I am embarrassed as hell! I tested a series of simple functions between managed and unmanaged code, from returning a simple int to a struct, to a vector of structs and to a list of structs. Even passed these fields in as a parameter.
They all worked.
What did I do wrong???
Idiot that I was I forgot I had not yet implemented the original stupid function. All my tests, were, of course, implemented! So my original discovery function was declared but had no implementation. AARRGG! All day I have spent on this.
(I would have never guessed that was the reason for the error message.)
Maybe this will save someone else who gets the same sets of messages for the same reason.