My application displays a folder structure in a tree. The user can browse the contents in these folders, and drag content into the folders.
However, some of these folders are readonly (meaning no content can be dragged into them). I'm looking for a clear way of depicting this to the user, so they are aware which folders they can drag to before actually having to try it, i.e. hover state. Something not too alarming or interrupting (no big red crosses overlayed on top of the folder icons, for instance), preferably a recognized standard.
Specifically, we are talking about an Outlook add-in showing a SharePoint folder tree here, but I'd much rather get the answer from a generic viewpoint. From the top of my head, I simply don't seem to know any applications that visually mark readonly folders.
I was trying to think of any applications that I've seen that display the notion of something being read only visually, but not many things came to mind.
One that did is SQL Server. When a database is read only, it is greyed-out in the UI (as well as it explicitly saying read only):
I think this works quite well as you are in effect saying that this node/folder is unavailable for the current action (ie. dragging files to it).
As others have mentioned, a lock icon also works well (I've seen this done in Visual Studio when a file is read only).