Universal Windows Platform Apps and C++/CLI (VS 2015 RC1)

Question

I have some C++/CLI code which derives from the .NET System Namespace classes.

Is there a way to reuse this code for Universal Windows Platform Apps?

I can't get a reference to the System Namespace in C++, though in C# it is possible. It looks like there is only support for C++/Cx code and not for managed C++/CLI.

Answer 1

The syntax and keywords of the C++/CX extension resembles C++/CLI a great deal. But that's where similarity ends, they have nothing whatsoever in common. C++/CX gets compiled directly to native code, just like native C++. But C++/CLI is compiled to MSIL, the intermediate language of .NET. Their syntax looks so similar because they both solve the same problem, interfacing C++ to a foreign type system. .NET's in the case of C++/CLI, WinRT in the case of C++/CX.

Which is the basic reason why you cannot use the System namespace, it is a .NET namespace. You instead use the std namespace, along with the Platform and Windows namespaces for WinRT specific types. The compiler cannot import .NET reference assemblies with the /ZW compile option in effect, only WinRT metadata files, the ones with the .winmd filename extension. Which are an extension to the COM .tlb type library file format, the ones you previously had to import with the #import directive.

Which in itself is another major source for confusion, the internal .winmd file format was based on the format of .NET metadata. Most .NET decompilers can show you the content of a .winmd file because of this. But again just a superficial similarity, it is completely unrelated to a .NET assembly. It can just contain declarations, not code. Best to compare it to a .h file you'd use in a native C++ project. Or a .tlb file if you previously had exposure to COM.

Knowing how COM works can be very helpful to grok what this is all about. It is in fact COM that lies at the core of WinRT, the basic reason why your C++/CX project can be easily used by a program written in a completely different language like Javascript or VB.NET. A WinRT app is actually an out-of-process COM server. A class library or WinRT component is actually an in-process COM server. COM object factories work differently, the scope is limited to the files named in the package manifest. C++/CX is part of the language projection that hides COM, along with the C++ libraries you link that implement the Platform namespaces. WinRT would be still-born if programmers had to write traditional COM client code. You still can in native C++, the WRL library does little to hide the plumbing.

WinRT readily supports code written in a managed language like C# or VB.NET, the language projection is built into the framework and highly invisible. But not C++/CLI, a structural limitation. A Store/Phone/Universal app targets a subset of the .NET Framework named .NETCore. Better known these days as CoreCLR, the parts that were open-sourced. Which does not support module initializers, critical to C++/CLI.

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Enough introduction and getting to the answer: no, you have no use for your C++/CLI code and you'll have to rewrite it. You'll have a decent shot at porting the native C++ code that your C++/CLI wrapper interfaced with, as long as it observes the api limitations. You should always start there first, given that it is easy to do and instantly tells you if your native C++ code is using verboten api functions, the kind that drains a battery too quickly or violates the sandbox restrictions.

The ref class wrappers however have to be significantly tweaked. Little reason to assume that will be a major obstacle, it could still structurally be similar. Biggest limitations are the lack of support for implementation inheritance, a COM restriction, and having to replace the code that used .NET Framework types with equivalent C++ code. The typical hangup is that there tends to be a lot of it, the original author would normally have favored the very convenient .NET types over the standard C++ library types. YMMV.