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Multi-Tenant Architecture for MVC 4

I need to create/publish a complex MVC site to several clients, each with mostly shared functionality but also custom stuff, such as client specific controllers / views / business logic etc. Most insist on hosting the site themselves, and have functionality they don't want others to know about.
Following reading this SO post and this, I've created a means for MVC Multi-tenancy, which seems to handle most scenarios.
As I can't attach to Stack Overflow, I have posted it here (no need to read it all - it's mostly screenshots!).
The basis is to have a generic project, referenced by several client projects. The client project can then have a similar structure to the generic and take precedence when I wish to use overriding code/controls/views.
As it's the foundation of the whole thing, I don’t want to implement something only to find everyone else does it in an easier/ better way.
My question is a bit wooly, but simple - Is there a better way?

Your architecture seems about right. I would just say that you have to be really careful about the generic thing and how the client's addons will talk to the core application.
I would do that by having a base project in the CVS with client's specific branchs referencing it (as in git submodule) so everyone can beenfit from the core.
Rolling out production and QA versions are also something that must be dealt with caution as you can end up with different version far from each other depending on the client.

How to make the framework and the dependent applications loosely coupled?

I have a specific case and I want to know the best practice way to handle it.
I make a specific .NET framework (web application). This web application acts like a platform or framework to many other web applications through the following methodology :
We create our dependent web applications (classes for the project business, rdlc reports) in a separate solutions then build them.
After that we add references to the resulted dll in the framework.
And create set of user controls (one for each dependent web application) and put them in a folder in the framework it self.
It works fine but any modification to a specific user control or any modification to any one of the dependent web applications. We have to add the references again and publish the whole framework !!
What I want to do is make those different web applications and the framework loosely coupled. So I could publish the framework one and only one and any modifications to the user controls or the different web applications just publish the updated part rather than the whole framework .
How to refactor my code so I can do this?
The most important thing is :
Never publish the whole framework if the change in any dependent application, just publish the updated part belongs to this application .

If loose coupling is what you are after, develop your "framework(web application)" to function as a WCF web service. Your client applications will pass requests to your web services and receive standard responses in the form of predefined objects.
If you take this route, I recommend that you implement an additional step: Do not use the objects passed to your client applications directly in your client code. Instead, create versions of these web service objects local to each client application and upon receiving your web service response objects, map them to their local counterparts. I tend to implement this with a facade project in my client solution. The facade handles all calls to my various web services, and does the mapping between client and service objects automatically with each call. It is very convenient.
The reason for this is that the day that you decide to modify the objects that your web service serves, you only have to change the mapping algorithms in your client applications... the internal code of each client solution remains unchanged. Do not underestimate how much work this can save you!
Developing WCF web services is quite a large subject. If you are interested, a book that I recommend is Programming WCF Services. It offers a pretty good introduction to WCF development for those who come from a .NET background.

I totally agree with levib, but I also have some tips:
As an alternative to WCF (with its crazy configuration needs), I would recommend ServiceStack. Like WCF it lets you receive requests and return responses in the form of predefined objects, but with NO code generation and minimal configuration. It supports all kinds of response formats, such as JSON, XML, JSV and CSV. This makes it much easier to consume from f.ex. JavaScript and even mobile apps. It even has binaries for MonoTouch and Mono for Android! It is also highly testable and blazing fast!
A great tool for the mapping part of your code is AutoMapper, it lets you set up all your mappings in a single place and map from one object type to another by calling a simple method.
Check them out! :)

Decades of experience says: avoid the framework and you won't have a problem to solve.
Frameworks evolve like cancer. The road to hell is paved with good intentions, and a good portion of those good intentions are embodied in a colossal tumour of a framework all in the name of potential re-use that never really happens.
Get some experience and knowledge when it comes to OO and design, and you'll find endless solutions to your technical problem, such as facades, and mementos, and what have you, but they are not solutions to your real problem.
Another thing, if you are using MS technology, don't bother with anything beyond what .NET offers. Stick with what the MS gods offer because as soon as you digress and become committed to some inhouse framework, your days are numbered.

What is the best way expose key classes/methods my core API to 3rd party developers?

