Question

My ComboBox:

<ComboBox
    SourceUpdated="MyComboBox_SourceUpdated"
    ItemsSource="{Binding ...}"
    SelectedValue="{Binding Path=SelectedValueMember, NotifyOnSourceUpdated=True}"
    SelectedValuePath="..."
    DisplayMemberPath="..."
/>

And my SourceUpdated handler:

private void MyComboBox_SourceUpdated(object sender, DataTransferEventArgs args) {
    if (/* user answers no to an 'are you sure?' prompt */) {
        // TODO: revert ComboBox back to original value
    }
}

I’m having difficult reverting the ComboBox back to its original value (in the “TODO” comment of my code above).

The first and most obvious thing I tried was to simply change back the value of the SelectedValueMember of the DataContext, assuming the binding would update the ComboBox:

MyDataContext.SelectedValueMember = original_value;

When I do this, in the debugger I can see that it is indeed updating the SelectedValueMember value, but the ComboBox does not change back – it holds onto the new value.

Any ideas?

---

AngelWPF’s answer below works and is probably the neatest and clearest way.

I did however, find a non-obvious solution:

private void MyComboBox_SourceUpdated(object sender, DataTransferEventArgs args) {
    if (/* user answers no to an 'are you sure?' prompt */) {
        Dispatcher.BeginInvoke(new Action(() => {
            MyDataContext.SelectedValueMember = original_value;
        }));
    }
}

Just by putting the operation on a second UI thread by means of BeginInvoke, it works! I could be wrong, but my hunch is that to avoid binding update loops, the actions directly in Source/Target Updated handlers are not reacted to by bindings.

Answer 1

For explicit conditional update of a source in a binding, use the updatesource trigger as Explicit and handle the selection changed event of combobox to perform the sync with the source based on the message box result.

    <StackPanel DataContext="{StaticResource MyObject}">
      <ComboBox ItemsSource="{Binding MyData}" 
             DisplayMemberPath="Name" 
             SelectedValuePath="ID"
             SelectedValue="{Binding Path=MyID,
                                     Mode=TwoWay,
                                     UpdateSourceTrigger=Explicit}"
             SelectionChanged="ComboBox_SelectionChanged">
      </ComboBox>
      <TextBlock Text="{Binding Path=MyID}"/>
    </StackPanel>

So the ComboBox is bound to a collection of items (called MyData) having Name and ID as properties. The selected value of the ComboBox is bound to another property called MyID. Now notcie that UpdateSourceTrigger=Explicit in the SelectedValue binding.

The way I sync the MyID when the selection changes, with the selected value of the combobox ONLY if user selects Yes on a message box as below…

    private void ComboBox_SelectionChanged
      (object sender, SelectionChangedEventArgs e)
    {
        var bndExp
            = BindingOperations.GetBindingExpression(
                (ComboBox) sender, Selector.SelectedValueProperty);
        if (bndExp != null && bndExp.ParentBinding != null)
        {
            if (MessageBox.Show(
                  "Are you sure?",
                  "Sure?",
                  MessageBoxButton.YesNo) == MessageBoxResult.Yes)
            {
                ((MyObject) bndExp.DataItem).MyID
                    = (int)((ComboBox) sender).SelectedValue;
            }
        }
    }

Thsi way there is no need to revert. The source updates explicitly and you have all the control over it.