C++/CLI wrapping a C library: cheatsheet

Because once in a while I write a C++/CLI wrapper around a C library, but since I do it so infrequently I forget everything each time. Here is a list of pointers (haha) for common pitfalls and problems.

Compiling and debugging

You have to wrap the .h into an #ifdef __cplusplus extern “C” :

You also need to remove the /clr option for C files.
Right click your C files > Properties > C/C++ > General > Common Language RunTime Support > No CLR Support.

In your managed class, to include an unmanaged C header, use managed(push|pop)

Because Linux and Windows get along so well, you may also have to redefine a few C methods in your header file.

If you wish to create a C# unit test project (using xUnit for instance), with Visual Studio 2012 and later, you will have to do a few things:

  • In the C++/CLI project’s properties, in Debugging, choose the Debugger Type “Mixed”
  • In the Visual Studio options, Debugging, check “Managed C++ Compatibility Mode” (in VS2012) / “Use Managed Compatibility Mode” (in VS2013).

To build using MSBuild, you may have to change the toolset to VS2012: project properties > General > Platform Toolset = Visual Studio 2012 (v110). And set the Configuration Type to Dynamic Library (.dll) while you’re at it.

Managed and unmanaged objects

gcnew instantiates a managed objects, which will be garbage collected. Always use it for the managed objects.

To easily call the native structures through managed classes, use the internal constructor and ToNative methods.

Creating a web API using ASP.Net WebApi over a legacy database

I’m trying to create a coherent API to access a crazy legacy database, do I decided to use an Asp.Net WebApi project. Since I want to get a somewhat sane result, I’ll test everything.

First things first: create a new WebApi project. It’s a type of project, located under Asp.Net MVC websites.
Don’t forget to update the project’s nuget packages ASAP, since doing it too late will make you tear you hairs out with broken links and missing files.

I will not go over creating API controllers, models etc, because there are a billion websites to help you with that, and it’s pretty straightforward. Instead, I will try to regroup all “best practices” when dealing with a web API in a single place, applied to the problems of dealing with a legacy database. I will also not dwell on the various concepts (data transfer objects, dependency injection, etc), you can read more about them at length with a quick Google search.

Version your API

If your project has any kind of risk associated with it (which means: once in production, will breaking it make you lose money?), you should version your API. So that your website can use the latest and most awesome version, but an executable running somewhere lost and forgotten in the bowels of your middle office doesn’t suddenly (and probably silently) crash once a method returns differently-named fields.

For that, there are several method, each one with various cons and pros. You can read a nice summary (and how to include all main methods at once in your project) here.
I decided to use URL versioning, because it’s the most simple to use and understand, and the most “visible”. Working with various contractors that use various languages and technologies, I always have to choose the less painful option.

To implement API URL-based versioning, the most simple, painless option is to use attribute-based routing. Make sure you have the AttributeRouting.WebApi Nuget package, and just add attributes to your controller’s methods:

I advise you against using namespace-based versioning (as seen here), because it makes your life a living nightmare. Everything you will be using (like Swagger) will assume your API is not versioned, so you have to keep default routes, and namespace-based versioning doesn’t allow that (unless you succeed in modifying the linked class, which I didn’t try).

Create a public data layer

Publish Data Transfer Objects (DTO)

One annoying thing when you try to “publish” entities from Entity Framework, is that you get all the foreign keys relationship stuff in your JSON result.
It also strongly links your underlying model with your API, which may be OK, but is not awesome, especially considering that our model is crazy (still working with a legacy DB, remember?).

So, in order to solve these problems (and also because I am not fond of publishing the underlying data model directly), there is a design pattern called Data Transfer Objects.

To implement this pattern, let’s create a Model project, create the Entity Framework entities inside, and publish them through custom and “clean” objects.

Always be remembering that we’re dealing with a legacy DB. Our crazy fields with random casing, cryptic names and insane types (for instance, everything in my main table is a varchar 100, even dates, booleans and numbers) would really love a nice grooming. You’re in luck, because tools exist to make your life easier.

We will use AutoMapper to bind the entities to the custom data objects. This awesome tool will do much of the grunt work for us.

