Symfony 2.7：无法加载类型

I am creating a simple contact form. My custom type starts this way:

<?php

namespace MyCompany\AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\Extension\Core\Type\TextareaType;
use Symfony\Component\Form\Extension\Core\Type\EmailType;
use Symfony\Component\Validator\Constraints\Email;
use Symfony\Component\Validator\Constraints\NotBlank;

class ContactType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {

... and my controller starts this way:

<?php

namespace MyCompany\AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

class ContactController extends Controller
{
    public function contactAction(Request $request)
    {
        // Create the form according to the FormType created previously.
        // And give the proper parameters
        $form = $this->createForm('MyCompany\AppBundle\Form\ContactType',null,array(
            // To set the action use $this->generateUrl('route_identifier')
            'action' => $this->generateUrl('myapplication_contact'),
            'method' => 'POST'
        ));

... but I find that my application is unable to load the type. I get the following exception message:

Could not load type "MyCompany\AppBundle\Form\ContactType" 500 Internal Server Error - InvalidArgumentException

What can I do to start debugging this?

Since you are using Symfony 2.7 the proper way to create a form in the controller using a form type class is:

<?php

namespace MyCompany\AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

use MyCompany\AppBundle\Form\ContactType; // add this

class ContactController extends Controller
{
    public function contactAction(Request $request)
    {
        // Create the form according to the FormType created previously.
        // And give the proper parameters
        $form = $this->createForm(new ContactType(), null, array(
            // To set the action use $this->generateUrl('route_identifier')
            'action' => $this->generateUrl('myapplication_contact'),
            'method' => 'POST'
        ));

Refer to the symfony documentation

In Symfony 2.7 you will have to send the instance like this in $this->createForm

new MyCompany\AppBundle\Form\ContactType