The ResourceSet manages access to resources via IEnumerable access which is ultimately used to return resources to the front end.
Resources are read once and cached into an internal Items field. A ResourceReader instance is specific to a ResourceSet and Culture combination so there should never be a need to reload this data, except when explicitly clearing the reader/resourceset (in which case Items can be set to null via ClearResources()).
public sealed IDictionaryEnumerator GetEnumerator();