更新ObservableCollection列表时发生未处理的异常

Question

im trying to load data from a webservice into a sfDataGrid using parameters i get from OnNavigatedTo

first it loads a dummy row into the grid because i can´t get the data OnNavigatedTo yet then it runs the OnNavigatedTo and gets the data from the webservice and gose into adding the data to the ObservableCollection that the grid has ItemsSource set to

using Prism.Commands;
using Prism.Mvvm;
using Prism.Navigation;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;

namespace PrismUnityApp2.ViewModels
{

public class HovedsideViewModel : BindableBase, INotifyPropertyChanged , INavigationAware
{
    MobileService.VaksServiceClient ws = new MobileService.VaksServiceClient();
    INavigationService _navigationService;
    public event PropertyChangedEventHandler PropertyChanged;


    private DateTime selectedDate;
    public DateTime SelectedDate
    {
        get { return selectedDate; }
        set
        {
            if (selectedDate != value)
            {
                selectedDate = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this,
                        new PropertyChangedEventArgs("SelectedDate"));
                }
            }
        }
    }

    private DateTime minimumSelectedDate;
    public DateTime MinimumSelectedDate
    {
        get { return minimumSelectedDate; }
        set
        {
            if (minimumSelectedDate != value)
            {
                minimumSelectedDate = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this,
                        new PropertyChangedEventArgs("MinimumSelectedDate"));
                }
            }
        }
    }

    public DelegateCommand Navigatetest { get; private set; }
    public DelegateCommand NavigateToBestilling { get; private set; }

    public string latitude { get; set; }
    private string _longitude;

    public string longitude
    {
        get { return _longitude; }
        set
        {
            _longitude = value;
            ws.GetToemmeDatasAsync(latitude, longitude, DateTime.Today.AddDays(-7)); // needs to be set to the 1 of november as its the only day with data
        }
    }




    public HovedsideViewModel(INavigationService navigationService)
    {
        try
        {
            toemmeListe = new ObservableCollection<ToemningData>();
        }
        catch (Exception ex)
        {

            throw;
        }
          SetRowData();

        MinimumSelectedDate = DateTime.Now;
        SelectedDate = DateTime.Now.AddDays(1);

        _navigationService = navigationService;
        Navigatetest = new DelegateCommand(Navigate);
        NavigateToBestilling = new DelegateCommand(_NavigateToBestilling);
    }



    private void _NavigateToBestilling()
    {
        _navigationService.NavigateAsync("Bestilling");
    }

    private void Navigate()
    {
        _navigationService.NavigateAsync("Scanner");
    }

    public void OnNavigatedFrom(NavigationParameters parameters)
    {

    }

    public void OnNavigatedTo(NavigationParameters parameters)
    {
        latitude = parameters["lat"].ToString();  // if the phone inst near latitude "55,7070631" and longitude "12,4235393" then remove the part where it gets data from parameters
        longitude = parameters["lon"].ToString();  // and use the two lines under this
        //latitude = "55,7070631";
        //longitude = "12,4235393";
    }

    public void SetRowData()
    {
        ws.GetToemmeDatasCompleted += Ws_GetToemmeDatasCompleted;
        ws.GetToemmeDatasAsync(latitude, longitude, DateTime.Today.AddDays(-7));   // needs to be set to the 1 of november as its the only day with data 
    }


    private void Ws_GetToemmeDatasCompleted(object sender, MobileService.GetToemmeDatasCompletedEventArgs e)
    {

        int numOfRows = 0;
        try
        {
            numOfRows = e.Result.Count();   // awdawd
        }
        catch (Exception)
        {
            numOfRows = 0;
        }
        //ToemmeListe.Clear();
        if (numOfRows > 0)
        {
            foreach (var toemming in e.Result)
            {
                ToemningData _ToemningData = new ToemningData();
                _ToemningData.Fraktion = toemming.Fraktion;
                _ToemningData.ToemmeID = toemming.ToemmeId;
                _ToemningData.Type = toemming.type;
                _ToemningData.Vaegt = toemming.weight;
                ToemmeListe.Add(_ToemningData);  // failes when it trys to add a new row   // An unhandled exception occured.
            }
        }
        else  // if there isnt any data then set one row saying that there isnt any data
        {
            ToemningData _ToemningData = new ToemningData();  
            _ToemningData.Fraktion = "Igen Data";
            _ToemningData.ToemmeID = -1;
            _ToemningData.Type = "Igen Data";
            _ToemningData.Vaegt = "Igen Data";
            ToemmeListe.Add(_ToemningData);
        }
    }

    private ObservableCollection<ToemningData> toemmeListe;

    public ObservableCollection<ToemningData> ToemmeListe
    {
        get { return toemmeListe; }
        set { this.toemmeListe = value; }
    }

