Making sure the hlink object is available outside the scope of your function foo by making it an output argument probably solves your problem. it worked for me:
About Link Objects
The link object that linkprop returns stores the mechanism that links the properties of different graphics objects. Therefore, the link object must exist within the context where you want property linking to occur (such as in the base workspace if users are to interact with the objects from the command line or figure tools).
The following list describes ways to maintain a reference to the link object.
- Return the link object as an output argument from a function and keep it in the base workspace while interacting with the linked objects.
- Make the hlink variable global.
- Store the hlink variable in an object's UserData property or in application data
