Develop Reference

How to Develop Dynamic Plug-In Based Functionality in C#

I've been looking around for different methods of providing plug-in support for my application. Ideally, I will be creating a core functionality and based on different customers developing different plug-ins/addons such as importing, exporting data etc... What are the some methods available for making a C# application extensible via a plug-in architecture?
Lets make up an example. If we have a program that consists of a main menu ( File, Edit, View, et al. ) along with a TreeView that displays different brands of cars grouped by manufacturer ( Ford, GM, for now). Right clicking on a car displays a context menu with the only option being 'delete car'.
How could you develop the application so that plug-ins could be deployed so that you could allow one customer to see a new brand in the TreeView, let's say Honda, and also extent the car context menu so that they may now 'paint a car'?
In Eclipse/RCP development this is easily handled by extension points and plug-ins. How does C# handle it? I've been looking into developing my own plug-in architecture and reading up on MEF.

MEF would be a good place to start.
Glenn Block's article Managed Extensibility Framework: Building Composable Apps in .NET 4 with the Managed Extensibility Framework provides a good overview.
BTW, don't be fooled by the title - you can also get MEF for .NET 3.5 SP1.

Visual Studio 2010 uses MEF, so I think its a safe bet this is the preferred way to go at MS. System.Addin always seemed a bit heavy, but it might be a better choice if you need addins to always work and your codebase is constantly evolving.
If you care about isolating addins, you should read up on AppDomains. I've got a demo project which I made to help learn how to deal with isolating assemblies within an AppDomain here, which you might find interesting. Quick facts about isolation: Only your types should ever cross the boundary and these types should be sealed, run screaming from cross domain event handling, and addins should NEVER extend MarshallByRefObject.

Recommended migration strategy for C++ project in Visual Studio 6

For a large application written in C++ using Visual Studio 6, what is the best way to move into the modern era?
I'd like to take an incremental approach where we slowly move portions of the code and write new features into C# for example and compile that into a library or dll that can be referenced from the legacy application.
Is this possible and what is the best way to do it?
Edit: At this point we are limited to the Express editions which I believe don't allow use of the MFC libraries which are heavily used in our current app. It's also quite a large app with a lot of hardware dependencies so I don't think a wholesale migration is in the cards.
Edit2: We've looked into writing COM-wrapped components in C# but having no COM experience this is scary and complicated. Is it possible to generate a C# dll with a straight-C interface with all the managed goodness hidden inside? Or is COM a necessary evil?

