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POC# Plugin pattern without interfaces
 

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    <p>I've encountered the need to implement a plugin pattern that doesn't fit with anything I've seen elsewhere and I'm just wondering whether I'm looking at it the wrong way or whether anyone else has encountered the same problem and might have a soluton.</p> <p>Essentially, we have a system that comprises of a core assembly, and a number of modules that plug into it. Some of the modules rely on other modules, but the need has arisen to potentially remove or replace some of those dependencies from time to time and I'd like to avoid recompiles as far as possible.</p> <p>The system is a bespoke CMS and the modules are plugins providing features within the CMS. For example, we have a comments module and several content modules such as a news module, a blogs module etc. that can include commenting functionality. My problem is that some customers may not purchase the comments module, so I either need to find a way to prevent the dependent modules from depending on the existence of a comments module and, in some cases, may need to cater for a modified version of the comment module.</p> <p>We're loading the modules at runtime and, at present, to avoid interdependencies between the modules, we're handling this using interfaces that are held in the core CMS assembly. My concern is that to avoid having to modify the core CMS assembly every time we create new modules where a dependency could exist, I need to use something a lot looser than interfaces and implementations of those interfaces.</p> <p>I'm considering the following:</p> <ul> <li>Core assembly contains an object that allows the registration and unregistration of shared input/output messages (for example "Comments.AddComment" or "Comments.ListComments") </li> <li>When modules are loaded, they advertise the services they require and the services they provide (for example, a news module would require the "Comments.AddComment" message and any variant of the comments module would provide the "Comments.AddComment" message). </li> <li>Any objects or data that are passed to these messages will inherit from a very loose base class or implement an interface that exposes a property of type IDictionary that is contained within the core assembly. Alternatively, the contract for a message will require only a parameter of type object and I pass anonymous objects into them from the provider/consumer.</li> </ul> <p>The downside is obviously losing strong typing, but the plus is that I don't rely on a strict interface implementation or require the inclusion of modules that may not exist at runtime.</p> <p>Plugins are loaded via Reflection, checking referenced assemblies and looking for classes implementing a given interface. MEF and dynamic types aren't an option as I'm restricted to .NET 3.5.</p> <p>Can anyone suggest anything better, or perhaps a different way of thinking about this problem?</p>
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    1. COIf your using .NET V4 then you might want to consider looking at the new MEF (Managed Extensibility Framework) stuff that's in there. This article should give you a head start : http://www.codecapers.com/post/Close-Encounters-of-the-MEF-Kind.aspx
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    2. COThanks. Have used MEF before and a combination of MEF and dynamic classes would solve this problem in a very neat way, however, as I mention above, I'm tied to .NET 3.5 on this system.
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    3. COI might, just be able to help you there :-) I had to do something similar, and I did my own MEF like implementation but in 3.5, I'll need to see if I can find the code though....
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