultBinary” Node.

baseDataType

NodeId

“i=22” [Structure]

structureType

StructureType

Structure_0 [Structure without optional fields]

fields [0]

StructureField

name

String

“A“

description

LocalizedText

Description of A

dataType

NodeId

“i=6” [Int32]

valueRank

Int32

-1 (Scalar)

arrayDimensions

UInt32[]

null

maxStringLength

UInt32

0

isOptional

Boolean

false

fields [1]

StructureField

name

String

“B“

description

LocalizedText

Description of B

dataType

NodeId

“i=6” [Int32]

valueRank

Int32

-1 (Scalar)

arrayDimensions

UInt32[]

null

maxStringLength

UInt32

0

isOptional

Boolean

false