Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

Shimmy Weitzhandler — Wed, 25 Jan 2012 03:30:07 GMT

Won't work on attached DPs though

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

wekempf — Fri, 22 Apr 2011 15:24:18 GMT

DrWPF, as usual your suggestion is simple and functional. However, there's a minor hole. This solution won't work for readonly _attached_ properties. Honestly, most of my readonly DPs are attached DPs.

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

David Anson — Fri, 12 Nov 2010 23:39:05 GMT

jariza,

That's a great point, thanks for bringing it up! On the other hand, you might consider having a dedicated instance of the _changingMyReadOnly/_restoringMyReadOnly variables for each read-only DependencyProperty in a class so they're all truly independent. Without analyzing it too much, I feel like this latter approach might be a bit easier to get right and more straightforward to think about.

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

jariza — Fri, 12 Nov 2010 20:48:25 GMT

Wouldn't that be a problem if you have several read-only properties in the same class? You might want to set other properties when one of them changes, so I would add

_changingMyReadOnly = false;

right before

myControl.OnMyReadOnlyChanged((int)e.OldValue, (int)e.NewValue);

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

DrWPF — Thu, 01 Apr 2010 18:12:06 GMT

The biggest issue I see with your solution, SharpGIS, is that it does not create a readonly DP. Your approach only makes it more difficult to modify the value, but its not a true readonly property. For example, you can change the value of the DP by creating a two-way binding to the property and changing the bound value (e.g., bind the Text property of a TextBox to your property using a TwoWay binding and then change the text in the control and it will update your property).

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

David Anson — Wed, 31 Mar 2010 22:08:11 GMT

SharpGIS,

Your InternalsVisibleTo solution excites and revolts me - at the same time. :) I'll be doing a follow-up blog post to this one and I'll include your suggestion along with DrWPF's.

Thanks very much for sharing!

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

David Anson — Wed, 31 Mar 2010 20:09:38 GMT

SharpGIS,

I'm saying that WPF will not allow "SetValue(MyReadOnlyProperty, 5)" to succeed on a read-only DependencyProperty (you need to use the private DependencyPropertyKey instead) and therefore that the same code should not work for "simulated read-only" properties on Silverlight, either.

Regarding your InternalsVisibleTo suggestion, I've Twittered you just now to discuss this further. I'll post a follow-up comment here after we've come to an agreement. :)

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

Morten Nielsen — Wed, 31 Mar 2010 19:57:38 GMT

btw are you now saying that it's "illegal" to do this:

myObject.SetValue(MyReadOnlyProperty,5) ?

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

Morten Nielsen — Wed, 31 Mar 2010 19:51:26 GMT

There's a MUCH easier way:

[assembly: InternalsVisibleTo("System.Windows, PublicKey=00240000048000009400000006020000002400005253413100040000010001008d56c76f9e8649383049f383c44be0ec204181822a6c31cf5eb7ef486944d032188ea1d3920763712ccb12d75fb77e9811149e6148e5d32fbaab37611c1878ddc19e20ef135d0cb2cff2bfec3d115810c3d9069638fe4be215dbf795861920e5ab6f7db2e2ceef136ac23d5dd2bf031700aec232f6c6b1c785b4305c123b37ab")]

public int MyReadOnly

{

get { return (int)GetValue(MyReadOnlyProperty); }

private set

{

SetValue(MyReadOnlyProperty, value);

}

internal static readonly DependencyProperty MyReadOnlyProperty = DependencyProperty.Register(

"MyReadOnly", typeof(int), typeof(MyControl), new PropertyMetadata(0));

re: Sometimes you just gotta do the best you can [Tip: Read-only custom DependencyProperties don't exist in Silverlight, but can be closely approximated]

David Anson — Wed, 31 Mar 2010 17:04:00 GMT

DrWPF,

Wow, that is a great suggestion! I'm emailing you now to discuss a couple of things about it, but once that's settled, it sounds like I need another DependencyProperty tip to discuss your solution.

Thanks very much!