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An ExtensionObject is encoded as sequence of bytes prefixed by the NodeId of its DataTypeEncoding and the number of bytes encoded.

An ExtensionObject may be encoded by the application which means it is passed as a ByteString or an XmlElement to the encoder. In this case, the encoder will be able to write the number of bytes in the object before it encodes the bytes. However, an ExtensionObject may know how to encode/decode itself which means the encoder shall calculate the number of bytes before it encodes the object or it shall be able to seek backwards in the stream and update the length after encoding the body.

When a decoder encounters an ExtensionObject it shall check if it recognizes the DataTypeEncoding identifier. If it does, then it can call the appropriate function to decode the object body. If the decoder does not recognize the type it shall use the Encoding to determine if the body is a ByteString or an XmlElement and then decode the object body or treat it as opaque data and skip over it.

The serialized form of an ExtensionObject is shown in Table 14.

Table 14 – Extension Object Binary DataEncoding

Name	Data Type	Description
TypeId	NodeId	The identifier for the DataTypeEncoding node in the Server's AddressSpace. ExtensionObjects defined by the OPC UA specification have a numeric node identifier assigned to them with a NamespaceIndex of 0. The numeric identifiers are defined in A.3. Decoders use this field to determine the syntax of the Body. For example, if this field is the NodeId of the JSON Encoding Object for a DataType then the Body is a ByteString containing a JSON document encoded as a UTF-8 string.
Encoding	Byte	An enumeration that indicates how the body is encoded. The parameter may have the following values: 0x00 No body is encoded. 0x01 The body is encoded as a ByteString. 0x02 The body is encoded as a XmlElement.
Length	Int32	The length of the object body. The length shall be specified if the body is encoded.
Body	Byte [*]	The object body. This field contains the raw bytes for ByteString bodies. For XmlElement bodies this field contains the XML encoded as a UTF-8 string without any null terminator. Some binary encoded structures may have a serialized length that is not a multiple of 8 bits. Encoders shall append 0 bits to ensure the serialized length is a multiple of 8 bits. Decoders that understand the serialized format shall ignore the padding bits.

ExtensionObjects are used in two contexts: as values contained in Variant structures or as parameters in OPC UA Messages.

A decoder may choose to parse an XmlElement body after decoding; if an unrecoverable parsing error occurs then the decoder should try to continue processing the stream. For example, if the ExtensionObject is the body of a Variant or an element in an array that is the body of Variant then this error can be reported by setting value of the Variant to the StatusCode Bad_DecodingError.