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copied!<p>Setting aside the question of how to dynamically attach a property or interface, it sounds like what you're attempting to do is augment existing classes with additional data. A very typical solution to this problem is to use a <code>Dictionary<Something, SomethingExtra></code> and store it in some service class that maintains the mapping. Now, when you need access to SomethingExtra, you just ask the service class for the associated information.</p> <p>Advantages:</p> <ol> <li><p>The implementation is easier to understand and maintain than a solution using reflection and dynamic proxy generation.</p></li> <li><p>If you need to derive from the type being extended, the class can't be sealed. Associating information externally works fine with sealed classes.</p></li> <li><p>You can associate information without having to be responsible for the construction of the augmented object. You can take instances created anywhere and associate the new information.</p></li> </ol> <p>Disadvantages:</p> <ol> <li><p>You need to inject the service instance that maintains the mapping. You can do this via an IoC framework if you're using one, manual injection (usually passed through a constructor) or, if there's no other alternative, via a Singleton static accessor.</p></li> <li><p>If you have a very large number of instances and they are being rapidly created/removed, the Dictionary overhead could become noticeable. I suspect you would need some very heavy load before the overhead becomes noticeable compared to a proxying implementation.</p></li> </ol>

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