Question

The following code works as you’d expect — MyProperty on the model is updated when the user picks a new item in the dropdown.

comboBox1.DataBindings.Add("SelectedValue", myModel, "MyProperty", true,
DataSourceUpdateMode.OnPropertyChanged);

The following, however, doesn’t work the same way and the model update isn’t triggered until the input focus moves to another control on the form:

comboBox1.DataBindings.Add("SelectedItem", myModel, "MyProperty", true, 
DataSourceUpdateMode.OnPropertyChanged);

Does anybody know why? I don’t even know where to start investigating the cause. Pointers in the right direction to start the investigation or an outright explanation would be equally appreciated.

Aside: for my purposes, I ended up binding to both SelectedItem and SelectedValue. This way I get instant model updates based on UI changes (through the SelectedValue binding), and UI updates based on programmatic model changes (through the SelectedItem binding).

Answer 1

The ComboBox control inherits from the ListControl control.

The SelectedItem property is a proper member of the ComboBox control. The event that is fired on change is ComboBox.SelectionChangeCommitted

ComboBox.SelectionChangeCommitted

Occurs when the selected item has changed and that change is displayed in the ComboBox.

The SelectedValue property is inherited from the ListControl control.
As such, this property will fire the ListControl.SelectedValueChanged event.

ListControl.SelectedValueChanged

Occurs when the SelectedValue property changes.

That said, they won’t fire the INotifyPropertyChanged.PropertyChanged event the same, but they will anyway. The only difference is in the firing event. SelectedValueChanged is fired as soon as a new selection is made from the list part of the ComboBox, and SelectedItemChanged is fired when the item is displayed in the TextBox portion of the ComboBox.

In short, they both represent something in the list part of the ComboBox. So, when binding either property, the result is the same, since the PropertyChanged event is fired in either case. And since they both represent an element from the list, the they are probably treated the same.

Does this help?

EDIT #1

Assuming that the list part of the ComboBox represents a property (as I can’t confirm since I didn’t write the control), binding either of SelectedItem or SelectedValue affects the same collection inside the control. Then, when this property is changed, the same occurs in the end. The INotifyPropertryPropertyChanged.PropertyChanged event is fired on the same property.