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    <blockquote> <p>In C# can you set constraints on events?</p> </blockquote> <p>Nope.</p> <blockquote> <p>I want to limit the types that can subscribe to a particular event based on the interface they implement.</p> </blockquote> <p>Event listeners are <em>instances of delegates of a type compatible with the event type</em>. "Types" are not listeners of events.</p> <blockquote> <p>I.e. I want an object "Employee" implementing IPerson to subscribe to an event handler but another object "Truck" implementing a totally different interface to be restricted. </p> </blockquote> <p>An "employee" object doesn't listen to an event in the first place. The only thing that listens to an event is a <em>delegate object</em>. </p> <p>Are you saying that you only want to accept delegates that are delegates to methods that happen to be instance methods of a particular class? That is a very strange thing to do and <strong>I recommend that you not attempt to do so</strong>. </p> <p>It is not possible to prevent that <em>at compile time</em>, but if you are really hell-bent on doing this strange thing, you can do it at runtime. </p> <p>To do so, as the other answers note, you can create adder and remover accessor methods for the event, and then put a check in the adder that verifies that the receiver of the delegate is of a type you approve of, and throw an exception if it is not. </p> <p>But again, this sounds like an extraordinarily bad idea. It is up to the listener to decide whether or not they are interested in the event, not up to the event source to vet its listeners. Users have the reasonable expectation that any accessible event can be listened to by any delegate of a compatible type.</p> <p>Perhaps you can explain why you want to do this strange thing; there might be a better way to achieve your aim.</p>
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    1. USEric Lippert
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    1. COhmm... so in reading your response it seems like you feel this is a bad idea and it is strange. Am I understanding correctly?
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    2. CO@dbobrowski: Yes. Why do you want to do this? The editor publishing the newspaper can choose to *publish* an edition or *not publish* it, but does not typically decide *who cannot subscribe*. The subscriber doesn't control the publishing schedule, but can decide whether or not to subscribe. That's the relationship people expect publishers and subscribers to have; changing that relationship is a surprising and unusual thing to do, so I'd imagine you'd have a pretty compelling reason to do so.
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    3. CO@dbobrowski every system including the .NET and C# language is designed to support common scenarios that should solve most of the problems you may encounter. If you try and do something the system is not designed for it may indicate the flaw of the system, but it's much more likely that you are trying to do something strange and not recommended.
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