I’m sure we’ll get interface sooner or later to LabVIEW, in the mean time we have to implement it ourselves in different ways. 
Here a thread and one example I use: http://lavag.org/topic/10621-interface/ 
  
I’ve not documented the Interface class template and how to use it, that comes with GDS. 
But here is a quick instruction, this design only makes sense to Referenced Objects. 
  
 
  
Add Interface Methods 
 
  
Create a Normal class that should implement the interface 
 
  
Add a design pattern: Interface 
 
  
This is what gets created 
 
 
  
Interface.zip 
  
Cheers, 
Mike