CODEWITHSUNDEEP
c#xamarinexceptionxamarin.forms
John
John

Reputation: 78

Handling exceptions in xamarin

I've encountered a few dificulties trying to handle some errors in the best way possible. One of my cases for example is NullReferenceException.

To be more clear let me explain it in a few words. When I make a call to a server to receive some pieces of information, in some cases the server might have some problems and it will return of course null.

What I've done is I display an alert to let the user know that he can try later again. After this I try to send him in the previous page for example. And after all this my app still crushes.

What I'd like to do is to simply display the alert and then let the user stay in the same page without the application crushing.

These are a few pieces of my code:

tasks.cs

 public async Task<List<Idea>> GetIdeaAsync(string accesToken)
  {
      List<Idea> ideas = null;
      try
      {
         var client = new HttpClient();
         client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accesToken);

         var json = await client.GetStringAsync("http://www.getdata.de/api/ideas/");
         var ideas = JsonConvert.DeserializeObject<List<Idea>>(json);
      }
      catch (Exception ex)
      {
        await Application.Current.MainPage.DisplayAlert("Server Error", "There has been an server error. Please try later.", "OK");
              if (ideas == null)
              {
                  await Application.Current.MainPage.Navigation.PopAsync(); //actually I would like to stay in the same page
              }
      }
      return ideas;
  }

view.xaml.cs

private async void Button_Clicked(object sender, EventArgs e)
 {
    Tasks ts = new Tasks();
    var ideas = await ts.GetIdeasAsync();
    if (ideas == null)
    {
        Debug.WriteLine("hello");
        //do nothing since the display alert is already shown
    }
    else
    {
      //code here
    }

I would really appreciate if anyone can guide me to a "best-practice" approach. Thanks :)

Upvotes: 0

Views: 2960

Answers (2)

John
John

Reputation: 78

The problem it was pretty obvious actually since I after catch-ing the exception I would later continue with the code. So what I did was :

public async Task<List<Idea>> GetIdeaAsync(string accesToken)
{
  List<Idea> ideas = null;
  try
  {
     var client = new HttpClient();
     client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accesToken);

     var json = await client.GetStringAsync("http://www.getdata.de/api/ideas/");
     var ideas = JsonConvert.DeserializeObject<List<Idea>>(json);
  }
  catch (Exception ex)
  {
    await Application.Current.MainPage.DisplayAlert("Server Error", "There has been an server error. Please try later.", "OK");
          if (ideas == null)
          {
              //actually I would like to stay in the same page
              return null; //-- added this line 
          }
  }
   return ideas;
}

Maybe it's not the best idea but it's working for me. Any other approach would be highly appreciated. :)

Upvotes: 0

sme
sme

Reputation: 4153

You are declaring ideas in the try block, and then trying to access it in the catch block, where it is out of scope. (Visual Studio should give an Intellisense error)

Also, whenever manipulating the UI, you should always do it on the main thread. so move your DisplayAlert() code into 

Device.BeginInvokeOnMainThread(async () => 
{
    // await DisplayAlert(); move it into here
});

In addition, any PopAsync or PushAsync calls should also be done on the main UI thread. But calling PopAsync after an asynchronous call to an API not be a good idea, as the user may have already pressed the back button by the time the call returns.

As for the NullReferenceException, check to see if json is null before passing it to the DeserializeObject() function.

Upvotes: 1

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