You are right that the methods in the interface are not invoked. This however is expected because they are not called from inside the collection nor the NewItemPlaceholderPosition is bound or set
. There is no one who interacts with those methods and properties. This entirely depends on your custom implementation.
As for the interface itself - it is supported by the RadTabControl. The interface only defines a structure. If the underlying collection respects that structure you will be able to use the collection with the RadTabControl.
Could you please provide us more details regarding your requirements? What exactly you want to happen when you use the custom collection? What exactly should be the behavior of the collection?
Pavel R. Pavlov
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