Question

Say you have a button whose command property is bound to some ICommand of the current item of some collection.

When the collection is null, the button remains enabled and clicking it seems to be a no-op. I want instead that the button remains disabled. I figured out the following to keep buttons disabled whenever the collection is null. It however seems a bit too convoluted for something that could perhaps be accomplished in a more natural, simpler, and more MVVM like.

Hence the question: is there a simpler way to keep that button disabled, ideally where no code-behind is used?

.xaml:

<Button Content="Do something" >
    <Button.Command>
        <PriorityBinding>
            <Binding Path="Items/DoSomethingCmd"  />
            <Binding Path="DisabledCmd" />
        </PriorityBinding>
    </Button.Command>
</Button>

.cs:

public class ViewModel : NotificationObject
{
    ObservableCollection<Foo> _items;

    public DelegateCommand DisabledCmd { get; private set; }

    public ObservableCollection<Foo> Items { 
        get { return _items; } 
        set { _items = value; RaisePropertyChanged("Items"); } 
    }

    public ViewModel()
    {
        DisabledCmd = new DelegateCommand(DoNothing, CantDoAnything);
    }

    void DoNothing() { }
    bool CantDoAnything()
    {
        return false;
    }
}

Edit:

A couple of notes:

1. I am aware that I can use lambda expressions, but in this example code I do not.
2. I know what a predicate is.
3. I don’t see how doing something with DoSomethingCmd.CanExecute will do anything to help as there is no DoSomethingCmd to access while there is no current item.
4. So I’ll re-center my question: How can I avoid using the DisabledCmd? I am not interested in moving up the DoSomethingCmd as it is not what I am looking for. I wouldn’t be asking this question otherwise.

Another edit:

So I basically adopted this answer as a solution: WPF/MVVM: Disable a Button's state when the ViewModel behind the UserControl is not yet Initialized?

It is, I believe, exactly what hbarck proposes.

Answer 1

I’d do it similar to akjoshi, only I’d use a normal Trigger instead of a DataTrigger, and I’d check on Button.Command to be null. Since it always makes sense to disable a Button that has no Command (especially in MVVM, where there are no click eventhandlers), it would also be a good idea to include this trigger into a default style for Buttons, in order to have this behaviour on all Buttons across the application… I don’t see a reason to use a dummy command.