Error when implementing the contract in Laravel 5.2

I am following this link to implement it

I did below steps to implement the Contract in my existing class.

Below is the class where I will write some logic also before sending it to controller

namespace App\Classes\BusinessLogic\Role;
use App\Classes\DatabaseLayer\Role\RoleDb;
use App\Classes\Contract\Role\IRole;

class RoleBL implements IRole {

    public function All() {
        return (new RoleDb())->All();
    }
}

Database Function

namespace App\Classes\DatabaseLayer\Role; 

class RoleDb {
    public function All() {
        $Roles = \App\Models\Role\RoleModel
                ::all();
        return $Roles;
    }
}

Interface

namespace App\Classes\Contract\Role;

interface IRole {
    public function All();
}

Service Provider class

namespace App\Providers\Role;

class RoleServiceProvider extends \Illuminate\Support\ServiceProvider {

    public function register()
    {
        $this->app->bind('App\Classes\Contract\Role\IRole', function($app){
            return new \App\Classes\BusinessLogic\Role\RoleBL($app['HttpClient']);
        });
    }
}

Finally in config/app.php in provider wrote below line.

App\Providers\Role\RoleServiceProvider::class

Controller - Constructor

protected $roles;
public function __construct(\App\Classes\Contract\Role\IRole $_roles) {
    parent::__construct();
    $roles = $_roles;
}

Controller Action method

public function index(IRole $roles) {
    $RoleTypes = $roles->All();
}

So far everything works fine if I keep Interface as parameter in method.

if I try to use the variable $roles in index method and remove the variable, it is always null.

Please guide me if I missed anything?

via Chebli Mohamed