C# 3.0 Cookbook, 3rd Edition

9.6. Observing Additions and Modifications to Dictionaries

Problem

You have multiple objects that need to observe modifications to objects that implement IDictionary<K,V>. When an item is added or modified in the dictionary-based collection, each of these observer objects should be able to vote to allow or disallow the action. In order for an action to be allowed to complete, all observer objects must state if they are vetoing the action. If even one observer object votes to disallow the action, the action is prevented.

Solution

Use the ObservableDictionaryObserver class implemented in Example 9-5 to observe additions and modifications to the ObservableDictionary class (shown in Example 9-4) object that is registered with this object. The ObservableDictionary class is a generic wrapper for collections that implement IDictionary<K,V> and allows itself to be observed by the ObservableDictionaryObserver class.

The ObservableDictionaryEventArgs class is a specialization of the EventArgs class, which provides the IDictionary<K,V> key and value being added or modified to the ObservableDictionaryObserver object, as well as a Boolean property, KeepChanges. This flag indicates whether the addition or modification in the ObservableDictionary object will succeed or be rolled back. The MakeObservableDictionary extension method for IDictionary<K,V> wraps up the code for creating an ObservableDictionary from an IDictionary instance. Example 9-4 illustrates the two classes and the extension method. ...