public static IObservable<T> DownloadAsync<T>( this WebClient client, Uri address, Func<object,T> selector)
{ 
    return Observable.Create<T>(observer => 
                       { 
                            Guid token = Guid.NewGuid();
                            OpenReadCompletedEventHandler handler = (sender, args) =>
                           { 
                                if (args.UserState != token) return;
                                if (args.Cancelled) observer.OnCompleted(); 
                                else if(args.Error !=null) observer.OnError(args.Error); 
                                else { observer.OnNext(selector(args.Result)); observer.OnCompleted();  } 
                      }; 
                      client.OpenReadCompleted += handler; 
                     try { 
                           client.OpenReadAsync(address, token); 
                      } 
                     catch (Exception ex)
                    { 
                        observer.OnError(ex); 
                    } 
                    return () => client.OpenReadCompleted -= handler; }
             );
}

public static T DefaultSelector<T>(object result)
{
       return result as T;
}

public Repository<T> : IRepository<T> 
{
     public Observable<T> LoadAsync<T>(Uri uri)
     {
           return LoadAsync<T>(uri, DefaultSelector);
     }

    protected Observable<T> LoadAsync<T>(Uri uri, Func<object,T> selector)
    {
          return new WebClient().DownloadAsync(uri, selector);
    }
}

public ViewModel<T> : IViewModel<T>
{
       private IRepository<T> _repository;

      public ViewModel(IRepository<T> repository)
      {
          _repository = repository;
      }

       public T Model
       {
            get { .......... }
            set  {  ...........}
        }

      public void Load(Uri uri)
      {
           _repository.LoadAsync<T>(uri).SubscribeOnDispatcher(
                                  e = > {  Model = e; }, 
                                  ex => { //обработчик ошибок }
             );
       }