Item added to ObservableCollection is successfull but still throws exception - xaml

Question

``I am utterly confused on why I am getting this error:
System.NullReferenceException occurred
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=Xamarin.Forms.Platform.UAP
StackTrace:
at Xamarin.Forms.Platform.UWP.WindowsBasePlatformServices.get_IsInvokeRequired()
at Xamarin.Forms.ListProxy.OnCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
at Xamarin.Forms.ListProxy.WeakNotifyProxy.OnCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
at System.Collections.ObjectModel.ObservableCollection1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
at System.Collections.ObjectModel.ObservableCollection1.InsertItem(Int32 index, T item)
at System.Collections.ObjectModel.Collection`1.Add(T item)
at ViewModels.ScanBadgesViewModel.Add(BadgeScan result)
InnerException:
The error results from the following line:
EmployeeIds.Add(badge.EmployeeId)
NOTE:
This error is observed on a Xamarin.Forms Windows 10 Universal app (Preview).
If I comment out the ListView element inside the XAML, then I no longer receive the null exception.
If I only comment out the ItemTemplate of the ListView, then I still observe the null exception.
XAML:
<Grid Grid.Row="4" Grid.RowSpacing="3" Grid.ColumnSpacing="3" BackgroundColor="Silver">
<ListView ItemsSource="{Binding EmployeeIds}" SelectedItem="{Binding SelectedEmployeeId}"
BackgroundColor="Black">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<ViewCell.View>
<Label Text="{Binding Value}" TextColor="Yellow" XAlign="Start" />
</ViewCell.View>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</Grid>
ViewModel Property:
ObservableCollection<EmployeeId> _employeeIds = new ObservableCollection<EmployeeId>();
public ObservableCollection<EmployeeId> EmployeeIds
{
get { return _employeeIds; }
set
{
if (_employeeIds != value)
{
_employeeIds = value;
OnNotifyPropertyChanged();
}
}
}
Entities:
public class EmployeeId
{
public EmployeeId(string employeeId) { Value = employeeId; }
public string Value { get; }
}
public class BadgeScan
{
public BadgeScan(string employeeId) { EmployeeId = new EmployeeId(employeeId); }
public BadgeScan(BadgeScan source, Predicate<string> validate) : this(source.EmployeeId.Value)
{
IsValid = validate.Invoke(source.EmployeeId.Value);
}
public EmployeeId EmployeeId { get; }
public bool IsValid { get; }
}
UPDATE
This line of code alters the behavior of my ObservableCollection.Add operation:
var administeredScan = new BadgeScan(result, validate);
The line simply creates a copy of the object.
var validate = DependencyManager.Resolve(typeof(Predicate<string>)) as Predicate<string>;
var administeredScan = new BadgeScan(result, validate);
var canAdd = CanAdd(administeredScan) &&
ScanMode == Entities.ScanMode.Add;
if (canAdd) Add(administeredScan);
break;
This still throws an exception even though an item is added to the collection:
Add(administeredScan);
However, this succeeds:
var result = obj as BadgeScan;
Add(result);
So creating a copy of an object to add to my observable fails. But adding the original object succeeds.

Answer 1

This is a Xamarin.Forms bug in regards to Windows Universal Platform (i.e. Windows 10).
Instead of invoking the Add operation on the ObservableCollection that my UI is data-bound to, I just create a new ObservableCollection for each Add operation and pass in a collection within the constructor.
Workaround:
_employeeIdsHack.Add(administeredScan.EmployeeId);
EmployeeIds = new ObservableCollection<EmployeeId>(_employeeIdsHack);

Answer 2

Your properties are read only. I would change the properties to have a private set
public EmployeeId EmployeeId { get; private set; }
public bool IsValid { get; private set;}

Answer 3

Your second BadgeScan constructor doesn't initialize the EmployeeId property. In this line:
EmployeeIds.Add(badge.EmployeeId);
...it would seem you might be adding a null object to the collection. That in itself shouldn't be a problem, but you're using EmployeeId.Value in a data binding. My guess is the NRE is connected with this.
Update: #ScottNimrod says it's a Xamarin bug, in which case the NRE might not be connected with this after all. Even so: Is the non-setting of the EmployeeId intentional?