WPF ComboBox MVVM strange behavior - wpf

Question

I'm having a strange behavior when I associate and combobox to my viewmodel. The behavior is the following, when I change the selected value of the combo I do a validation of the new value and if this new value is invalid I keep the old value and discard the new one, in this way I don't raise the Inotifypropertychanged, but the getter from the property associated to the combobox is call anyway, this gets the old value that I want to show, but instead the combobox shows the new value, even though the selectedvalue of the combo as the old and correct value, I checked in debug mode. I don't know how can I solve this because I never saw this kind of behavior, any suggestions would be much appreciated.
This is the code of the XAML
<ComboBox Height="23" Name="cbxStatus" HorizontalAlignment="Left"
ItemsSource="{Binding Path=Status, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"
SelectedItem="{Binding Path=SelectedStatus, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
DisplayMemberPath="Value" Width="130" VerticalAlignment="Center"
IsEnabled="{Binding Path=StatusEnable, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
this is the viewmodel code, the property
public Config SelectedStatus
{
get
{
if (ApplicationAction == ApplicationAction.Add)
{
base.Object.State = configManager.BudgetInitStatus();
StatusEnable = false;
}
else
{
StatusEnable = true;
}
return base.Object.State;
}
set
{
if (base.Service.CanChangeBudgetStatus(base.Object, value))
{
base.Object.State = value;
base.Object.IsDirty = true;
}
RaiseOnPropertyChanged("SelectedStatus");
RaiseOnPropertyChanged("AssociateOrderButtonVisibility");
}
}
Thanks for the help

Answer 1

As indicated in my comment to Jay, the problem here is that WPF is setting the value and not listening to your change notification (which it is, after all, expecting). What you need to do is raise the property change notification outside the context of the current message. You could do this using the dispatcher, for example:
set
{
if (!valid)
{
// value is unchanged
Dispatcher.BeginInvoke(delegate { this.OnPropertyChanged(...) });
return;
}
// value is changed here
}
This will ensure the current data binding message is executed, then a separate message tells WPF that, "actually, the value you just provided to my setter is no longer the current value".
You could also use SynchronizationContext if you prefer. Either way, I admit it's a little hacky. Unfortunately, I don't know of a nice way around this. The fact is, WPF assumes that the value it passes to your setter is the effective value of the property. No amount of property change notifications within the context of the binding operation will convince it otherwise.

Answer 2

Setting your binding to
IsAsync=True
will work.

Answer 3

When the user changes the value in the combobox, it gets changed in the combobox, irrespective of your viewmodel.
If you change that value back and do not raise a property change notification, your viewmodel and view will be out of sync.
In short, when you reject the selected value you still need the property change notification.