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How can I test the backward compatibility of API between .net assemblies

I have an assembly that provides an API and is used by some other assemblies. I need to verify that a newer version of API dll is still compatible with the older assemblies that were using the older version of API.
I've found a couple of questions that ask the same, but there are no answers that resolve my problem:
Tool to verify compatibility of a public APIs
Tool for backwards compatibility for the C#/.NET API
Suggested tools can only compare two assemblies and say if there are possible breaking changes in API, but not if the newest API really breaks the older assembly that uses it.
I'd like to find a tool or write a test that will be able to check whether each of the older dlls can work with my new API dll.
As for the changes in API more likely that I will only extend it, but even though it still can break the code in older assemblies. Some of the examples of such changes can be found here:
A definite guide to API-breaking changes in .NET
.NET: with respect to AssemblyVersion, what defines binary compatibility?
For now the only solution I see is to compile the source code of the older assemblies with the newest API, but I would like to do it only with assemblies and add them as part of my unit tests. Is there any better way I can handle that?
edit:
I'm looking for a tool that will be able to automate the process of verifying the backward compatibility between .net assemblies. (command line or with some api too)

What you want is to do a diff and generate a the list of breaking changes. Then you want to search if of your assemblies does use any of the broken APIs. You can do this with ApiChange tool to do the diff and to find any affected users of it.
To make it more concrete. If you have removed a method from an interface then you need to find all implementers and users of this method in classes which uses the interface method or any class that does implement this method.
ApiChange can search for implementers and users of specific methods on the command line with the commands -whoimplementsinterface and -whousesmethod. It is not automated at the command line but you can directly use the ApiChange.Api.dll to automate this queries.
Edit1:
I just forgot: The ApiChange tool has actually the functionality you are interested in already. It is the option
-ShowrebuildTargets -new -old [-old2 ] -searchin
We did use it in our department with good results. The only gotcha are the XML Intellisense files. If another target does not use the removed method but references it inside the XmlDoc the compiler will write a warning that a non existing method was referenced. This is quite hard to catch and would involve to parse the intellisense docu files as well. But this is quite an edge case.

I've spent the day looking around for an answer to this. It seems like the tools referenced on the related (unhelpfully closed) questions are now dead or as good as. But I've just taken a look at Telerik's assembly diff tool JustAssembly and this looks much better than rolling your own, which, if you look at their library seems to be a whole heap of work and likely to go wrong.
They have a UI which isn't of that much help from the point of view of integrating into your CI build, it is pretty basic, but you can build the library from source, which I've just done and the library looks like it has everything you need to get yourself up and running pretty quickly.

Approach to share code between Compact Framework, Silverlight, WP7 and full .NET runtime

I am creating libraries that I will use across Compact Framework, Silverlight, WP7 and the full .NET runtime. I am aware that the question around sharing between Compact Framework and full .NET, or between Silverlight and full .NET has been asked many times and I have been reading all the answers around that, however this situation is further complicated because I have to use VS2010 for Silverlight/WP7 and VS2008 for Compact Framework.
I therefore need to use multiple solutions along with multiple projects for this.
Is there a suggested "best-practice" approach for managing this, I am aware that I can create the multiple solutions/projects using add-file-as-link functionality to maintain the project, however this becomes a manual process open to error which I'd like to avoid.
Has anyone had any experience with automating the build of seperate frameworks, for example creating and maintaining a single .NET project, but having a custom build action which tweaks the solution and project files automatically and building several output assemblies for the different required frameworks. I am aware there is an added complications with making sure the correct references are generated.
Is there an existing framework that achieves this, I've had a search around but can't see anything. Alternatively is there an appetite for the creation of such a framework?

This session from PDC2010 might help:
http://blogs.microsoft.co.il/blogs/arik/archive/2010/10/31/pdc-2010-3-screen-coding-sharing-code-between-windows-phone-silverlight-and-net.aspx
Microsoft have announced that the Portable Library Project will be available H1 this year - so it should be available real soon - maybe at Mix?
Until then, the best advice seems to be to create a SL 3 class library project for most of your sharing - WP7, WPF, SL-Web, .Net desktop - but you'll then still need to do something special for WM6 (but for WM6 I still need to do lots of special things anyway - like it still insists on using VS2008!)

How should I store my custom classes?

