public abstract class WPFViewModelCommon : WPFViewModelBase
   {

      //  public abstract UserVM User{get; set;} // так + override в дочернем классе - работает
        private UserVM _user;
        public override UserVM User
       {
         get { return _user; }
        set { _user = value; OnPropertyChanged("User"); }
       }

       private string _textblockcontent;
       public string TextBlockContent
       {
           get { return _textblockcontent; }
           set { _textblockcontent = value; OnPropertyChanged("TextBlockContent"); }
       }

       private string _buttoncontent;
       public string ButtonContent
       {
           get { return _buttoncontent; }
           set { _buttoncontent = value; OnPropertyChanged("ButtonContent"); }
       }

        private ImageSource _avataronform;
        public ImageSource AvatarOnForm
        {
            get { return _avataronform; }
            set
            {
                _avataronform = value; OnPropertyChanged("AvatarOnForm");
            }
        }

       public abstract ICommand FinalAction { get; set; }

       protected void LoadAvatarMethod()
       {
           IFileService fileService = new ImageFileService();
           IDialogService dialogService = new WPFClient.FileService.DialogService();

           try
           {
               if (dialogService.OpenFileDialog() == true)
               {
                   User.Avatar = fileService.Open(dialogService.FilePath);  // падает здесь
                   (fileService as ImageFileService).Dispose();
                   dialogService.ShowMessage("Файл загружен");
                   using (var stream = new FileStream(dialogService.FilePath, FileMode.Open))
                   {
                       AvatarOnForm = BitmapFrame.Create(stream, BitmapCreateOptions.None, BitmapCacheOption.OnLoad);
                       OnPropertyChanged("AvatarOnForm");
                   }
               }
               (dialogService as DialogService).Dispose();
           }

           catch (Exception ex)
           {
               dialogService.ShowMessage(ex.Message);
           }

       }
    }




    public class WPFViewModelRegistration : WPFViewModelCommon

{


    public string LoadButtonContent { get; set; }

    public ICommand LoadAvatarCommand { get; set; }
    public override ICommand FinalAction { get; set; }


    public WPFViewModelRegistration()
    {
        FinalAction = new RelayCommand(arg => RegistrationMethod());
        LoadAvatarCommand = new RelayCommand(arg => LoadAvatarMethod());
        TextBlockContent = "Окно регистрации";
        ButtonContent = "Зарегистрироваться";
        LoadButtonContent = "Загрузить аватар";
        User = new UserVM();
        if (client==null)
        {
            client = new HttpClient();
            HttpClientSettings();
        }
    }

    private async void RegistrationMethod()
    {

        //post query

        if (User.Login == null||User.Login == ""||User.Password == null || User.Password == "")
        {
            System.Windows.Forms.MessageBox.Show("Неверные данные. Не введен логин или пароль!");
            return;
        }

        HttpResponseMessage testresponse = await client.GetAsync(new StringBuilder()
                                                               .Append("api/userapi")
                                                               .Append("?")
                                                               .Append("login=")
                                                               .Append(User.Login)
                                                               .Append("&")
                                                               .Append("password=")
                                                               .Append(User.Password).
                                                               ToString());

        if (testresponse.StatusCode == HttpStatusCode.NotFound)
        {
            HttpResponseMessage response = await client.PostAsync("api/userapi", new StringContent(JsonConvert.SerializeObject(User)));
            if(response.StatusCode == HttpStatusCode.OK)
            {
                System.Windows.Forms.MessageBox.Show("Поздравляю! Вы успешно зарегистрировлаись в системе");
            }
            else
            {
                System.Windows.Forms.MessageBox.Show("Ошибка сервера. Повсторите ввод данных");
            }
        }
        else if (testresponse.StatusCode == HttpStatusCode.OK)
        {
            System.Windows.Forms.MessageBox.Show("такой пользователь уже есть в системе. Введите другие данные");
            return;
        }
        else
        {
            System.Windows.Forms.MessageBox.Show("Ошибка сервера. Повсторите ввод данных");
            return;
        }


    }


}