The event payload class for OPC Data Access is the DAItemChangedPayload<TValue>, where TValue is the type of the OPC values. This class contains an embedded DAVtqPayload<TValue> object, which contains the actual OPC value, timestamp, and quality. This object is always present; in case of errors, it contains default values.

In addition, there are Server and Item string properties, identifying the source of the event. For a single OPC observable, they always contain the same value, but they become useful once you start combining events from different OPC sources into one stream.