Any way to un-register a WPF dependency property?

Question

I\'m running into an unusual problem in my unit tests. The class I\'m testing creates a dependency property dynamically at runtime and the type of that dependency property c

Answer 1

I don't think you can un-register a dependency property but you can redefine it by overriding the metadata like this:

MyDependencyProperty.OverrideMetadata(typeof(MyNewType), 
                     new PropertyMetadata());

Answer 2

I had similar issue just yesterday when trying to test my own DependencyProperty creating class. I came across this question, and noticed there was no real solution to unregister dependency properties. So I did some digging using Red Gate .NET Reflector to see what I could come up with.

Looking at the DependencyProperty.Register overloads, they all seemed to point to DependencyProperty.RegisterCommon. That method has two portions:

First to check if the property is already registered

FromNameKey key = new FromNameKey(name, ownerType);
lock (Synchronized)
{
  if (PropertyFromName.Contains(key))
  {
    throw new ArgumentException(SR.Get("PropertyAlreadyRegistered", 
      new object[] { name, ownerType.Name }));
  }
}

Second, Registering the DependencyProperty

DependencyProperty dp = 
  new DependencyProperty(name, propertyType, ownerType, 
    defaultMetadata, validateValueCallback);

defaultMetadata.Seal(dp, null);
//...Yada yada...
lock (Synchronized)
{
  PropertyFromName[key] = dp;
}

Both pieces center around DependencyProperty.PropertyFromName, a HashTable. I also noticed the DependencyProperty.RegisteredPropertyList, an ItemStructList<DependencyProperty> but have not seen where it is used. However, for safety, I figured I'd try to remove from that as well if possible.

So I wound up with the following code that allowed me to "unregister" a dependency property.

private void RemoveDependency(DependencyProperty prop)
{
  var registeredPropertyField = typeof(DependencyProperty).
    GetField("RegisteredPropertyList", BindingFlags.NonPublic | BindingFlags.Static);
  object list = registeredPropertyField.GetValue(null);
  var genericMeth = list.GetType().GetMethod("Remove");
  try
  {
    genericMeth.Invoke(list, new[] { prop });
  }
  catch (TargetInvocationException)
  {
    Console.WriteLine("Does not exist in list");
  }

  var propertyFromNameField = typeof(DependencyProperty).
    GetField("PropertyFromName", BindingFlags.NonPublic | BindingFlags.Static);
  var propertyFromName = (Hashtable)propertyFromNameField.GetValue(null);

  object keyToRemove = null;
  foreach (DictionaryEntry item in propertyFromName)
  {
    if (item.Value == prop)
      keyToRemove = item.Key;
  }
  if (keyToRemove != null)
  propertyFromName.Remove(keyToRemove);
}

It worked well enough for me to run my tests without getting an "AlreadyRegistered" exception. However, I strongly recommend that you do not use this in any sort of production code. There is likely a reason that MSFT chose not to have a formal way to unregister a dependency property, and attempting to go against it is just asking for trouble.

Answer 3

Had an issue with a ContentPresenter with different Datatemplates where one of them had a DependencyProperty with a PropertyChangedCallback When changing ContentPresenters content to another DataTemplate the callback remained.

In the UserControls Unloaded event i called:

BindingOperations.ClearAllBindings(this);
Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Normal, new DispatcherOperationCallback(delegate { return null; }), null);

That worked for me

Answer 4

I was facing scenario where I created a custom control that inherits from Selector which is meant to have two ItemsSource properties, HorizontalItemsSource and VerticalItemsSource.

I don't even use the ItemsControl property, and don't want the user to be able to access it.

So I read statenjason's great answer, and it gave me a huge POV on how to remove a DP.
However, my problem was, that since I declared the ItemsSourceProperty member and the ItemsSource as Private Shadows (private new in C#), I couldn't load it at design time since using MyControlType.ItemsSourceProperty would refer to the shadowed variable.
Also, when using the loop mentioned in is enswer above (foreach DictionaryEntry etc.), I had an exception thrown saying that the collection has changed during iteration.

Therefore I came up with a slightly different approach where the DependencyProperty is hardcodedly refered at runtime, and the collection is copied to array so it's not changed (VB.NET, sorry):

Dim dpType = GetType(DependencyProperty)
Dim bFlags = BindingFlags.NonPublic Or BindingFlags.Static

Dim FromName = 
  Function(name As String, ownerType As Type) DirectCast(dpType.GetMethod("FromName",
    bFlags).Invoke(Nothing, {name, ownerType}), DependencyProperty)

Dim PropertyFromName = DirectCast(dpType.GetField("PropertyFromName", bFlags).
  GetValue(Nothing), Hashtable)

Dim dp = FromName.Invoke("ItemsSource", GetType(DimensionalGrid))
Dim entries(PropertyFromName.Count - 1) As DictionaryEntry
PropertyFromName.CopyTo(entries, 0)
Dim entry = entries.Single(Function(e) e.Value Is dp)
PropertyFromName.Remove(entry.Key)

Important note: the above code is all surrounded in the shared constructor of the custom control, and I don't have to check wether it's registered, because I know that a sub-class of Selcetor DOES provide that ItemsSource dp.

Answer 5

If everything else fails, you can create a new AppDomain for every Test.

Answer 6

If we register name for a Label like this :

Label myLabel = new Label();
this.RegisterName(myLabel.Name, myLabel);

We can easily unregister the name by using :

this.UnregisterName(myLabel.Name);