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Microservices standard Healthcheck and API response

I am wondering about how should i implement certain things. I am new to the Microservices architecture, but i think i understood how it works in a certain way. But i am very confused in the shared code, and shared projects thing.
I am developing a new system which contains about 50 microservice including ApiGateWays as well. I am working only in ASP.net Core
I want to standardize the requests and responses. I created a new project which contians only models, so no logic included, just a few interface and some classes.
Is it hurting the core principals of microservice architecture any ways?
I know in a certain point it does, but i develop only in ASP.Net core, and it is a very basic no logic model collection, which standard in the whole system, so if i would create a node.js project then i would use the same model there as well.
The other thing is the HealthCheck middleware. I am thinking to create this in a shared project and i would just reference it from the microservices. This would contain logic, so i am confused, i think that if i would copy and paste it to everywhere than it would not be better than creating a shared project for it. It must be a standard thing, so if i have to change it for some reason then i would have to change it every single microservice as well.
What do you think about this? It is a good practice to do it? I know this is not the best practice, i know if i would work in a team i won't try to use the HealthCheck in a shared project, because that would cause more issues than it would solve, because which team should develop that and so on...
And the last thing, somebody can help me to understand this whole code sharing in microservices, if it is possible, i read a lot of articles about it, but everybody says that you can share, but you should avoid it. Please help me if you have time for it, i think a lot of newbie how jumps into microservices, and developing alone has this confusion in his mind.
Thanks for your time and i hope this would help some newbie in Microservices, which has the same confusion in Sharing code between microservices

In the microservices world, there will still be a need for shared libraries. Healthchecks is one of them.
On MSDN docs this is described in detail.
https://learn.microsoft.com/en-us/aspnet/core/host-and-deploy/health-checks?view=aspnetcore-3.1
A healthchecks shared library would that use that code and provide ways for your micriservices to register healthcheck endpoints with a call of one extension method.
Also that way the endpoints registered would be the same for all microservices.
Tell me if you need more info and I elaborate more.

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.

REST Web Services using MVC, is it a good idea?

In .Net, I think about a web service as being a project type that you select from the menu, define your classes and methods then .Net does all this black magic under the hood to allow someone on the other side of the world to reference my web service and start coding using my classes and methods directly within their visual studio.
So having this preconceived notion, when looking at writing REST web services using MVC 3 (I know MVC 4 has a REST api baked in but am waiting for a full release) I'm wondering all the usual stuff like "is this a good idea", "will this stand up to heavy use" and "am I just writing toy web services that other developers will laugh at".
Now I think a lot of my anxiety is probably down to microsoft not having wrapped a big, overly complicated, bloated, shiny REST package around it yet. So I'm looking to have my anxiety relieved hopefully by people telling me yes MVC web services are perfectly good things to create.
Any help?

I've done it a few times, I am still using it in production and haven't got any complaints.
I actually think its a nice solution because it so simple to setup and maintain.
Not this incredibly xml-configuration-heavy wcf stuff..

You might want to also have a look at the WebAPI stuff that is in the process of being released (.net 4.5):
http://weblogs.asp.net/scottgu/archive/2012/02/23/asp-net-web-api-part-1.aspx
It's very much to do with exposing plain html services.

I would suggest you take a look at ServiceStack: http://www.servicestack.net/. It 's not only quite mature, but it can help you produce cleaner code.

It really does depend on what you plan on doing with your application. Yes, you could write an MVC website that doubles as a RESTful service. However, you are then tying your UI layer very closely to your logic layer, and that is what you really need to consider. I am working on an MVC site with a ServiceStack REST service (already mentioned by #Ioannis) . The reason that I did not make MVC my REST service is because I did not want any changes in my UI to potentially affect any third party application that might be using my logic service. So, as long as you carefully consider the ramifications of making your site also your RESTful service, then either decision could be ok. :)

As others have mentioned here, ServiceStack provides a solid, terse REST Web Services Framework allowing you to effortlessly develop typed, idiomatic C# API's end-to-end.
ServiceStack also includes a number of high-performance components that deeply integrates with ASP.NET MVC using the ServiceStack.Host.Mvc NuGet package.
To learn more about the benefits ServiceStack can add to your MVC project see:
http://www.servicestack.net/mvc-powerpack/

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)