public interface IService
    {
        Task<SessionModel> GetData(string id);
    }

    /// Provides a group of schedulers available to be used
    public interface ISchedulers
    {
       IScheduler Default {get;}
       IScheduler Dispatcher {get;}
    }

    public class MyVm
    {
        IService service;

        public MyVm(ISchedulers schedulers, IService service)
        {
            this.service = service;
            this.session = this.WhenAnyValue(x => x.SessionId)
                               .SelectMany(SearchSession)
                               .ObserverOn(schedulers.Default)
                               .ToProperty(this, x => x.Session);
        }

        private async Task<SessionModel> SearchSession(string id)
        {
            return await this.service.GetData(id);
        }
    }