Access List<Object> in task async

Question

I have this code to load and count data from API server;

class TestNetWork
  {
    private Task taskFillPicker;
    private List<CityItemDB> itemsCity;
    private CustomPicker cpCity; 

    public async Task FillPicker()
      {
          try {
            JObject res = await SuperFUNC.GET_CITY_ACTIVE_SENDER();
            if(res == null){
              //null
            }else{
              string message = res["message"].ToString();
              if(message.Equals("Success")){

                itemsCity.Clear();
                cpCity.Items.Clear();

                JArray data = (JArray)res["data"];
                int count = data.Count;
                for (int i = 0; i < count; i++) {
                  CityItemDB node = new CityItemDB();
                  node.cityId = Int32.Parse(data[i]["cityId"].ToString());
                  node.cityName = data[i]["cityName"].ToString();

                  itemsCity.Add(node);
                  cpCity.Items.Add(node.ToString());
                }
              }else{
              //null
              }
            }
          } catch (Exception ex) {
            Debug.WriteLine (TAG + " : " + ex.StackTrace);  
          }
      }

    public TestNetWork()
      {
          this.itemsCity = new List<CityItemDB> ();

          this.cpCity = new CustomPicker {
            HeightRequest = 40,
            TextColor = Color.FromHex("#5a5a5a"),
            Title = "City Choose",
          };

          taskFillPicker = FillPicker ();

          Debug.WriteLine (COUNT + " : " + itemsCity.Count);  
      }
  }

But console print me COUNT : 0, I'm sure code get and parse json from internet is correct, picker show full data but List<CityItemDB> itemsCity count 0.

Thank for read, sorry my english not good!

thumbmunkeys · Accepted Answer · 2015-09-18 09:24:26Z

1

You need to await the task, otherwise execution might continue before FillPicker has completed:

taskFillPicker = await FillPicker ();

As this code is in a constructor where await is not possible, I suggest moving it to a separate async method:

  public async Task Init()
  {          
      taskFillPicker = await FillPicker ();

      Debug.WriteLine (COUNT + " : " + itemsCity.Count);  
  }

You have to write a little bit more code to construct the object now:

 var n = new TestNetWork();
 await n.Init();

edited Sep 18, 2015 at 9:24

answered Sep 18, 2015 at 9:11

thumbmunkeys

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4 Comments

bulubuloa Over a year ago

can't use await in default contructor, sorry i edit the code in question

thumbmunkeys Over a year ago

@HoangQBH yes - my answer suggests a solution for that. Do you have troubles with understanding it?

bulubuloa Over a year ago

i know, but i use xamarin, defaule constructor can't be async

thumbmunkeys Over a year ago

Please read my answer carefully :) You have to create a separate method that is async and then call this method after you called the constructor

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