Returns a Inedo.Extensibility.UserDirectories.IUserDirectoryPrincipal for the user or group with the specified name.
The name of the principal to look up.
A corresponding Inedo.Extensibility.UserDirectories.IUserDirectoryPrincipal for the specified name, if the principal was found in the directory; otherwise null (Nothing in Visual Basic).
principalName is null (Nothing in Visual Basic) or empty.