Let’s pretend that in the actual DB, we have entities looking a bit like this:

Since having all fields as string and with random casing  is clearly insane (or lazy, or both, who knows?), we want to map it to an object like this:

In order to do that, we will use AutoMapper so that we don’t have to manually create object converters. It doesn’t prevent us to write code, though, it just means that all mapping code can be stored in a single place.

System types are best converted using system conversion, but in my case the datetime have some weird things going on, so I created my own converter.

Note that you can also write a custom converter to convert from and to Order and OrderDto. However, it makes you write mapping method “by hand” in various places like constructors, and I like having all the casting in one place, even though it can get quite long after a while. The mapping can also be much more complicated than the above example, as seen here for instance, and using AutoMapper helps a lot in these cases.

Use dependency injection to publish the DTOs

In order to help in the next step (creating test), we will use dependency injection in our controllers.

In your Model project, create a IEntitiesContainer interface with all the methods to fetch the data you need.

Then, still in your Model project, implement this interface in a class that will wrap around the actual Entity Framework container.

Note that tying the controller to the MyEntities() implementation is not the best approach, but solving it is well documented.

Note that we’re not using AutoMapper’s Project().To(), because it doesn’t work great with Entity Framework when using type conversion.

Now use this new layer in your controller:

Now we can use the controller with any implementation of IEntitiesContainer in our tests.

Test the controller

Now that we have a foundation for the project, we can finally do some yummy tests. I will use xUnit (because it’s a great tool) and FluentAssertions (because it makes tests easier to read and write).

I will not go over testing the model, since it’s pretty well documented. However, testing the WebApi controller requires a bit more work.

In order to not hit the actual DB, we’ll use the dependency injection we’ve set up. Create an implementation of the IEntitiesContainer in your test project. I suggest using an awesome method that I found on a blog: create a mostly-empty implementation, for which you will provide the implementation on a case-by-case basis.

Then your controller test can use this fake like this:

The TestsBoostrappers.SetupControllerForTests() method configures the routes of the controller. If we don’t do that, the controller won’t know what to do.

One last, but very important, point of interest on your tests. In order to follow the REST principles, after certain actions, your API should return some specific data. For instance, after an item creation, it should return the URL to this new item. Your POST method may look like this:

This simple this.Url creates an URL based on your routes and such, but if you’re using WebApi 1 like me (because you’re stuck on .Net 4.0 in Visual Studio 2010), all documented solutions won’t work, and will make you want to flip your desktop. In order to solve that, I simply used another dependency injection to wrap the UrlHelper class, and provide my own implementations in my tests.

Document your API

Documenting your API is pretty easy using Swagger. An Nuget package wrapping the implementation for .Net is available as Swashbuckle. Add it to your API project, and then you can access the documentation through /swagger/.

Creating a C++/CLI wrapper around a C library

So, following my previous post, I have decided that I might try C++/CLI after all, since the SWIG way goes nowhere except “down in flames”, and I really need to implement it.

Turns out that creating a C++/CLI wrapper around a fairly well-made C library is not that hard.

Considering the following .h file from the C library (which are the methods and structures to interface):

Add the .h and .c files to a new C++ CLR DLL project, then add a new cpp file, in which we will replicate all the C structures as classes.
We have to name our classes differently than the C structures, otherwise there will be conflicts, both with the compiler, and in our heads.

The annoying thing when like me, you know pretty much nothing about C/C++, is that you never know where to specify pointers and stuff, and it’s mostly trial and error (it says it can’t build, let’s add a *… nope, a & ? Yes, that seems to work).

A few things to note:

  • Provide internal ToNative() methods on the structure wrappers to ease up marshalling from managed objects to native structures.
  • Also provide internal constructors taking a native structure as parameter, to ease up marshalling from native to managed.
  • The “^%” syntax (for the “images” parameter of the “Pack” method) builds to a “ref” in the .Net method signature. Otherwise the calling C# can’t get the values back.

As you can see, it’s not that difficult after all. I guess for an experienced C/C++ developer it would be even more simple.