I have been searching the C# API documentation, but have not found anything. Is there a way to get the field name of the asset type field of a utility network feature class? Is it safe to assume it will always be "ASSETTYPE"?
If you have a row and want to get the asset type, you can always call UtilityNetwork.CreateElement(), followed by Element.AssetType. That's certainly guaranteed. But if you're trying to create a row and want to fill in the asset type, the best answer for now is to use the field called ASSETTYPE. I'm not aware of a case where this would not work.
I agree that the API should provide a more formal mechanism for getting the asset type field name. I'll put it in the backlog for a future release.