I have an application that I have designed and this app has a pretty decent core dll that contains an API that my main view's exe uses. I would like to allow other developers to access this core dll as well but I don't want them to have as much access as me since it would be a security risk. What is the standard way of exposing my core dll? Are there any particular design patterns I should be looking at?
I'm using C#
Edit: my question was a little vague so here is some clarification
My program is deployed as a windows exe which references the core.dll. I want other people to create extensions which dynamically get loaded into my program at start up by loading dlls in the /extensions directory. The 3rd party dlls will inherit/implement certain classes/interfaces in my core.dll. I only want to give 3rd parties limited access to my core but I want to give my exe additional access to the core.
I should mention that this is the first time I have written a program that imports DLLs. Perhaps this whole method of allowing users to add extensions is wrong.

How do I modify/expose my API for
other developers?
To deliberately allow other developers to work with an API you've built touches on many things, which can be broken into two areas:
Resources (documentation, samples, etc) that makes it easier for them to understand (yes - basically an SDK).
Architecting, constructing and deploying your solution so that it's easy to actually work with.
Examples include:
By packing it in a way that suits re-use.
By using naming conventions and member names that others can easily follow.
Documentation, samples.
Providing the source code (as open source) if you're happy for them to modify it.
I would like to allow other developers
to access this core dll as well but I
don't want them to have as much access
as me since it would be a security
risk.
Ok, so this gets us right into the second area - the actual solution.
The problem you have is not a trivial one - but it's also quite do-able; I'd suggest:
Looking into existing material on plugins (https://stackoverflow.com/questions/tagged/plugins+.net)
Personally, I've found using attributes and Dependency Inversion to be a great approach.
There's also stuff like the Managed Extensibility Framework which you should consider.
The big issue you face is that you're into serious architecture territory - the decisions you make now will have a profound impact on all aspects of the solution over time. So you might not be able to make an informed decision quickly. Still - you have to start somewhere :)

The "design patterns" in terms of an API are more related to things like REST.
I don't want them to have as much
access as me since it would be a
security risk
Then i would (for the sake of maintenance), layer on top of the core DLL extra logic to prevent this.
The thing is, the "clients" call the API, not the Core DLL.
"How" the API accesses the Core DLL is under your full control. Just only expose operation contracts that you wish.

Since you're using C#, I would look at Microsoft's Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries and use FxCop to in-force many of them (latest version here). This won't be all you'll likely need, but it would help put you in the right direction.
Also, take a look at the freely available distillation of Framework Design Guidelines by the same author.

Architecture for new ASP.NET web application

I'm maintaining an application which currently is just a web service (built with WCF) and a database backend. The web service is built in layers with a linq-to-sql data access part with core functionality in an own assembly and on top of that the web service assembly which contains the WCF code. The core assembly also handles all business logic rules (very few actually).
The customer now wants a Web interface for the application instead of just accessing it through other applications which are consuming the web service. I'm quite lost on modern web application design, so I would like some advice on what architecture and frameworks to use for the web application. The web application will be using the same core assembly with business rules and the linq-to-sql data access layer as the web service.
Some concepts I've thought about are:
ASP.NET MVC (or MVC-2)
Webforms
AJAX controls - possibly leting the AJAX controls access the existing web service through JSON.
Are there any more concepts I should look into? Which one is the best for a fresh project?
The development tools are Visual Studio 2008 Team Edition for Developers targeting .NET 3.5. An upgrade to Visual Studio 2010 Premium (or maybe even Ultimate) is possible if it gives any benefits.

