When you develop application for Microsoft StreamInsight, you are responsible for defining event payloads, i.e. .NET data structures that hold the data associated with StreamInsight events. The fields in the payload are user-defined and their types are based on the .NET type system.

There are various rules and limitations regarding the design of event payloads for Microsoft StreamInsight. For details, refer to StreamInsight documentation, e.g. the “Event Structure” article (http://msdn.microsoft.com/en-us/library/ee378905(v=sql.111).aspx).

The objects provided as notifications by OPC observables are not directly suitable as StreamInsight event payloads. You need to define the event payload structure for StreamInsight according to the needs of your application. There is no general rule as to what to use as StreamInsight event payload – it all depends on the specifics of the application. You can, for example

• Pick up a single field from the notification provided by the OPC observable – for example, the actual data value. This may be very useful in applications that only need the data values, but beware of special cases such as communication errors etc.
• Create a custom .NET data structure, and define its fields and organize the data in it precisely as your application requires. Then, convert the notifications provided by the OPC observable, to instances of this >NET event payload structure.
• Use ready-made OPC Event Payload classes provided by the StreamInsight Extensions for OPC Data Client. These classes are already designed to conform to StreamInsight rules, and payload instances can be easily created by converting them from the notifications coming from OPC observables.

The following paragraphs describe the OPC Event Payload classes for various OPC specifications.