What you describe is by design - both for WriteValue and for subscriptions.

Synchronous methods that work on a single operand (such as WriteValue) throw UAException or a derived exception in case of any errors during the operation, and that is what you need to catch. There is no way to reliably check that upfront, because things may change with time quickly.

Synchronous methods that work with multiple operands (with "Multiple" in their name) never throw, except for usage errors (program bugs such as passing in null where it is not allowed). Any errors are reported in the output array - each element has an .Exception property that is either null (OK) or not (an error). There is no way to reliably check upfront.

Asynchronous methods like SubscribeXXXX also never throw, except for usage errors . All errors are reported through the event notifications or callbacks.

Best regards