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Windows Phone 8: Native Code Support

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Peter Kuhn
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Peter Kuhn
Joined Jan 05, 2011
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9 comments   /   posted on Nov 27, 2012
Categories:   Windows Phone

Developing software for Windows Phone always meant developing managed code. Whether it was traditional development in Silverlight to create your apps or if you decided to build games in XNA, you never left the cozy world of a managed runtime environment. While usually this is something that gives you much more benefits than drawbacks, there are situations where having the possibility to use unmanaged code would be very desirable. Unfortunately, until now using managed code was not a conscious decision you could make as the developer, but rather a limitation of the platform that was imposed on you, and a choice that you didn't have.

With version 8 of Windows Phone, Microsoft opens up the platform to unmanaged third party code and allows you to develop in C++. I have hinted at this in the first part of the series already when I presented you the simplified overview of the new platform infrastructure:

image_thumb3

As you can see, traditional developing in .NET (or, as it was called back then, Silverlight :)) is only one of the available options. We now have a whole set of native APIs at hand that we can also use, and we are free to choose whatever programming model matches our situation and requirements best.

Making the Decision

When I ask people in my workshops and trainings about what they think should be driving reasons behind using native code as opposed to managed code, 90% of the time the first answer that is mentioned is "performance". Up to this day, developers still think of performance as the primary motivation why someone should choose native development over let's say writing the same in C#. The interesting part for me is that apparently the background of a developer doesn't seem to influence that very much. Not only people you'd expect to come up with these arguments (with a background in C++ development for example) think that, but also junior developers who never ever had to write or use native code in a real project. Of course it's true that managed code takes a performance penalty, and there are scenarios where this actually is the decisive argument. However, the truth is that developers very often overestimate the performance benefit they receive from using native code. I often do code reviews and analyses of applications, and usually bad performance is the result of things that are not tied to managed code or the underlying runtime, but rather flaws of the involved architecture and software design. Rethinking your software's structure is in almost all cases what you need to do, and only switching to a different programming model wouldn't result in any measurable improvements. So if you think that support for native code in Windows Phone means you now should rewrite your existing C# projects in C++ to get an enormous boost in performance, please think again – that is definitely not what you're supposed to do, and it definitely will result in a huge disappointment.

With this out of the way, you may ask yourself what valid reasons to go native actually are? Well, here is my personal list of considerations, ordered by descending priority. Yours of course may differ, but in the end it more or less always comes down to variations of these.

  • Reusability: You have some legacy code around that you really don't want to rewrite but still use in your app.
  • Portability: You want to use the code you're about to write on multiple platforms.
  • Personal Preference: You are a developer who really favors C++ over e.g. C# or you have significantly better skills in C++.
  • Performance: You really are in need for these last bits of performance benefits and know what you're doing.

Now that we have talked about the whys, it's about time to get into the details of how you use native code.

A Closer Look at What's Available

First of all, let's take take a look at the options we have in Visual Studio. When you inspect the available templates for Windows Phone and C++, you can see that the ones that create new applications are very much focused on Direct3D:

image

Surprisingly, creating a "Direct3D with XAML" C++ application actually creates a C# main project and a separate Windows Runtime Component in C++. The basic idea here is very similar to how you mixed XNA and Silverlight in Windows Phone 7: Silverlight did take the leading role there and allowed you to embed XNA on individual pages. With the mentioned template in Windows Phone 8, the main application is based on .NET for Windows Phone and makes use of specific controls in XAML (in particular the DrawingSurfaceBackgroundGrid) to embed Direct3D in a similar way. The actual rendering in the sample then is handed off to the native component. This mixture of technologies probably is the typical use case: it allows you to benefit from both the flexibility and comfort of XAML with managed code (for example for menus and other details) as well as the power of native code at the same time.

If you want to create an app that really has no connection to C# and managed code anymore, you can select the second template that already has the "native only" reference in its name, which then results in a pure unmanaged application. The other available C++ templates are there to create libraries, either native ones that are dynamically or statically linked, or single Windows Phone Runtime Components which we now will take a closer look at.

It's All About Communication

For interop and communication between the managed and unmanaged world, just like in Windows 8, the Windows Runtime is the magic glue that allows you to let different technologies interact with each other in a very natural way. Particularly, we can integrate unmanaged C++ implementations into managed apps by creating simple Windows Runtime wrappers. They can easily be consumed from C# and feel very ".NET-like".

image

It's really not obvious you're not using normal .NET classes and objects when you interact with WinRT components, because you don't have to resort to traditional ways of interop or COM communication for that.