I'd like to take an incremental
approach where we slowly move portions
of the code
That's the only realistic way to do it.
First, what kind of version control do you use? (If you use branching version control that allows you to make experiments and see what works, while minimizing the risk of compromising your code; others are OK also, but you'll have to be really careful depending on what you are using).
Edit: I just saw you are using SVN. It may be worthwile to move to mercurial or git if you have the liberty to do that (the change provides a quantum leap in what you can do with the code-base).
and write new features into C# for
example and compile that into a
library or dll that can be referenced
from the legacy application.
That's ... not necessarily a good idea. C# code can expose COM interfaces that are accessible in C++. Writing client code in C++ for modules written in C# can be fun, but you may find it taxing (in terms of effort to benefits ratio); It is also slow and error-prone (compared to writing C# client code for modules written in C++).
Better consider creating an application framework in C# and using modules (already) written in C++ for the core functionality.
Is this possible and what is the best
way to do it?
Yes, it's possible.
How many people are involved in the project?
If there are many, the best way would be to have a few (two? four?) work on the new application framework and have the rest continue as usual.
If there are few, you can consider having either a person in charge of this, or more people working part-time on it.
The percentage of people/effort assigned on each (old code maintenance and new code development) should depend on the size of the team and your priorities (Is the transition a low priority issue? Is it necessary to be finished by a given date?)
The best way to do this would be to start adapting modules of the code to be usable in multiple scenarios (with both the old code and the new one) and continue development in parallel (again, this would be greatly eased by using a branching distributed version control system).
Here's how I would go about it (iterative development, with small steps and lots of validity checks in between):
Pick a functional module (something that is not GUI-related) in the old code-base.
Remove MFC code (and other libraries not available in VS2010 Express - like ATL) references from the module picked in step 1.
Do not attempt to rewrite MFC/ATL functionality with custom code, unless for small changes (that is, it is not feasible to decide to create your own GUI framework, but it is OK to decide to write your own COM interface pointer wrapper similar to ATL's CComPtr).
If the code is heavily dependent on a library, better separate it as much as possible, then mark it down to be rewritten at a future point using new technologies. Either way, for a library heavily-dependent on MFC you're better off rewriting the code using something else (C#?).
reduce coupling with the chosen module as much as possible (make sure the code is in a separate library, decide clearly what functionality the module exposes to client code) and access the delimited functionality only through the decided exposed interface (in the old code).
Make sure the old code base still works with the modified module (test - eventually automate the testing for this module) - this is critical if you need to still stay in the market until you can ship the new version.
While maintaining the current application, start a new project (C# based?) that implements the GUI and other parts you need to modernize (like the parts heavily-dependent on MFC). This should be a thin-layer application, preferably agnostic of the business logic (which should remain in the legacy code as much as possible).
Depending on what the old code does and the interfaces you define, it may make sense to use C++/CLI instead of C# for parts of the code (it can work with native C++ pointers and managed code, allowing you to make an easy transition when comunicating between managed .NET code and C++ native code).
Make the new application use the module picked in step 1.
Pick a new module, go back to step 2.
Advantages:
refactoring will be performed (necessary for the separation of modules)
at the end you should have a battery of tests for your functional modules (if you do not already).
you still have something to ship in between.
A few notes:
If you do not use a distributed branching version control system, you're better off working on one module at a time. If you use branching/distributed source control, you can distribute different modules to different team members, and centralize the changes every time something new has been ported.
It is very important that each step is clearly delimited (so that you can roll back your changes to the last stable version, try new things and so on). This is another issue that is difficult with SVN and easy with Mercurial / Git.
Before starting, change the names of all your project files to have a .2005.vcproj extension, and do the same for the solution file. When creating the new project file, do the same with .2010.vcxproj for the project files and solution (you should still do this if you convert the solutions/projects). The idea is that you should have both in parallel and open whichever you want at any point. You shouldn't have to make a source-tree update to a different label/tag/date in source control just to switch IDEs.
Edit2: We've looked into writing
COM-wrapped components in C# but
having no COM experience this is scary
and complicated.
You can still do it, by writing wrapper code (a small templated smart pointer class for COM interfaces wouldn't go amiss for example - similar to CComPtr in ATL). If you isolated the COM code behind some wrappers you could write client code (agnostic of COM) with (almost) no problems.
Is it possible to generate a C# dll
with a straight-C interface with all
the managed goodness hidden inside? Or
is COM a necessary evil?
Not that I know of. I think COM will be a necessary evil if you plan to use server code written in C# and client code in C++.
It is possible the other way around.

Faced with the same task, my strategy would be something like:
Identify what we hope to gain by moving to 2010 development - it could be
improved quality assurance: unit testing, mocking are part of modern development tools
slicker UI: WPF provides a modern look and feel.
productivity: in some areas, .NET development is more productive than C++ development
support: new tools are supported with improvements and bugfixes.
Identify which parts of the system will not gain from being moved to C#:
hardware access, low-level algorithmic code
pretty much most bespoke non-UI working code - no point throwing it out if it already works
Identify which parts of the system need to be migrated to c#. For these parts, ensure that the current implementation in C++ is decoupled and modular so that those parts can be swapped out. If the app is a monolith, then considerable work will be needed refactoring the app so that it can be broken up and select pieces reimplemented in c#. (It is possible to refactor nothing, instead just focus on implementing new application functionality in c#.)
Now that you've identified which parts will remain in C++ and which parts will be implemented in c#, (or just stipulate that new features are in c#) then focus turns to how to integrate c# and c++ into a single solution
use COM wrappers - if your existing C++ project makes good use of OO, this is often not as difficult as it may seem. With MSVC 6 you can use the ATL classes to expose your classes as COM components.
Integrate directly the native and c# code. Integrating "legacy" compiled code requires an intermediate DLL - see here for details.
Mixing the MFC UI and c# UI is probably not achieveable, and not adviseable either as it would produce a UI mix of two distinct styles (1990s grey and 2010 vibe). It is simpler to focus on achieving incremental migration, such as implementing new application code in c# and calling that from the native C++ code. This keeps the amount of migrated c# code small to begin with. As you get more into the 2010 development, you can then take the larger chunks that cannot be migrated incrementally, such as the UI.

First, your definition of modern era is controversial. There's no reason to assume C# is better in any sense than C++. A lot has been said on whether C# helps you better avoid memory management errors, but this is hardly so with modern facilities in C++, and, it's very easy to do mess with C# in terms of resource acquisition timing, that may be dependent on what other programs are doing.

