Silverlight Dependancy Property Change Event Problem - silverlight

Question

I have a custom control that has a Dependancy Property...it has a few but let us say Dragable is my problem. The property is a boolean and I want to execute a piece of code each time it changes...a toggle.
I have two options, both shown below
[Category("Modal Options")]
public bool Dragable
{
get { return (bool)GetValue(DragableProperty); }
set { SetValue(DragableProperty, value); toggleDragable(); }
}
// Using a DependencyProperty as the backing store for Dragable. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DragableProperty =
DependencyProperty.Register("Dragable", typeof(bool),
typeof(PlussWindow), new PropertyMetadata(false));
private void MakeDragable()
{
this.dragBehavior.Attach(this.LayoutRoot);
}
private void MakeUnDragable()
{
this.dragBehavior.Detach();
}
public virtual void toggleDragable()
{
if (this.Dragable)
{
MakeUnDragable();
}
else
{
MakeDragable();
}
}
or
[Category("Modal Options")]
public bool Dragable
{
get { return (bool)GetValue(DragableProperty); }
set { SetValue(DragableProperty, value); }
}
// Using a DependencyProperty as the backing store for Dragable. This enables animation, styling, binding, etc...
public static readonly DependencyProperty DragableProperty =
DependencyProperty.Register("Dragable", typeof(bool),
typeof(PlussWindow), new PropertyMetadata(false, (o, e) => { (o as PlussWindow).toggleDragable(); }
));
private void MakeDragable()
{
this.dragBehavior.Attach(this.LayoutRoot);
}
private void MakeUnDragable()
{
this.dragBehavior.Detach();
}
public virtual void toggleDragable()
{
if (this.Dragable)
{
MakeUnDragable();
}
else
{
MakeDragable();
}
}
Each method results in a 'Object reference not set to an instance of an object'
I usually use binding to get over this problem, e.g Visibility or Text are easily done, but for custom functionality I need to enable this in code.
How do I do this, noting that the propertychanged method is static?

Answer 1

Try this:
public bool Dragable
{
get { return (bool)GetValue(DragableProperty); }
set { SetValue(DragableProperty, value); }
}
public static readonly DependencyProperty DragableProperty =
DependencyProperty.Register("Dragable", typeof(bool), typeof(PlussWindow), new PropertyMetadata(false, new PropertyChangedCallback(onDragableChange)));
private static void onDragableChange(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
bool validate = (bool)e.NewValue;
PlussWindow win = (PlussWindow) d;
if (validate)
{
win.dragBehavior.Attach(this.LayoutRoot);
}
else
{
win.dragBehavior.Detach();
}
}