I've gotten to the point where I have made a few classes that I have found to be rather useful for a variety of different projects, they're either extensions of the already existing .Net ones or something entirely new.
Although I may not use them for EVERY project I would most certainly use them again at some point, my questions is what is the best way to keep these stored?
I was thinking about compiling them into a .dll that I can simply reference if necessary but at the moment there are only about 4 different classes, I've always thought that a .dll is more suited towards a larger amount of classes.
Would it just be simpler to store them somewhere in the cloud so I can access them from pretty much any computer?
What has worked best for you?
Edit: I'll be using more than one computer as I sometimes use the university computer facilities.
The classes range from memory management helper classes in XNA to niche functions in regular .Net/C#

If the classes don't fit together naturally as an assembly, keep the source files somewhere like Github and include them in your projects where needed. You can always rearrange them into components at a later date, when you feel it's worthwhile.

Are these classes in any way related? If you want to use one of them, do you need the others? If not, then those don't belong in a common package together.
Robert C. Martin provides some decent introduction in the chapter "Principles of Package and Component Design" of his book "Agile Software Development". There is also a C# adapted version with very similar content called "Agile Principles, Patterns and Practices in C#".
What I'm just saying is, packaging components is not only about thinking components X and Y are "cool enough" to be reused, but also about how you organize things and how well libraries or packages fit into the big picture.

You could compile them as a DLL and install them to the GAC. Then you can reference the DLLs from any project you need, just like any native C# library.
And I agree with Jim Brissom. Compile only the classes that go together as one assembly.

I keep my common classes in sourcegear and then share them into any projects as required.

Building An App With Plug-in Support

I'm starting a new project which would greatly benefit from program add-ons. The program in its most basic form reads data from a serial port and parses it into database records. Examples of add-ons that could be written would be an auto-archive add-on, an add-on to filter records, etc. I'm writing both the program and the add-ons, but some customers need custom solutions, so instead of branching off and making a completely separate program, add-ons would be great. The simplest add-on would probably be a form who's constructor takes an object reference, manipulates the object in some way, then closes.
Unfortunately, I have absolutely no idea where to start coding, and almost as little idea where to search. Everything I search for turns up browser add-ons. From what I have gathered, I need to look into dynamic loading DLLs. Besides that, I'm clueless. Does anyone have any good resources or examples I that they know of?
I'm happy to provide more details, but this project is in its inception, so I don't have a ton of specific details (specifics kind of defeats the point of add-ons, too.)

You should seriously consider using the Managed Extensibility Framework (MEF) to handle your plugin architecture. It requires thinking about things a little differently, but it is well worth the mind-stretch.

This is a simple example to illustrate the basic technique.
codeproject.com - Plugin Architecture using C#
This article demonstrates to you how
to incorporate ... as a
plugin for another application or use
it as a standalone application.
in .NET 4 you now have the Managed Extensibility Framework (MEF) to do much of the plumbing.
In .NET 3.5 you had the System.AddIn but it was deemed by many to be far too complex.
codeproject.com - AddIn Enabled Applications with System.AddIn
AddIns (sometimes called Plugins) are
seperately compiled components that an
application can locate, load and make
use of at runtime (dynamically). An
application that has been designed to
use AddIns can be enhanced (by
developing more AddIns) without the
need for the orginal application to be
modified or recompiled and tested

You really need to look at Managed Extensibility Framework (MEF). This is specifically designed to help support add-ons and other extensibility.

A very basic description (basically, your plugins must implement a special interface):
http://martinfowler.com/eaaCatalog/plugin.html
Much better article, in C#:
http://www.drdobbs.com/184403942;jsessionid=TVLM2PGYFZZB1QE1GHPCKHWATMY32JVN

I think Reflection will play a major role.
I expirimented with an app that had a plugin folder. A filesystem watcher would watch the folder, and when a new DLL was placed in it, it would use reflection to determine which types of plugins it included, loaded them, and added them to the list of available classes, etc.

Try using the term 'add-in' or 'plug-in' for your research instead of 'add-on'. That should help some.
If you're using .Net 4, there's an add-in namespace in the framework that will get you partway there.
Writing plug-in support for an app is no simple task. You'll have to maintain pretty strict separation-of-concerns across your interfaces, you'll need to provide an interop library that defines ALL of the supported plug-in types, and you'll want to do some research into dependency injection & inversion of control, in addition to the previously-suggested reflection research.
It sounds like you might have a busy weekend doing research.

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.