Autofac 抛出 NullReferenceException 错误

有一类：

 public class FileRepository : IFileRepository
 {
      private readonly string _connectionString;
      private readonly IUserStore _usersRepository;

      public FileRepository(string connectionString, IUserStore user)
      {     
           _connectionString = connectionString;      
           _usersRepository = user;
      }
      public void GetInfo()
      {
          _usersRepository.Get();
      }
 }

第二类：

 public class ShareRepository : IShareRepository
 {
      private readonly string _connectionString;
      private readonly IFileRepository _fileRepository;

      public ShareRepository(string connectionString, IFileRepository 
             fileRepository)
      {
           _connectionString = connectionString;
           _fileRepository = fileRepository;
      }

      public void GetTest()
      {
           _fileRepository.GetInfo()
      }          
 }

执行时调用了类中_fileRepository.GetInfo()的一个方法，该方法又使用DI并执行， 这里出现错误：GetInfo()FileRepository_usersRepository.Get()

  System.NullReferenceException: "Ссылка на объект не указывает на экземпляр объекта."

我的 autofac 配置：

  var connectionString = "......."

  builder.Register(ctx => new FileRepository(connectionString, new 
      UserRepository(connectionString)))
            .As<IFileRepository>();

  builder.Register(ctx => new ShareRepository(connectionString,
       new FileRepository(connectionString))).As<IShareRepository>();

添加：

 builder.RegisterType<UserRepository>().As<IUserStore>.InstancePerRequest()