You must select repeating group or field

I’ve been working on InfoPath form which utilize WCF. And most operations work without any problem. Piece of cake. But when I create last method and tried to use it with combo box of my InfoPath form I get an error “You must select repeating group or field” during setting up. I was trying all possible solutions without any result. Also there was no clue on the internet, all suggestion or solution didn’t work for my problem. Finally I’ve decided to rewrite the operation. I was using  [DataContract] with [DataMember] in the first version of the operation, which described fields in database. And it was possible to get it, but not with InfoPath.

The solution was implementing List <string> as return type for the operation. When I set it and rewrite code InfoPath without any problem bind this data with combo box control.
[adblockingdetector id=”5770e6ed85560″]