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When using an optional attribute in a product configuration model it is not possible to write an expression that checks if the optional attribute was actually used. The only way right now is to make the attribute mandatory and use a default value like "empty".

Please extend the expression syntax with a IsNull() function to check if an optional attribute is used.
STATUS DETAILS
Needs Votes
Ideas Administrator

Thanks for your feedback! If it gets voted, we will consider adding it to our long term backlog

Sincerely, 

Beatriz Nebot Gracia

Program Manager, Microsoft