If you move straight from 6 to 2010 you may end up with some messed up project settings. If this isn't a fairly large project, and it's one of few that you need to convert, then that should be fine. Just open it in 2010, and follow the conversion wizard. Make sure to back up your project first, and verify your project settings when you're done.
In my opinion though the best way is to convert it step by step through each iteration of Visual Studio. I had to modernize 1400 projects from 2003 to 2010, and the best way that I found was to convert everything to 2005, then to 2008, and then finally to 2010. This caused the least amount of issues to arise for me.
If you only have 6 and the newest Visual Studio you may end up just having to try and go straight to the new one using the wizard. Expect some manual cleanup before everything builds correctly for you again.
Also, one more time, BACK IT UP FIRST! :)

High-level C++ code calling low-level C# code doesn't look like a good idea. The areas where .NET languages are better, are user interface, database access, networking, XML files handling. Low-level stuff like calculations, hardware access etc. is better to keep as native C++ code.
Moving to .NET, in most cases it is better to rewrite UI completely, using WPF or Windows Forms technologies. Low-level stuff remains native, and different interoperability technologies are used to connect C# and native code: PInvoke, C++/CLI wrappers or COM interoperability. After some time, you may decide to rewrite low-level native components in C#, only if it is really necessary.
About compiling native C++ code in VS2010 - I don't see any problems. Just fix all compilation errors - new compilers have more strict type checking and syntax restrictions, and catch much more bugs at compilation time.

Not sure why so many folks are advocating for COM. If you haven't already got a lot of COM in there, learning how to do it on the C++ side is going to hurt, and then you're using the slowest possible interop from the managed side. Not my first choice.
Ideally you have refactored your UI from your business logic. You can then build a new UI (WPF, WinForms, ASP.NET, web services that support some other client, whatever) and call into your business logic through P/Invoke or by writing a C++/CLI wrapper. #mdma has good advice for you assuming that the refactoring is possible.
However if you were paying me to come in and help you my very first question would be why do you want to do this? Some clients say they don't want to pay C++ devs any more, so they want all the C++ code gone. This is a scary objective because we all hate to touch code that works. Some clients want to expose their logic to ASP.NET or Reporting Services or something, so for them we concentrate on the refactoring. And some say "it looks so 1999" and for them I show them what MFC looks like now. Colours, skinning/theming including office and win7 looks, ribbon, floating/docking panes and windows, Windows 7 taskbar integration ... if you just want to look different, take a look at MFC in VS 2010 and you might not have to adjust any code at all.
Finally to make non-Express versions of VS 2010 affordable look into the Microsoft Partner Program. If you have sold your software to at least 3 customers who still speak to you, and can get through the Windows 7 logo self test (I have got VB 6 apps through that in a day or two) then you can have 5-10 copies of everything (Windows, Office, VS) for $1900 or so a year, depending on where you live.

To start I'd try and keep as much code as possible to avoid a rewrite. I'd also remove all unused code before starting the conversion.
Since VC++ 6.0 Microsoft changed the MFC libraries and the C++ Standard Library.
I recommend to start building your DLLs with no dependencies, then looking at your third party libraries, and then rebuild one dependent DLL/EXE at a time.
Introduce unit tests to make sure the behaviour of code does not change.
If you have a mixed build, using different versions of VC++, you need to guard against passing resources (file handles) between DLLs that use different versions of the VC runtime.

If at all financially possible I would strongly consider just paying the money for the version of Visual Studio that you need because you could very well lose more money on the time you spend. I do not know enough about the express editions to give a good answer on them but when integrating some code from a subcontractor that was written in C++ I used C++ / CLI. You will probably be able to reuse most of your codebase and will be familiar with the language but you will also have access to managed code and libraries. Also if you want to start writing new code in C# you can do that. The biggest problem I had with it was that in VS 2010 there is no intellisense in C++ / CLI.

Visual Studio 6 is legendary for being buggy and slow. Moving into the modern era would best be done by getting a new compiler. What is probably the easiest thing to do is write the legacy app into a DLL, then write your exe into C# and use P/Invoke. Then you never have to touch the old code again- you can just write more and more in C# and use less and less of the old DLL.
If your old code is very heavily OO, you can use C++/CLI to write wrapper classes that allow .NET to call methods on C++ objects, and collect them too if you use a reference counted smart pointer.

You can use C# to write your new components with a COM or COM+ (System.EnterpriseServices) wrapper, which will be callable from your existing C++ code.

