Question

I am gathering data in a separate Task and I want to data bind the result to a GUI component using an ObservableCollection<>. So my code goes something like this:

private ObservableCollection<MyItem> _items;
public ObservableCollection<MyItem> Items
{
  get { return _items; }
  set
  {
    if (_items.Equals(value))
    {
      return;
    }
    _items = value;
    RaisePropertyChanged("Items");
  }
}

private void LoadData()
{
  Task task = Task.Factory.StartNew(() =>
  {
    ObservableCollection<MyItem> itms = _htmlParser.FetchData(...);

    Dispatcher.CurrentDispatcher.Invoke((Action)delegate
    {
      Items = itms;
    });
  });
}

The data is fetched from a component doing some HTTP requests. The error I am getting is:
Must create DependencySource on same Thread as the DependencyObject.

I am using the MVVM Light toolkit framework. I also tried to send the result as a message, but that ended up in the same error message. Any ideas or pointers?

EDIT: Here’s the issue:

public class MyItem
{
  public string Id { get; set; }
  public string Name { get; set; }
  public BitmapImage Image { get; set; }  // <--- A big No No because it inherits from the DependencyObject
  public Uri Uri { get; set; }
}

I changed the BitmapImage to a byte[] data type.

Answer 1

The exception you’re getting (“Must create DependencySource on same Thread as the DependencyObject”) indicates that something’s being created on a background thread and used on the UI thread. Are there any UI controls being created and stored in the collection for use by the UI?

I see that the ObservableCollection itself is being created on a background thread, but I don’t think that’s the issue — unfortunately I’m not in the office to code and confirm that.