ComboBox Binding Confusoin

[ryan baldwin]

I have a WPF combobox in which I'm binding the SelectedIndex to a model
class I've developed. Assume something similar to the following (but
with different names, etc... this is a simplified version):

<ComboBox Name="myCombo" ItemsSource="{Binding ComboSource}"
SelectedIndex="{Binding IndexOfCombo, Mode=OneWay}" />

In my model I occasionally will set IndexOfCombo back to 0, and will
then publish a NotifyPropertyChanged for IndexOfCombo. This sets the
selected index of the combobox to 0 as I would expect.... UNLESS the
user has already selected something int he combobox. For whatever
reason, it's as though the combobox will ignore the
NotifyPropertyChanged if the user has selected an item in the combobox.

Is this behaviour expected? Is there a way to prohibit the combobox
from ignoring the NotifyPropertyChanged event? I'm very confused as to
why this is happening.

Any help would be gracefully accepted, as this issue has burned my
entire morning...

Thanks!

- ryan.

 

[Zhi-Xin Ye [MSFT]]

Dear ryan,

I think this is a by design feature, when changes the selection of the
ComboBox manually, the value of the SelectedIndex property is changed,
naturally you would expect to retrieve the value later, but if the binding
remained, it could be updated before you had a chance to retrieve that
value, if you're developing a multithreaded system, this would cause
undesired effects.

So the system tends to discard the binding when the target property is
manually changed. You can check this feature by checking the return value
of the ComboBox.GetBindingExpression() method, if you don't manually change
the selection of the ComboBox, this method returns the BindingExpression
that you're currently used, but if you manually change the selection of the
ComboBox, this method returns null, which means the binding is discarded.

If you want the binding continue to work after manually changing the
selection, you can reset the binding, something like this:

private void button1_Click(object sender, RoutedEventArgs e)
{
if
(this.comboBox1.GetBindingExpression(ComboBox.SelectedIndexProperty) ==
null)
{
this.comboBox1.SetBinding(ComboBox.SelectedIndexProperty,
binding);
}
m.IndexOfCombo = 1;
}

If any of my explanation is unclear, please let me know.

Sincerely,
Zhi-Xin Ye
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[rbaldwin]

Hey Zhi-Xin,

Thanks for the explanation. It made sense.

- ryan.