Dependency Properties or Attached Properties

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SilverSpud » Blog Archive » Dependency properties, conceptually explained — Wed, 31 Oct 2007 02:12:58 GMT

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re: Dependency Properties or Attached Properties

Ian Griffiths — Sun, 23 Sep 2007 16:14:19 GMT

...in fact it turns out to me more complex than I thought, as I discovered since I wrote the previous reply.

It is possible for a property to declare that it's only supposed to be applicable to child elements. If the Get accessor has the AttachedPropertyBrowsableForChildrenAttribute applied, it's declaring that it's only meaningful when applied to children of the defining type.

Not that anything enforces this right now. Indeed, it's only intended as a design-time hint for where the property should be suggested.

Silverlight Cream for September 21, 2007

WynApse — Fri, 21 Sep 2007 18:40:03 GMT

Silverlight Cream for September 21, 2007

re: Dependency Properties or Attached Properties

jesseliberty — Thu, 20 Sep 2007 12:24:29 GMT

Ian,

Thank you for the very thoughtful analysis.

The use of the term "derivation" was too casual; the term has a specific meaning and I should have chosen a different word.

It is valuable, however, to have the second blog entry corrected; I frankly didn't know that about WPF, and I appreciate your taking the time to sort that out.

-jesse

re: Dependency Properties or Attached Properties

Ian Griffiths — Thu, 20 Sep 2007 11:08:16 GMT

A couple of points:

Attached properties in XAML in general have nothing to do with either derivation or containment. So when you say that you can have property

"that derives from an ancestor element (such as a canvas)"

that's not really what's going on from the XAML perspective. It so happens that in the Silverlight XAML vocabulary all the attached properties happen to be defined by ancestor elements, but that's just a coincidence. It's not a feature of attached properties in general, it just happens to be a feature of the small set of attached properties Silverlight happens to support.

If you look at the WPF XAML vocabulary, it includes all the attached properties supported by Silverlight, but it also includes a load more. For example, there's the FocusManager.FocusedElement attached property. You'll never see anything be the descendent of a FocusManager, because FocusManager is a static class - you can't create one so it can't be the ancestor of anything in a visual tree. And yet you can set the FocusManager.FocusedElement on elements in a WPF visual tree.

(So Josh's description is wrong I'm afraid. He's talking about WPF, which has several attached properties that don't match his description.)

So attached properties have nothing to do with containment. An attached property is simply one that can be applied to an element of a different type than the type that defines the property. So Canvas defines a Left property, but allows that property to be applied to other things such as Rectangle. And when Canvas's Left property is applied to a non-Canvas element such as Rectangle, we call that an attached property.

The attached properties defined by Canvas only mean anything useful when they are attached to the children of a Canvas. However, in WPF, you're free to attach them to anything you like. They are simply ignored if you use them anywhere other than on the children of a Canvas.

And derivation doesn't come into the picture at all. I'm not sure why you mentioned that.

Silverlight Cream for September 19, 2007

WynApse — Wed, 19 Sep 2007 16:26:21 GMT

Silverlight Cream for September 19, 2007

Dependency Properties or Attached Properties

Test — Wed, 19 Sep 2007 14:28:57 GMT

Silverlight lets you create a property in an object (such as a rectangle) that derives from an ancestor

Dependency Properties or Attached Properties - Jesse Liberty - Silverlight Geek

Dependency Properties or Attached Properties - Jesse Liberty - Silverlight Geek — Wed, 19 Sep 2007 13:49:40 GMT

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