Definitely dig into ASP.NET MVC2.
All of our projects are now being developed using ASP.NET MVC2. It's not just highly scalable. It's highly testable as well. Which leads to way better maintainable apps in the long term.
WebForms vs. MVC2 points - (speaking out of experience):
Scalability:
In our company we had a lot of applications using WebForms which then were updated and changed by us as needed by our customers.
I think your customer will be requesting more changes on the application in near future. Making calls to other services, and maybe you'll have to rework parts of the final product to match their wishes.
And with the upcoming Cloud Computing and the Windows Azure platform you'll probably need to keep up with your code.
ASP.NET MVC absolutely supports the concept of being able to scale up your application any time you want.
I remember one of our customers walking up on me asking me for an extension for their app (they have a member management system) and the feature would contain something like a link to export the current view as a csv file so they could do office stuff with it (mostly serial letters).
It wasn't really a big problem setting that feature up. (took around 2 hours including writing tests) - let's go to tests:
Testability:
Using WebForms we didn't really have much interest writing tests because it was a pure pain to do so.
I remember writing some tests to have at least some proofs but let's drop that topic.. (:p)
We had tests for our custom classes but we couldn't really test all the EventHandlers within the WebForms.
Our CodeBase is way cleaner and saver to use thanks to this testable environment. I just check out some of the code, modify it, run all the tests and: Oh, it broke on strange behavior! - Let's fix that again. Earlier, I remember sitting with my co-worker debugging and trying to find those bugs for hours.
With ASP.NET MVC2 we are now lacking tests!
We ask all kinds of people (even the non-Web ones) to provide test-cases we could feed into our TestSuite.
And yeah, there are some AJAX-Controls too:
AJAXability:
You asked about AJAX Controls and in conjunction with ASP.NET MVC I highly recommend you to check out Telerik ASP.NET MVC UI Controls.
If that isn't something for you, we also make extensive use of jQuery and jQuery UI
With ASP.NET MVC and the HTML Views, those libraries aren't just a pleasure to use, they just look amazingly beautiful.
There is no random-html-tag-id-value autogeneration anymore!
But what I like most is: You can finally really re-use your code again.
There is so much more to those frameworks than just that, there is the T4 templating system. Auto-Scaffolding for your ViewModels / DomainModels with the Html.EditorFor() method and of course there is a great and easy way to use the IoC and DI paradigms.
Assuming that you have asked the question with mostly .NET Framework related tags, you'll probably stick with it.
Just to keep the post complete, there are also other frameworks that are just as good (or even better):
Ruby on Rails
Django
CakePHP
And many many more!

There's also DynamicData which may be appropriate if you need simple CRUD access to your data.

The Web Service Software Factory (WSSF) might come in handy in your situation.
This will allow you to define your contracts (XML entity returned (if XML you choose), etc.) while designing the server/client communication using WCF (or standard Web Service communication protocol).
WSSF favors either ASP.NET MVC or ASP.NET MVP. A simple example of the MVP architecture is shown here, plus this article.
As for me, I often come with a hybrid-like architecture using a bit of both MVC and MVP, as both have different strong points which combined together fill each other's improvement points.

I'd also recommend looking into Silverlight.
http://www.silverlight.net/learn/

Just my opinion to use MVC on Client sites and WebForms inside administration pages(site)

Building a highly modular business application with WPF?

I'm fleshing out a WPF business application in my head and one thing that sparked my interest was how I should handle making it incredibly modular. For example, my main application would simply contain the basics to start the interface, load the modules, connect to the server, etc. These modules, in the form of class libraries, would contains their own logic and WPF windows. Modules could define their own resource dictionaries and all pull from the main application's resource dictionary for common brushes and such.
What's the best way to implement a system of this nature? How should the main interface be built so that the modules it loads can alter virtually any aspect of its user interface and logic?
I realize it's a fairly vague question, but I'm simply looking for general input and brainstorming.
Thanks!

Check out Composite Client Application Guidance
The Composite Application Library is designed to help architects and developers achieve the following objectives:
Create a complex application from modules that can be built, assembled, and, optionally, deployed by independent teams using WPF or Silverlight.
Minimize cross-team dependencies and allow teams to specialize in different areas, such as user interface (UI) design, business logic implementation, and infrastructure code development.
Use an architecture that promotes reusability across independent teams.
Increase the quality of applications by abstracting common services that are available to all the teams.
Incrementally integrate new capabilities.

First of all you might be interested in SharpDevelop implementation. It is based on it's own addin system known as AddInTree. It is a separate project and can be used within your own solutions for free. Everything is split across different addins where addins are easily added/removed/configured by means of xml files. SharpDevelop is an open source project so you'll have a chance examining how the infrastructure is introduced, as well as service bus and cross-addin integrations. The core addin tree can be easily moved to WPF project without implications.
Next option is taking "Composite Client Application Guidance" (aka Prism, aka CompositeWPF) already mentioned earlier. You will get the Unity (Object builder) support out-of-box, Event Aggregation as well as the set of valuable design patterns implemented.
If you want to perform some low level design and architecture yourself the MEF will be the best choise (though working with all three I personally like this one). This is what VS 2010 will be based on so you might be sure the project won't lose support in future.
My advice is elaborating on these approaches and selecting the best and efficient one that perfectly suits your needs and the needs of your project.

Have a look at Prism