Question

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

<ComboBox SelectedItem="{Binding SelItem, Mode=TwoWay}" ItemsSource="{Binding MyItems}">

When the user selects a new Item in the View ComboBox, I want to display a prompt and verify that they want to make the change.

In the SetItem Property setter in the View Model, I display a Dialog to confirm the selection. When they say yes, it works fine.

My problem is, when the user clicks on “No” I am not sure who to get the ComboBox
to revert back to the previous value. The Property in the ViewModel has the correct
older value, however in the View the ComboBox displays the newly Selected Value.

I want the user to select an item, confirm they want to go ahead with it, and if they
decide not to, I want the ComboBox to revert back to the previous item.

How can I accomplish this?
Thanks!

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.