WPF ComboBox SelectedItem - change to previous value

Question

I have a ComboBox that has the SelectedItem bound to the ViewModel.

Answer 1

When the user says "no", WPF is unaware that the value has changed. As far as WPF is concerned, the value is whatever the user selected.

You might try raising a property changed notification:

public object SelItem
{
    get { ... }
    set
    {
        if (!CancelChange())
        {
            this.selItem = value;
        }

        OnPropertyChanged("SelItem");
    }
}

The problem is, the change notification happens within the same context of the selection event. Thus, WPF ignores it because it already knows the property has changed - to the item the user selected!

What you need to do is raise the notification event in a separate message:

public object SelItem
{
    get { ... }
    set
    {
        if (CancelChange())
        {
            Dispatcher.BeginInvoke((ThreadStart)delegate
            {
                OnPropertyChanged("SelItem");
            });
            return;
        }

        this.selItem = value;
        OnPropertyChanged("SelItem");
    }
}

WPF will then process this message after it's done processing the selection changed event and will therefore revert the value in the view back to what it should be.

Your VM will obviously need access to the current Dispatcher. See my blog post on a base VM class if you need some pointers on how to do this.