How to update Target object from markup extension in SL5? - silverlight-5.0

Question

I'm trying this code in a markup extension in Silverlight 5.
public override object ProvideValue(IServiceProvider serviceProvider)
{
_target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
_depObj = _target.TargetObject as DependencyObject;
_depProp = _target.TargetProperty as DependencyProperty;
return GetText(TextId, DefaultText);
}
depObj seems to be provided correctly, however depProp seems not to be of type
DependencyProperty (The type is Sytem.String (of System.Reflection.PropertyInfo)). The cast results in null.
depProp is representing to the right Text or Content property, but without it being a
DependencyProperty I cannot set its value.
Any input greatly appreciated.
SiKo

Answer 1

Not sure why its coming up as a PropertyInfo but you can of course set the value with code something like:
if (TargetProperty is DependencyProperty)
{
var dependencyObject = target as DependencyObject;
if (dependencyObject != null)
{
var newValue = GetValue();
dependencyObject.SetValue(TargetProperty as DependencyProperty, newValue);
}
}
else if (TargetProperty is PropertyInfo)
{
var pi = target as PropertyInfo;
pi.SetValue(target, GetValue(), null);
}

Answer 2

The approach #Berryl gave above could work. But if you also need to get to the DependencyProperty, you can use something like this. This code also demonstrates the subtle differences between silverlight and WPF, but both can work great.
For silverlight or WPF only, you can remove the parts before or after the #else to simplify the code a little bit.
But the part that resolves the dependency property is in the middle (assigning _property).
public override object ProvideValue(IServiceProvider serviceProvider)
{
IProvideValueTarget Target = serviceProvider.GetService(typeof(IProvideValueTarget)) as IProvideValueTarget;
if (Target == null) {
throw new InvalidOperationException(
"Cannot resolve the IProvideValueTarget. Are you binding to a property?");
}
/* we need the dependency property (for bindings) */
#if SILVERLIGHT
PropertyInfo Property = (PropertyInfo)Target.TargetProperty;
/* resolve the dependency property */
FieldInfo DepPropertyInfo = Target.TargetObject.GetType()
.GetField(Property.Name + "Property", BindingFlags.Static | BindingFlags.Public);
_property = (DependencyProperty)DepPropertyInfo.GetValue(/* obj = */ null);
#else
_property = Target.TargetProperty as DependencyProperty;
#endif
if (_property != null) {
#if SILVERLIGHT
if (Property.Name.StartsWith("Text")) {
UpdateSourceTrigger = UpdateSourceTrigger.Default;
#else
if (_property.Name.StartsWith("Text")) {
UpdateSourceTrigger = UpdateSourceTrigger.LostFocus;
#endif
}
else {
UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
}
}