A Practical Sample

I want to show you an example of how you can easily integrate an existing C++ code base in your managed C# app by making use of such a Windows Runtime Component. First of all, since the C++ app templates very much focus on Direct3D, I create a new C# XAML application instead.

image

Then, in a second step, I simply add another project to my solution, but this time I choose the C++ "Windows Phone Runtime Component" template. After the project is created, I add a reference to it to the main project.

image

As you can tell from the names in the screenshots, I'm going to re-use an existing Mandelbrot algorithm that is written in C++. The motivation behind this could be any of the above mentioned reasons: in my case I found some existing code and I'm simply reusing it for demonstration. But it could very well be a particularly fast algorithm I want to integrate, or some code that I want to share between multiple platforms.

The code of the Windows Phone Runtime Component is explained easily:

winrt_component

In line 3, I include the header file for the existing native implementation that I want to use. In line 13, I declare a member for that legacy type that I can then use later on. These two things could be part of any other C++ code and are not particularly interesting.

The details that are special to Windows Runtime Components however are highlighted in line 7 and line 11. First, the class needs to be decorated with the "ref" keyword as well as be a sealed class. The first one tells the compiler that this is a WinRT class that needs to be treated in a special way, the latter one simply is a technical restriction for these kinds of components. In line 11 you can see a wrapper method that I introduced and that will do nothing else but invoke the legacy implementation contained in the "Mandelbrot" class (see below). However, the argument types need some comments. As you can see, I'm using a "WriteOnlyArray", which is a special type that determines that the native code will be able to write to the array. The type is followed by a hat symbol (^) that may remind you of managed C++ (which this isn't!). This is the notation for WinRT types that are automatically reference counted.

Obviously, when I pass on the call to the original native implementation, I cannot use the WinRT types, because that old code knows nothing about those. Instead, I use the "Data" member to access the underlying type that my legacy code expects and understands (here: double*):

image

That's basically all you have to do:

  • Create wrapper methods for all the functionality you want to expose
  • Make sure you only use compatible WinRT types in the public API of your component

For the latter one, you should take a look at the available documentation for proper type mapping [1], and of course the compiler will also hint you at errors in your code.

Consuming WinRT Components From C#

As mentioned before, the consuming end of this feels very natural to managed developers. You simply have to add a reference to the project containing your Windows Runtime Component and then can start using your component right away:

csharp_invocation

The first highlighted line (1.) creates a new instance of the Windows Runtime component, the second marked line (2.) invokes the wrapper method which internally in turn invokes the original native implementation. As you can see, there's virtually no difference to how you would use normal .NET types – this is the magic of WinRT's projection layers that translate everything into natural .NET types behind the scenes for you.

In the sample, I continue to convert the result of the native call to a WriteableBitmap which then is displayed on the XAML page. Et voilà, we have a Mandelbrot set in our managed application, coming straight out of a ten year old piece of native C++ code:

mandelbrot

Limitations And Opportunities

One thing to note is that obviously you cannot use native code which has dependencies on particular, incompatible features that are not available on Windows Phone. This somewhat limits the possibilities to reuse existing legacy code especially if it is not designed very well. However, the ability to use native code now also opens a lot of new opportunities that simply weren't possible before. For example, it's now much easier to bring interesting projects like the Unity 3D engine over to the platform [2], which in turn will make it more attractive for developers too.

I hope you enjoyed this quick detour to native development and how you can benefit from it on Windows Phone 8. The sample code may be downloaded at the top of this article.

Have fun!


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Comments

  • NotConvinced

    Re: Windows Phone 8: Native Code Support


    posted by NotConvinced on Dec 07, 2012 07:55

    I have been asking the same question for months on various sites around the Internet since I first saw Joe Belfiore's video where he claimed that Windows Phone 8 allows native development. His body language did not convey conviction, at least to me, so I had a fellow engineer check if this was true, and he said that the so-called 100% native applications *still* run inside an interpreted sandbox. I am in a weird situation where I cannot install VS2012 or Windows 8 to check myself, so I would be very grateful if someone could give a definitive answer to this question:

    When a Windows Phone 8 application image is built, what is its structure, and more importantly, what is the execution environment? Is there any managed code, WHATSOEVER, involved in the execution of that application?

