从wp8中的可观察集合中删除项目

Question

I have an observable collection linked to a LongListItemSelector and this parts works fine.

I use the checkbox selection method, which builds a list in the background and this list is passed as a parameter when I click on my Delete button. In the Delete button method defined in my ViewModel, I have the following code:

foreach (FavoriteItemViewModel item in selectedFavorites)
{
    FavoriteItemViewModel itemToDeleted = this.Favorites
        .FirstOrDefault<FavoriteItemViewModel>(m =>
            m.Description.ToLowerInvariant() == item.Description.ToLowerInvariant());
    if (itemToDeleted != null)
        this.Favorites.Remove(itemToDeleted);
}

Where selectedFavorites is of type List<Object> and contains the items selected via a checkbox.

Each of the "object" is of type FavoriteItemViewModel . As the "object" contained in the list are slightly different from the one contained in my Favorites ObservableCollection, I make sure to first find the item in Favorites based on their Description as this should match.

If found, I delete it but I can this that the item is not actually being removed from Favorites observable collection.

I understand you cannot use a For Each to loop through an observable collection and try to remove an item directly, but I'm not doing that.

I've tried different scenarios but getting nowhere with it.

Any ideas what I'm doing wrong?

Thanks.

Answer 1

解决问题后，唯一可以做的就是使用removeall linq而不是其他列表。

Favorites.RemoveAll(a => selectedFavorites.Exists(w => ((FavoriteItemViewModel)w).Description == a.Description));

Answer 2

I figured out the problem... Couple of mistake on my part!

First problem: How my property Favorites was defined:

    public ObservableCollection<FavoriteItemViewModel> Favorites
    {
        get
        {
            return _favorites ?? (this.LoadFavorites(App.MainViewModel.Favorites));
        }
        set
        {
            _favorites = value;
        }
    }

The get part never actually set the internal variable, thus reloading the data over and over again from the Favorites collection held in my MainViewModel.

I changed it from:

return _favorites ?? (this.LoadFavorites(App.MainViewModel.Favorites));

to

return _favorites ?? (_favorites = this.LoadFavorites(App.MainViewModel.Favorites));

Doooh!!

The second problem:

So while the following is not directly removing the object from my observable collection

private void DeleteAction(object actionParameter)
{
    List<object> selectedFavorites = actionParameter as List<object>;

    foreach (FavoriteItemViewModel item in selectedFavorites)
    {
        FavoriteItemViewModel itemToDelete = 
        this.Favorites.FirstOrDefault<FavoriteItemViewModel>
        (m => m.Description.ToLowerInvariant() == item.Description.ToLowerInvariant());

        if (itemToDelete != null)
          this.Favorites.Remove(itemToDelete);
   }
 }

The parameter actionParameter passed via the CommandParameter ie List, is the List of selected objects from my LongListSelector (Custom one to handle MVVM). This List is directly affected whenever an item is selected or unselected.

I assumed that this parameter would have been passed by value but this doesn't appear to be the case and is in fact passed by reference.

So calling

this.Favorites.Remove(itemToDelete)

does not work either as the second I remove it from the observable collection, it also affect the local variable selectedFavorites list which was assigned the actionParameter variable and I get the typical error:

An exception of type 'System.InvalidOperationException' occurred in 
mscorlib.ni.dll but was not handled in user code

Additional information: Collection was modified; enumeration 
operation may not execute.`

If I use a for loop, while I don't get the error, assuming that I have 2 items selected, the second I remove the first item from the Favorites observable collection, selectedFavorites.count drops from 2 to 1 and causes my for loop to finish too early.

So the only work around was to first build a separate list based on the selectedFavorites list then simply use a for each loop.

The final code looks like this:

    private void DeleteAction(object actionParameter)
    {
        List<object> selectedFavorites = actionParameter as List<object>;
        List<FavoriteItemViewModel> myList = new List<FavoriteItemViewModel>();

        Favorites.RemoveAll(a => selectedFavorites.Exists(w =>     ((FavoriteItemViewModel)w).Description == a.Description));

        myList = null;
    }

And this works as expected!

Answer 3

You can use prameterized constructor of List<T> to construct new List containing items from existing List without manual foreach loop. Something like this should be enough :

private void DeleteAction(object actionParameter)
{
    List<object> selectedFavorites = 
                    new List<object>(actionParameter as List<object>);
    foreach (var item in selectedFavorites)
    {
        this.Favorites.Remove((FavoriteItemViewModel)item);
    }
}

Normally We wouldn't need to call Clear() and set selectedFavorites to null because the variable it self declared in local context, so it will be qualified for garbage-collection soon after the method execution finished.