ui-router resolve

ui-router-resolve.js

$stateProvider.state('main', {
    abstract: true,
    url: '/:account',
    resolve: {
      auth: function ($q, $state, $stateParams, $location, AccountRepo, Security) {
        return AccountRepo.auth().then(function (user) {
          // If not logged in redirect to login with path preserved
          if (user === false) {
            $state.go('account.login', {redirect: $location.url()});
            return $q.reject('not logged in');
          }

          // If account is set in path, set it in the SecurityService otherwise redirect to change account
          if ($stateParams.account) {
            Security.setAccount($stateParams.account);
          } else {
            $state.go('account.changeaccount');
            return $q.reject('no account selected');
          }

          return user;
        });
      }
    },
    templateUrl: 'app/Main.tpl.html',
    controller: 'MainCtrl'
  });

how can we use resolve to protect a state ?
Suppose i create a auth service ! how to redirect if the auth fails in resolve ? i am trying to use $state.go inside resolve but it is not working ?

Not quite sure why but $state.go doesn't work inside a resolve (probably because it's already in the middle of a state change). You can use $location.path() instead.