I may be reading into this more than necessary but you wouldn't necessarily get a tree view (in the sense of a tree view control) from the web service. Rather you would basically be looking for the names of the folders that are associated with the parent and child nodes in this control. You would essentially be passing the strings rather than an instance of a control. Could you give a few more details about your workflow and expectations? I've mainly used .NET and Java for writing web services. What you choose would of course depend on what you're comfortable with. Depending upon what you are starting with and what the client of your web service would be may also affect what you choose for writing the web service.
