ok, found it... this article describes the problem and fix:
http://code.msdn.microsoft.com/wpfdatabindingnull
just made a nullable bool to string converter to go from null<->"" and it is
happy.
"dave" wrote:
Quote:
> More info... i was missing something, there is an error generated when it
> tries to set the null value back to the xml:
>
> System.Windows.Data Error: 8 : Cannot save value from target back to source.
> BindingExpression:Path=Element[StatusSummary].Value; DataItem='XElement'
> (HashCode=51920462); target element is 'CheckBox' (Name=''); target property
> is 'IsChecked' (type 'Nullable`1')
> ArgumentNullException:'System.ArgumentNullException: Value cannot be null.
> Parameter name: value
> at System.Xml.Linq.XElement.set_Value(String value)
> at
> System.Xml.Linq.ComponentModel.XElementValuePropertyDescriptor.SetValue(Object component, Object value)
> at MS.Internal.Data.PropertyPathWorker.SetValue(Object item, Object value)
> at MS.Internal.Data.ClrBindingWorker.UpdateValue(Object value)
> at System.Windows.Data.BindingExpression.UpdateSource(Object value)'
>
>
> So it looks like the set_Value for a string can't handle the null for the
> checkbox in tristate mode.
>
>
>
>
>
> "dave" wrote:
> Quote:
> > I know the events for indeterminate state transitions are different, but in
> > the databinding you just get to specify the property you want to bind to and
> > the trigger mechanism for updating the source. Unfortunately you can't
> > specify the event to do the update unless you add your own event handler and
> > use the manual method to call updatesource. It appears that the trigger
> > mechanism doesn't recognize transitions to indeterminate, only to checked or
> > unchecked.
> >
> > "John Bundy" wrote:
> > Quote:
> > > haven't tried it as you have below, but i tried to check this intuitively
> > > with a checkchanged event updating a label.
> > > Me.Label1.Text = Me.CheckBox1.CheckState
> > > i found that the indeterminate value did not fire an event with then
> > > CheckBox1_CheckedChanged event, it did work with CheckBox1_CheckStateChanged.
> > > So you might try checking against that event instead
> > > --
> > > -John
> > > Please rate when your question is answered to help us and others know what
> > > is helpful.
> > >
> > >
> > > "dave" wrote:
> > >
> > > > I am binding a 3-state checkbox to an XElement:
> > > >
> > > > <CheckBox Content="Summary" IsChecked="{Binding
> > > > Path=Element[StatusSummary].Value, Mode=TwoWay,
> > > > UpdateSourceTrigger=PropertyChanged}" IsThreeState="True" IsEnabled="False"
> > > > Margin="0,79,0,0"/>
> > > >
> > > > it properly reports True and False when it is checked or unchecked. but it
> > > > retains the previous value when in the intermediate state. it would seem to
> > > > not be firing the propertychanged event to update the xelement when
> > > > transitioning to the indeterminate state. Is there a way to bind to get back
> > > > the null for indeterminate?