Plugin-like architecture in .NET

I'm trying to implement a plug-in like application. I know there are already several solution out there but this is just going to be proof of the concept, nothing more. The idea would be to make the application main application almost featureless by default and then let the plugins know about each other, having them have implement all the needed features.
A couple of issues arise:
I want the plugins at runtime to know about each other through my application. That wouldn't mean that at code-time they couldn't reference other plugin's assemblies so they could use its interfaces, only that plugin-feature initialization should be always through my main app. For example: if I have both plugins X and Y loaded and Y wants to use X's features, it should "register" its interest though my application to use its features. I'd have to have a kind of "dictionary" in my application where I store all the loaded plugins. After registering for interest in my application, plugin Y would get a reference to X so it could use it. Is this a good approach?
When coding plugin Y that uses X, I'd need to reference X's assembly, so I can program against its interface. That has the issue of versioning. What if I code my plugin Y against an outdated version of plugin X? Should I always use a "central" place where all assemblies are, having there always the up to date versions of the assemblies?
Are there by chance any books out there that specifically deal with these kinds of designs for .NET?
Thanks
edit: I think people are drifting away from the 2 questions I made. I can take a look at both MEF and #develop, but I'd like to get specifics answers to the questions I made.

I recommend looking into MEF. This is a new way of doing plugins in .NET. It is the recommend way of doing new addins for VS2010, for example. I've not used it myself, but what I've looked into about it looks great. Adding this as an answer on prodding of others :)

Look into the System.AddIn namespace. It's a little lower-level than MEF, and so should give you the "implement it myself" experience you're looking for.

There is a good book on building what you are looking for: Dissecting a C# Application: Inside SharpDevelop. Here's a link: http://www.icsharpcode.net/OpenSource/SD/InsideSharpDevelop.aspx
The SharpDevelop application is fully plugin-based and the book talks about how they built it, the pitfalls they faced, and how they overcame it. The book is freely available from the site, or you can buy it too.

Once I done it using this example. I loved it, but it was couple years ago, I think there might be better solutions now. As long as I remember the basic idea was that there is abstract class in your program, and your plug-ins inherit that class and compiled as DLLs... or something similar using Interfaces. Anyways that approach worked great for me. Later I added a filesystemwatcher so it could load those DLL plugins while it is running.
To load an Assembly
To get the types the assembly exposes

About the two specific issues you exposed:
1) I'm not sure what are you trying to achieve, but my guess is that you want to have lazy initialization of features, and maybe lazy loading of add-ins. If that's the goal, what you are proposing might work. So it could work like this:
The Y plugin provides a list of features it needs to use (it could be done for example through a specific interface implementation or through an xml manifest).
The X add-in implements an API which allows initializing a feature, with a method like Initialize(featureId).
The host application gets the feature list required by Y, loads/initializes the X plugin, and calls Initialize for each feature.
The host application also provides a GetFeature() method which Y can use to get a reference to a 'feature' object, which would be implemented in X.
However, if the plugin Y has direct access to the X API, I think it is unnecessary to have all that infrastructure for registering features. Y can just access the X features by directly using the X API, and Y would take care of lazy initializing each feature when required. For example, Y could just call SomeXFeature.DoSomething(), and the implementation of that class would initialize the feature the first time it is used.
2) If the API of an assembly changes, any assembly depending on it may break. Plugins are just assemblies which depend on other assemblies, so they will also break. Here are some things you can do to alleviate this problem.
Assign a version number to each plugin. This could be just the assembly version.
When loading a plugin, ensure that all dependencies can be properly satisfied (that is, all plugins on which it depends must be present and have the required version). Refuse to load the plugin if dependencies can't be satisfied.
Implement a plugin management tool, to be used for all plugin install/uninstall operations. The manager can check dependencies and report errors when trying to install plugins with unsatisfied dependencies, or when trying to uninstall a plugin on which other plugins depend.
Similar solutions are used by the Mono.Addins framework. In Mono.Addins, each add-in has a version number and a list of add-ins/versions on which it depends. When loading an add-in, the add-in engine ensures that all dependent add-ins with the correct versions are also loaded. It also provides an API and a command line tool for managing the installation of add-ins.

