Vista Tutorial - databinding to 3-state checkbox?

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?

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
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"dave" wrote:

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:

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:

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: