Hi

the Public empty constructor for ContentService appears to have gone in 7.3 the only constructor is now

 public ContentService(
            IDatabaseUnitOfWorkProvider provider,
            RepositoryFactory repositoryFactory,
            ILogger logger,
            IEventMessagesFactory eventMessagesFactory,
            IPublishingStrategy publishingStrategy,
            IDataTypeService dataTypeService,
            IUserService userService)

which means you need a lot of other stuff to call it

However i think calling ContentService this way is really only used when runing through the tests.

if you want to get to a version of content service for the current umbraco instance you should call

var contentService = ApplicationContext.Current.Services.ContentService;

You will then have an instance of content service you can do anything you need to with.