Sandboxing plugins with Managed Extensibility Framework

I'm working on an application where third party developers will be able to write plugins. I've been looking a little at Managed Extensibility Framework and it seems the right way to go.
One thing though, I want to prevent plugins from accessing the rest of the application freely (calling singletons etc) but would want to restrict to to communicate via some interface, ideally each plugin would have to "request" permission for different things like accessing other plugins and user data, is there a good way to do accomplish this?
Only thing I can think of otherwise is to have a security string passed to each method and obfuscate the hell out of the code but it seems like an ugly solution :P

What you need is a new AppDomain to be the sandbox for your plugin, but I don't think MEF supports loading exports into a separate AppDomain at this time (I'm sure someone will correct me if this is no longer the case).
If this is a serious concern for you, consider using the bits in the System.Addin namespace, and see this section on Activation, Isolation, Security, and Sandboxing for more information. It's a much more robust and secure alternative to MEF, but is far less flexible.
Update: Kent Boogaart has a blog post showing how you can use MEF and MAF together.

How can I create my custom Shell Context Handlers for Windows?

Problem
Language: C# 2.0 or later
I would like to register context handlers to create menues when the user right clicks certain files (in my case *.eic). What is the procedure to register, unregister (clean up) and handle events (clicks) from these menues?
I have a clue it's something to do with the windows registry, but considering how much stuff there is in .net, I wouldn't be surprised if there are handy methods to do this clean and easy.
Code snippets, website references, comments are all good. Please toss them at me.
Update
Obviously there is a slight problem creating context menues in managed languages, as several users have commented. Is there any other preferred way of achieving the same behaviour, or should I spend time looking into these workarounds? I don't mind doing that at all, I'm glad people have put effort into making this possible - but I still want to know if there is a "proper/clean" way of achieving this.

Resist writing Shell Extensions in managed languages - there are a multitude of things that could go bang if you pursue this route.
Have a browse through this thread for more details. It contains links to do it if really want, and sagely advice of why it can be done, but shouldn't.
http://social.msdn.microsoft.com/Forums/en-US/netfxbcl/thread/1428326d-7950-42b4-ad94-8e962124043e/
You're back to unmanaged C/C++ as your only real tools here.

This is not a good idea because of potential dependency issues between different versions of the .NET Framework. Your shell extension could be expecting one version, while a different version may have already been loaded by the application that's currently running.
This thread contains a good summary of the situation.

While others already mentioned that writing shell extensions in pure .NET is a bad idea due to framework conflicts, you should still note that:
There are 3rd party drivers out there (see Eldos or LogicNP) that do the unmanaged side for you, allowing you to write managed code that talks to the native driver, thus preventing shell-related CLR version conflicts.
A recent MSDN article mentioned that Microsoft has solved this problem for the CoreCLR, as used by Silverlight. They've accomplished this by allowing multiple versions of the CLR to run in the same process, thus fixing the problem. The author further stated that this fix in Silverlight will be rolled into future versions of the full CLR. (Meaning, in the future, it will be quite feasible to write shell extensions in managed code.)

I've done them before in C#. It ends up being a hell of a lot harder than it should be. Once you get the boilerplate code down, though, it is easy to roll out new items. I followed this link:
Link To Info

As the prior comments mention, it isn't the best idea to write shell extensions in managed languages, but I thought I'd share an Open Source project that is doing just that :)
ShellGlue is a managed shell extension that is actually quite helpful. The source also might be helpful to you if you're interested in pursuing writing a shell extension in C/C++.

Aside from the caveats that have been mentioned concerning the implementation of shell extensions in managed code, what you'd basically need to do is the following:
First, create a COM component in C# that implements the IShellExtInit IContextMenu interfaces. How to create COM components in C# is described here. How to implement the necessary interfaces is described in this article. While the description is for a C++ implementation, you can apply that knowledge to you C# version.
Your COM component will have GUID called the Class-ID or CLSID. You need to register that ID with your file type as a context-menu shell extension:
HKEY_CLASSES_ROOT\.eic\ShellEx\ContextMenuHandlers\MyShellExt
(Default) -> {YOUR-COMPONENTS-CLSID}
Also make sure that you registered your component correctly as described in the C# COM tutorial. You should find it in the registry under
HKEY_CLASSES_ROOT\CLSID\{YOUR-COMPONENTS-CLSID}
InprocServer32
(Default) -> C:\WINDOWS\system32\mscoree.dll
Class -> YourImplClass
assembly -> YourAssembly, version=..., Culture=neutral, PublicKey=...
...
Good luck...

As others have pointed out, shell extensions are not practical in windows development currently.
I asked a similar question recently which was answered with a link to a guide to do exactly what I wanted to do