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Mike, the worst thing about WM_COPYDATA is that client is obliged to provide a receiver window to be able to catch dll errors. Too pushy design, I would say, from dll client standpoint. Another point...

If it really does allow, then yes, absolutely. What makes you think it might be not?

No exception should be let out of the dll, though inside you may use whatever you prefer. The wrapper function should catch all probable exceptions and based on those return error codes explanatory...