Best way to manage services and exceptions

Question

I have a mobile app who consumes a api service. I have a class to manage all the operations (get, post, put) who throws exceptions if the result is not 200.

The app was originaly designed to be mostly offline so my services where like this:

        public async Task<bool> GetMailList(int idUser)
        {
            try
            {
                string uri = GlobalSettings.Instance.MailEndpoint + idUser;
                List<MailDto> listaMailDto = await _requestProvider.GetAsync<List<MailDto>>(uri);                    
                await StoreListMailToDB(MapperMail.MapMailList(listaMailDto));
                return true;
            }
            catch (ServiceAuthenticationException ex) 
            {
                Log.CreateErrorLog("Service.35 - ServiceAuthenticationException \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.errorBadUser);
                return false;
            }
            catch (HttpRequestExceptionEx ex)
            {
                Log.CreateErrorLog("Service.40 - HttpRequestExceptionEx \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.errorNoConection);
                return false;
            }
            catch(Exception ex)
            {
                Log.CreateErrorLog("Service.45 - Exception \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.error);
                return false;
            }
        }

So the service gets the data, and stores it to Sqlite. If was all ok I return true, if no I return false. And in the screen I call to Sqlite to retrieve the data.

The problem comes now because we want to change it to a mostly online app. I want to keep the structure where the services manages the exception getting the data when the function ends. I want something like this but I don't want to return nulls

  public async Task<List<MailDto>> GetMailList(int idUser)
        {
            try
            {
                string uri = GlobalSettings.Instance.MailEndpoint + idUser;
                List<MailDto> listaMailDto = await _requestProvider.GetAsync<List<MailDto>>(uri);  
                List<MailBo> listMappead = MapperMail.MapMailList(listaMailDto)                 
                StoreListMailToDB(listMappead);
                return listMappead;
            }
            catch (ServiceAuthenticationException ex) 
            {
                Log.CreateErrorLog("Service.35 - ServiceAuthenticationException \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.errorBadUser);
                return null;
            }
            catch (HttpRequestExceptionEx ex)
            {
                Log.CreateErrorLog("Service.40 - HttpRequestExceptionEx \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.errorNoConection);
                return null;
            }
            catch(Exception ex)
            {
                Log.CreateErrorLog("Service.45 - Exception \n" + ex.ToString());
                DependencyService.Get<IDialogs>().ShowMessage(AppResources.error);
                return null;
            }
        }

Any idea?

so what _do_ you want to return when there is nothing to return? — Ewan, Mar 13 '18 at 12:40
I want a solution to have the services "like" this, but I don't want to return nulls. For example I was thinking on use cancelation Token, cancel the task on exception and Check if the task was succefull — Mulflar, Mar 13 '18 at 12:42

score 1 · Answer 1 · answered Mar 13 '18 at 12:59

Best practice when calling services and they fail is to throw exceptions and let them bubble up.

also: don't use global variables
also: don't reference your DI container directly

eg:

public async Task<List<MailDto>> GetMailList(int idUser)
{
    string uri = this.MailEndpoint + idUser;
    var data =  await _requestProvider.GetAsync<List<MailDto>>(uri); 
    return this.mapper.MapMailList(data);
}

async Task MyAppButton_click()
{
    try 
    {
        var data = await this.service.GetMailList(this.userId);
        await this.localRepo.StoreListMailToDB(data)
        mailControl.Data = data;

    }
    catch(Exception ex)
    {
        Log.CreateErrorLog("Service.45 - Exception \n" + ex.ToString());
        this.Dialogs.ShowMessage(ex.Message);
    }
}

Best way to manage services and exceptions

1 Answers1