Cant insert a object into a silverlight databound combo box

Question

Hi

Until recently I had a combo box that was bound to a Linq queried IEnumerable of a DataService.Obj type in the bind method, and all worked fine

private IEnumerable<DataService.Obj> _GeneralList;
private IEnumerable<DataService.Obj> _QueriedList;


private void Bind()
{
    _GeneralList = SharedLists.GeneralList;
    _QueriedList = _GeneralList.Where(q =>q.ID >1000);

    cmbobox.ItemsSource = _QueriedList;
}

Then I had to change the method to insert a new obj and set that object as the default obj and now I get a "System.NullReferenceException: Object reference not set to an instance of an object" exception. I know this has to do with inserting into a linq queried ienumerable but I cant fix it. Any help will be gratefully received.

private IEnumerable<DataService.Obj> _GeneralList;
private IEnumerable<DataService.Obj> _QueriedList;

private void Bind()
{
    _GeneralList = SharedLists.GeneralList;
    _QueriedList = _GeneralList.Where(q =>q.ID >1000);

    cmbobox.ItemsSource = _QueriedList;

    DataService.Obj info = new DataService.Obj();
    info.ID = "0";
    (cmbobox.ItemsSource as ObservableCollection<DataService.Obj>).Insert(0,info);
    cmbobox.SelectedIndex = 0;
}

Thanks in advance

Answer 1

Try inserting your new item before binding to the control.

private IEnumerable<DataService.Obj> _GeneralList; 
private IEnumerable<DataService.Obj> _QueriedList; 

private void Bind() 
{ 
    _GeneralList = SharedLists.GeneralList; 
    _QueriedList = _GeneralList.Where(q =>q.ID >1000).ToList(); 

    DataService.Obj info = new DataService.Obj(); 
    info.ID = "0"; 
    _QueriedList.Insert(0,info); 

    cmbobox.ItemsSource = _QueriedList; 

    cmbobox.SelectedIndex = 0; 
} 

Answer 2

This expression :-

(cmbobox.ItemsSource as ObservableCollection<DataService.Obj>)

Will return null. The ItemsSource is what ever was assigned to it. In this case a LINQ supplied object that implements IEnumerable<DataService.Obj> hence the as returns null (LINQ knows nothing of ObservableCollection<T> and certainly doesn't use it).

See this question to create a ToObservableCollection extension method.

That said I'm gonna guess that your actual objective is to have "<N/A>" element at the top of the list right? If so try this:-

 cmbobox.ItemsSource=  Enumerable.Repeat(new DataService.Obj() {ID = 0}, 1)
                     .Union(_QueriedList));

This inserts as single DataService.Obj instance with an ID of 0 as the first item of an IEnumerable<DataService.Obj followed by all the items in _QueriedList. No need to attempt to insert a value into collection with this approach.