Question

In my viewmodel, I have a list (ObservableCollection) containing items. In the view, this list is displayed in an ItemsControl. In each row, there is a “Delete” button. I want the command behind the button to remove the item from the list.

<ItemsControl ItemsSource="{Binding myList}">
    <ItemsControl.ItemTemplate>
        ...
            <Button Command="{StaticResource myDeleteCommand}" CommandParameter="???">
                Remove item
            </Button>
        ...
    </ItemsControl.ItemTemplate>
</ItemsControl>

What do I pass as the command parameter?

• The item itself (Binding .)? Then I don’t have a reference to the list in the command, so I’d need to change my model such that each list item contains a back-reference to the list.
• The list? Then I don’t have a reference to the item.
• Both? Then I need to write a MultiConverter that translates the list plus the item into some custom object. Seems like a lot of overhead for such a simple task.

Any ideas? This seems like a fairly common scenario to me, so I guess there must be some well-established best-practice solution…

Answer 1

I’ve implemented such commands in that way, that I pass the Item as Parameter. The command self knows on which list it should operate. Either trough a delegate that calls a Delete Method in my ViewModel or the command receives the list of items in it’s constructor.

i.e. a Command with delegates

public sealed class SimpleParameterCommandModel<T> : CommandModel
{
    private readonly Action<T> execute;
    private readonly Func<T, bool> canExecute;

    public SimpleParameterCommandModel(string label, string tooltip, Action<T> execute, Func<T, bool> canExecute)
        : base(appCtx, dataCtx, label, tooltip)
    {
        if (execute == null) throw new ArgumentNullException("execute");
        this.execute = execute;
        this.canExecute = canExecute;
    }
    ...
}

usage:

private ICommand _DeleteCommand = null;
public ICommand DeleteCommand
{
    get
    {
        if (_DeleteCommand == null)
        {
            _DeleteCommand = new SimpleParameterCommandModel<IEnumerable<DataObjectModel>>                      ("Delete", "Delete selection from data store", 
                (items) => items.ToList().ForEach(i => DeleteItem(i)),
                (items) => items != null && items.Count() > 0 && AllowDelete);
        }
        return _DeleteCommand;
    }
}
public void DeleteItem(DataObjectModel item)
{
        if (item == null) { throw new ArgumentNullException("item"); }

    myCollection.Remove(item.Object);
}

EDIT: Forgot XAML

<Button Command="{Binding DeleteCommand, ElementName=...}" CommandParameter="{Binding}">
        Remove item
</Button>