  • MisterGoodcat

    Re: Windows Phone 8: Native Code Support


    posted by MisterGoodcat on Dec 07, 2012 10:48

    Here is a quick response:

    1. Nothing on Windows Phone runs in an "interpreted sandbox", not even on Windows Phone 7. Code is either JIT compiled or MDIL that is linked to native code. No interpreter is involved. More details in the "compile in the cloud" article of this series.
    2. Obviously, no matter what programming model we prefer, we have to deal with a secured environment where either the runtime or the operating system enforces restrictions on your code to prevent it from going rampage. Native development doesn't mean that you e.g. can freely access the file system or do similar things.
    3. I would be interested in how your "fellow engineer" was able to analyze the inner workings of code execution on Windows Phone, something that to my knowledge is not possible with reasonable effort.
    4. Not only Joe Belfiore but every other tech guy inside Microsoft who talked about this put a lot of emphasize on the fact that native means native, not C++/CLI or similar variants. Watch e.g. Peter Torrs talks for more.

    Even if there was an underlying sandboxed execution environment, the question still is why someone would care. You can write and build C++ code into native images, and the level of compatibility apparently is excellent, or else projects like SQLite, Ogre or Unity 3D would have a lot more trouble targeting Windows Phone 8. So as long as you are able to reuse code, use your standard libraries and get native performance, why bother to dig deeper?

    By the way, you can install Windows 8 in a virtual machine and test there if you don't have the possibility to use it on bare metal. I read that with some tweaks it's even possible to get the Windows Phone 8 emulators up and running.


  • mmluqman

    Re: Windows Phone 8: Native Code Support


    posted by mmluqman on Mar 28, 2013 16:39

    Thanks. It was useful for me.

  • Sulekha

    Re: Windows Phone 8: Native Code Support


    posted by Sulekha on Apr 02, 2013 13:13

    Hi,

    I am not able to call simple C++ Dll which is created in Windows 8 (Visual Studio 2012) through C# Visual Studio 2012 Windows 8 Phone App.I have used DLLImport method.In the button click while calling the function it is giving error - System Exception Not Null....Though the same code worked in Windows 8 Metro Apps.Please provide me any link of a simple example or tell me any better option.If needed I can post the code too.

    Please help.

    Regards,

    SB


  • harsini

    Re: Windows Phone 8: Native Code Support


    posted by harsini on Apr 07, 2013 21:46

    Hi.

    Thanks for your helpful sample.I have a problem passing string in a wp8 project and I appreciate you if you could help me.

    I want to pass a string from a winrt component to c#/Xaml .I pass a string by refrence to winrt and in winrt c++ I give a value to that string.I debug the project with no error  but at runtime no address receive in c++  and the string (which is passed with out keyword) doesn't change in C# part of program.

  • msankar

    Re: Windows Phone 8: Native Code Support


    posted by msankar on Jul 05, 2013 09:06

    Hi,

    I have following queries. Please respond.


    1. Is WIndows Phone 8 OS intended to be an OS only for smart Phones? Is it not targetted at WIndows Tablets?

    if not, for tablets, do they still use plan WIndows 8 (not windows phone8?)

    2. WIndows Phone 8 has native support to run Plain C++ code . SImilarly, does WIndows 8 support native C++ code ?

    3. How do I run a native executable on WIndows Phone 8 ?

    Thanks




  • MisterGoodcat

    Re: Windows Phone 8: Native Code Support


    posted by MisterGoodcat on Jul 08, 2013 09:04

    Hi. 

    1. Yes, Windows Phone is only for smart phones, not tablet - although both platforms certainly converge to some degree (e.g. Windows Phone supporting higher and higher resolutions, unification of APIs etc.)

    2. The concept is similar in that you wrap native code in a RT component; however, on a technical level they are not interchangeable (e.g. as binaries).

    3. You can't. You have to compile the source targeting Windows Phone, then it can execute as part of the app.

  • dsds

    Re: Windows Phone 8: Native Code Support


    posted by dsds on Oct 20, 2013 13:00

    Thank you for the great article.

    1) Can C native (not C++) be called using this method or should I use a C++ wrapper for this?

    2) How can C native code call managed code? Is it by registering callbacks? Do you know of a good article or tutorial for doing this?

    Thank you,

    David

  • Vineet24

    Re: Windows Phone 8: Native Code Support


    posted by Vineet24 on Oct 28, 2013 14:42

    Hi,

    I am trying to return a string from Windows Phone runtime component code to Windows phone XAML project, but don't why the WP runtime component isn't accepting any function with a return type string.Only functions with ret type int are accepted.

    Please help

    Vineet

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