An object to lookup user and group principals by name. A UserPrincipal represents an identity that may be used to determine access rights to objects in a file system. A GroupPrincipal represents a group identity. A UserPrincipalLookupService defines methods to lookup identities by name or group name (which are typically user or account names). Whether names and group names are case sensitive or not depends on the implementation. The exact definition of a group is implementation specific but typically a group represents an identity created for administrative purposes so as to determine the access rights for the members of the group. In particular it is implementation specific if the namespace for names and groups is the same or is distinct. To ensure consistent and correct behavior across platforms it is recommended that this API be used as if the namespaces are distinct. In other words, the lookupPrincipalByName should be used to lookup users, and lookupPrincipalByGroupName should be used to lookup groups.