WPF ObservableCollection: How to add a blank line in one form's combobox, but not actually affect the ObservableCollection?

Question

I have a static ObservableCollection in a Data Repository class. I use it to populate a combobox on one of my forms (which needs to be able to include a blank line which represents NULL).

I use the same ObservableCollection to populate a DataGrid, so I don't want the blank item in the actual ObservableCollection. How do I actually do this?

Oh, and the reason I want to do this is so that if I have both forms open and I delete an item from the ObservableCollection it should reflect that in both of the lists.

Answer 1

1. You can't select null value in combobox.
2. You have to use blank item to display it in the control.

3. I have the same problem and i'm using this solution in my current project:

   public class ObservableCollectionCopy<T> : ObservableCollection<T>
   {
   public ObservableCollectionCopy(T firstItem, ObservableCollection<T> baseItems)
   {
       this.FirstItem = firstItem;
       this.Add(firstItem);
       foreach (var item in baseItems)
           this.Add(item);
       baseItems.CollectionChanged += BaseCollectionChanged;
   }
   
   
   public T FirstItem { get; set; }
   
   
   private void BaseCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
   {
       if (e.NewItems != null)
           foreach (var newItem in e.NewItems.Cast<T>().Reverse())
               this.Insert(e.NewStartingIndex + 1, newItem);
       if (e.OldItems != null)
           foreach (var oldItem in e.OldItems.Cast<T>())
               this.Remove(oldItem);
   }
   }
   

New collection has one-way binding to base collection:

this.SelectableGroups = new ObservableCollectionCopy<GroupModel>(
                new GroupModel{Id = -1, Title = "Any group"},
                this.GroupsCollection);

Filtering:

if (this.selectedGroup != null && this.selectedGroup.Id != -1)
    this.MyCollectionView.Filter = v => v.SomeItem.GroupId == this.selectedGroup.Id;
else this.MyCollectionView.Filter = null;

Answer 2

You might be able to use the TargetNullValue property of a binding declaration to declare output for a null value.

<ComboBox ItemsSource={Binding Path=Collection, TargetNullValue="-------"}/>