CoerceValue not working correctly with POCO objects - wpf

Question

I've run into an issue which seems pretty strange and I'm not sure if this is a bug or a design feature of WPF.
I have a Dependency Object with a Dependency Property with constraints enforced via value coercion and everything works as expected when using this property in bindings with other dependency properties.
But things break down when trying to use the property in a binding against a POCO object. In this case the POCO is updated when the dependency property changes, but before value coercion is executed, leaving the POCO with an invalid/out-of-sync value.
Here's an example:
public class TestDO : DependencyObject
{
public static DependencyProperty NumberProperty =
DependencyProperty.Register("Number", typeof(int), typeof(TestDO),
new FrameworkPropertyMetadata(0, FrameworkPropertyMetadataOptions.None, OnNumberChanged, OnNumberCoerce));
public int Number
{
get { return (int)GetValue(NumberProperty); }
set { SetValue(NumberProperty, value); }
}
private static void OnNumberChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
Console.WriteLine("DepObj: NumberChanged {0} --> {1}", e.OldValue, e.NewValue);
}
private static object OnNumberCoerce(DependencyObject d, object value)
{
int cval = (int)value;
if (cval > 10)
cval = 10;
else if (cval < -10)
cval = 10;
Console.WriteLine("DepObj: OnNumberCoerce: {0} --> {1}", value, cval);
return cval;
}
}
public class TestPOCO
{
private int _pocoNumber = 0;
public int PocoNumber
{
get { return _pocoNumber; }
set { _pocoNumber = value; }
}
}
Now, we can create two instances of the types above and a binding and test setting the Number property. The binding updates the POCO object but it does so before value coercion.
// create a POCO object
var testPoco = new TestPOCO();
// create a dependency object
var testObj = new TestDO();
// create a binding between the NumberProperty of the Dependency Object and the
// PocoNumber of the POCO object
BindingOperations.SetBinding(testObj, TestDO.NumberProperty, new Binding("PocoNumber")
{
Source = testPoco,
Mode = BindingMode.TwoWay
});
// try setting a value of 100, to trigger value coercion
testObj.Number = 100;
Console.WriteLine("Poco Number = {0}", testPoco.PocoNumber); // displays 100
Console.WriteLine("DP Number = {0}", testObj.Number); // displays 10
The same is not true if the binding exists between two DependencyObjects. In that case everything works as expected. SetValue --> ValueCoercion --> PropertyChanged --> SetValue