Binding issues with RadioButton

Question

I have a seemingly simple task, that is giving me headaches and would appreciate some help.

What I want to accomplish is binding two boolean values to the IsChecked property of two RadioButtons, sharing the same GroupName (so only one is checked at a time).

The problem I'm facing is that when the Content of a ContentPresenter is about to change (through binding to SelectedItem of a ComboBox), the current content receives a call to a Property-setter with a value of the same property but from the view model that is about to become the new content. (!) The end result is that there is a change of the view model despite no click on the RadioButton bound to the property in question.

I've put together a sample app that shows the problem. To reproduce, run the app and follow these steps:

1. Select "One" in the combobox => MyPropery is checked, MyProperty2 is not.
2. Select "Three" in the combobox => MyPropery is checked, MyProperty2 is not.
3. While still having "Three" selected, click MyProperty2 => MyProperty2 is checked (also indicated in the debug output window)
4. Select "One" in the combobox => MyPropery is checked, MyProperty2 is not. Notice how the debug window shows how MyProperty2 of object "Three" is set to false here
5. Select "Three" in the combobox => Neither of the Radiobuttons are now checked (due to #4).

If between #3 and #4, you first select "Two" in the combobox to have the ContentPresenter display another view (as selected through the DataTemplate), the problem does not appear!?

Could someone please explain why the property is set at step #4 when the views are exchanged by the ContentPresenter, and what can be done about it?

Answer 1

DataTemplates are cached, so both One and Three use the exact same UserControl. You can verify this by adding a Loaded event to the control and switching between options.

When you switch to Two and back to Three, WPf will simply re-draw the item from cache, however switching to One and back to Three, it changes the DataContext behind the object. I think this is causing the problem because it seems to be clearing the 2nd RadioButton IsChecked prior to removing the DataContext, so the end result is that Property2 gets set to false. This does not happen if both One and Three have the 2nd radio button selected.

Usually in this kind of situation, I will have the VM contain an ObservableCollection<T> Items and an int SelectedIndex. I'll then draw the UI using a ListBox which has been overwritten to use RadioButtons for the items. This way only one item can be selected at a time, and there is only one property for storing the selected item.