    }
}

xaml

    <?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             xmlns:sfgrid="clr-namespace:Syncfusion.SfDataGrid.XForms;assembly=Syncfusion.SfDataGrid.XForms"
             x:Class="PrismUnityApp2.Views.Hovedside"
             xmlns:sys="clr-namespace:System;assembly=mscorlib"
             Title="test">

  <ContentPage.Content>

    <Grid>
      <Grid.RowDefinitions>
        <RowDefinition Height="20" />
        <RowDefinition Height="50"/>
        <RowDefinition  Height="*" />
        <RowDefinition Height="50" />
        <RowDefinition Height="50" />
      </Grid.RowDefinitions>

      <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="*"/>
      </Grid.ColumnDefinitions>

      <DatePicker Date="{Binding SelectedDate,Mode=TwoWay}" MinimumDate="{Binding MinimumSelectedDate,Mode=TwoWay}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" HorizontalOptions="Center">
        <DatePicker.Format>yyyy-MM-dd</DatePicker.Format>
      </DatePicker>



      <sfgrid:SfDataGrid x:Name="dataGrid"
                  AutoGenerateColumns="False"
                  ColumnSizer="Star"
                  SelectedIndex="1"
                  SelectionMode="Single"
                 ItemsSource="{Binding ToemmeListe}"
                   AllowSorting="True"
                          Grid.Row="2" 
                          Grid.Column="0" Grid.ColumnSpan="3"
                         >

        <sfgrid:SfDataGrid.Columns x:TypeArguments="syncfusion:Columns">
          <sfgrid:GridTextColumn HeaderText="Fraktion"
                                     MappingName="Fraktion" />     <!--Fraktion-->                                               
          <sfgrid:GridTextColumn HeaderText="Type"
                                     MappingName="Type" />          <!--Type--> 
          <sfgrid:GridTextColumn HeaderText="Vaegt"
                                   MappingName="Vaegt" />     <!--vægt-->
        </sfgrid:SfDataGrid.Columns>

      </sfgrid:SfDataGrid>

      <Button Text="Scan" FontSize="24" Grid.Row="3" Grid.Column="0" Command="{Binding Navigatetest}"/>
      <Button Text="Mail" FontSize="24" Grid.Row="3" Grid.Column="2"/>
      <Button Text="Manuel" FontSize="24" Grid.Row="4" Grid.Column="0" Grid.ColumnSpan="3" HorizontalOptions="Center" Command="{Binding NavigateToBestilling}"/>

    </Grid>


  </ContentPage.Content>

</ContentPage>

when it gets in the Ws_GetToemmeDatasCompleted after getting the data from the webservice it stops at ToemmeListe.Add(_ToemningData); in

foreach (var toemming in e.Result)
            {
                ToemningData _ToemningData = new ToemningData();
                _ToemningData.Fraktion = toemming.Fraktion;
                _ToemningData.ToemmeID = toemming.ToemmeId;
                _ToemningData.Type = toemming.type;
                _ToemningData.Vaegt = toemming.weight;
                ToemmeListe.Add(_ToemningData);  // failes when it trys to add a new row   // An unhandled exception occured.
            }

and says An unhandled exception occured.

Any help is deeply appreciated

im using visual studio 2015 community, xamarin, prism v6.2.0 Xam.Plugin.Geolocator v3.0.4 and Syncfusion mainly running it on a sony xperia z3 with Android version 6.0.1 and using API 23

Dropbox link to the full project

The solution to i went with

    using Prism.Commands;
using Prism.Mvvm;
using Prism.Navigation;
using PrismUnityApp2.ProxyClient;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.ServiceModel;
using System.Threading.Tasks;

namespace PrismUnityApp2.ViewModels
{


    public class HovedsideViewModel : BindableBase, INotifyPropertyChanged, INavigationAware
    {
        MobileService.VaksServiceClient ws = new MobileService.VaksServiceClient();
        INavigationService _navigationService;
        public event PropertyChangedEventHandler PropertyChanged;


        private DateTime selectedDate;
        public DateTime SelectedDate
        {
            get { return selectedDate; }
            set
            {
                if (selectedDate != value)
                {
                    selectedDate = value;

                    if (PropertyChanged != null)
                    {
                        PropertyChanged(this,
                            new PropertyChangedEventArgs("SelectedDate"));
                    }
                }
            }
        }

        private DateTime minimumSelectedDate;
        public DateTime MinimumSelectedDate
        {
            get { return minimumSelectedDate; }
            set
            {
                if (minimumSelectedDate != value)
                {
                    minimumSelectedDate = value;

