Question

I have a class like this:

public class Stretcher : Panel {      public static readonly DependencyProperty StretchAmountProp = DependencyProperty.RegisterAttached('StretchAmount', typeof(double), typeof(Stretcher), null);      public static void SetStretchAmount(DependencyObject obj, double amount)     {         FrameworkElement elem = obj as FrameworkElement;         elem.Width *= amount;          obj.SetValue(StretchAmountProp, amount);     } } 

I can set the stretch amount property in XAML using the attribute syntax:

<UserControl x:Class='ManagedAttachedProps.Page'     xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'      xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'     xmlns:map='clr-namespace:ManagedAttachedProps'     Width='400' Height='300'>      <Rectangle Fill='Aqua' Width='100' Height='100' map:Stretch.StretchAmount='100' />  </UserControl> 

and my rectangle is stretched, but I can’t use property element syntax like this:

<UserControl x:Class='ManagedAttachedProps.Page'     xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'      xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'     xmlns:map='clr-namespace:ManagedAttachedProps'     Width='400' Height='300'>      <Rectangle Fill='Aqua' Width='100' Height='100'>         <map:Stretcher.StretchAmount>100</map:Stretcher.StretchAmount>     </Rectangle> </UserControl> 

with the property element syntax my set block seems to be totally ignored (I can even put invalid double values in there), and the SetStretchAmount method is never called.

I know it’s possible to do something like this, because VisualStateManager does it. I’ve tried using types other than double and nothing seems to work.

Answer 1

I think I figured this out, although I’m not totally sure I understand the reason why it works.

In order to get your example to work I had to create a custom type called Stretch with a property called StretchAmount. Once I did that and put that inside the property element tags it worked. Otherwise it wasn’t called.

public class Stretch {     public double StretchAmount { get; set; } } 

And the property changed to..

public static readonly DependencyProperty StretchAmountProp = DependencyProperty.RegisterAttached('StretchAmount', typeof(Stretch), typeof(Stretcher), null);  public static void SetStretchAmount(DependencyObject obj, Stretch amount)  {      FrameworkElement elem = obj as FrameworkElement;      elem.Width *= amount.StretchAmount;      obj.SetValue(StretchAmountProp, amount);  } 

To get this to work in the scenario where you aren’t using a property element you would need to create a custom type converter to allow this to work.

Hope this helps, even though it doesn’t explain the why which I’m still trying to understand.

BTW – for a real brain teaser, take a look at the VisualStateManager in reflector. The dependency property and the setter for VisualStateGroups are both internal.