A resource index on a computing device identifies multiple resource instances (e.g., multiple user interface (UI) resource instances) of multiple resource items (e.g., of multiple UI resource items), each resource instance having one or more resource instance conditions. In response to a request for a resource item received from an application, a determination is made based on the resource index of one of the multiple resource instances that satisfy conditions associated with the request, and the one of the multiple resource instances is returned to the application. Additionally, the resource index can be used to identify potential errors in running an application in various potential contexts.