                    if (PropertyChanged != null)
                    {
                        PropertyChanged(this,
                            new PropertyChangedEventArgs("MinimumSelectedDate"));
                    }
                }
            }
        }

        public DelegateCommand Navigatetest { get; private set; }
        public DelegateCommand NavigateToBestilling { get; private set; }

        public string latitude { get; set; }
        private string _longitude;

        public string longitude
        {
            get { return _longitude; }
            set { _longitude = value; }
        }




        public HovedsideViewModel(INavigationService navigationService)
        {

            SetRowData(); // initialize the grid

            MinimumSelectedDate = DateTime.Now;  // this one will be revmoved later. its fine that they can go back and watch but not edite it
            SelectedDate = DateTime.Now.AddDays(1);

            _navigationService = navigationService;
            Navigatetest = new DelegateCommand(Navigate);
            NavigateToBestilling = new DelegateCommand(_NavigateToBestilling);
        }



        private void _NavigateToBestilling()
        {
            _navigationService.NavigateAsync("Bestilling");
        }

        private void Navigate()
        {
            _navigationService.NavigateAsync("Scanner");
        }

        public void OnNavigatedFrom(NavigationParameters parameters)
        {

        }

        public async void OnNavigatedTo(NavigationParameters parameters)
        {
            Xamarin.Forms.Device.BeginInvokeOnMainThread(async () =>
            {
                latitude = parameters["lat"].ToString();  // if the phone inst near latitude "55,7070631" and longitude "12,4235393" then remove the part where it gets data from parameters
                longitude = parameters["lon"].ToString();  // and use the two lines under this
                                                           //latitude = "55,7070631";
                                                           //longitude = "12,4235393";
                var ggg = await CallService(new MobileService.Toemning { Latitude = latitude, Longitude = longitude, date = new DateTime(2016, 11, 1) });  // the date is set to a day where there is data
                var justatest = 0;
                ToemmeListe.Clear();
                foreach (var toemming in ggg)
                {

                    ToemningData _ToemningData = new ToemningData();
                    _ToemningData.Fraktion = toemming.Fraktion;
                    _ToemningData.ToemmeID = toemming.ToemmeId;
                    _ToemningData.Type = toemming.Type;
                    _ToemningData.Vaegt = toemming.Weight;
                    ToemmeListe.Add(_ToemningData);
                }

            });

        }

        MobileService.Toemning ToWCFServiceTodoItem(MobileService.Toemning item)
        {
            return new MobileService.Toemning
            {
                Fraktion = item.Fraktion,
                ToemmeId = item.ToemmeId,
                Type = item.Type,
                date = item.date,
                Latitude = item.Latitude,
                Longitude = item.Longitude,
                Weight = item.Weight
            };
        }


        public async Task<ObservableCollection<MobileService.Toemning>> CallService(MobileService.Toemning item)
        {
            var todoItem = ToWCFServiceTodoItem(item);
            MobileService.VaksServiceClient client = new MobileService.VaksServiceClient(new BasicHttpBinding(), new EndpointAddress("http://vf-kssweb2/Vaks2Svc/VaksService.svc?singleWsdl"));
            var t = Task<ObservableCollection<MobileService.Toemning>>.Factory.FromAsync(
                    ((MobileService.IVaksService)client.InnerChannel).BeginGetToemmeDatas,
                    ((MobileService.IVaksService)client.InnerChannel).EndGetToemmeDatas,
                     todoItem, TaskCreationOptions.None);
            return await t;
        }

        public void SetRowData()
        {

            try
            {
                toemmeListe = new ObservableCollection<ToemningData>();
            }
            catch (Exception ex)
            {

                throw;
            }

            ToemningData _ToemningData = new ToemningData();
            _ToemningData.Fraktion = "Igen Data";
            _ToemningData.ToemmeID = -1;
            _ToemningData.Type = "Igen Data";
            _ToemningData.Vaegt = "Igen Data";
            ToemmeListe.Add(_ToemningData);
        }

        private ObservableCollection<ToemningData> toemmeListe;

        public ObservableCollection<ToemningData> ToemmeListe
        {
            get { return toemmeListe; }
            set { this.toemmeListe = value; }
        }




    }
}

Answer 1

You're not allowed to change an ObservableCollection from any thread but the one that created it (normally the UI thread). You have to marshall the Add operations back to that thread, preferably using async / await :

private async Task FillList()
{
    // on the ui thread here...

    // leaving it to wait for i/o operations or cpu-bound work on a threadpool thread
    var data = await _server.FetchData();

    // here we are back on the ui thread and update the list
    foreach (var item in data)
        TransformItemAndAddItToTheList( item );
}