Using and extending the Orcas marshal_as library

Introduction

The need to marshal between native and managed types is a very frequent scenario in mixed-mode programming. This is specially true when it comes to strings - when you've got MFC strings, COM strings, standard C++ strings and CLR strings and need to convert between those types. In fact that's what prompted me to write the StringConvertor class for managed-unmanaged string conversions. While I did realize that I had misspelled converter as convertor I decided to leave it like that so I got a unique class-name and that way I could avoid Google dilution (there are dozens of other StringConverter classes, specially Java based ones).

Anyway, in the Orcas release of Visual C++, the VC++ team have added a mixed-mode marshalling library which primarily consists of the marshal_as template function. While marshal_as is technically not limited to string conversions, that is its most important role as of now. Using it is very similar to using one of the C++ cast operators such as static_cast - though be aware that you are not really casting here, you are doing a conversion - which may or may not be a different thing from a mere cast.

Basic usage

Here's a code snippet that shows how to convert various native strings to System::String

String^ clrString;
    
const char* pcszHello = "hello world";
clrString = marshal_as<String^>(pcszHello);
    
wchar_t* pwszHello = L"hello wide world";
clrString = marshal_as<String^>(pwszHello);
    
bstr_t bstrtHello("hello bstr_t world");
clrString = marshal_as<String^>(bstrtHello);
    
std::string stdHello = "hello from std::string";
clrString = marshal_as<String^>(stdHello);
    
CString mfcString("hello from CString");
clrString = marshal_as<String^>(mfcString);
    
CComBSTR atrBSTR(L"hello from CComBSTR");
clrString = marshal_as<String^>(atrBSTR);

The reverse conversion is also similar.

String^ clrString = "Original System::String";
    
std::string stdHello = marshal_as<std::string>(clrString);
CString mfcString = marshal_as<CString>(clrString);
CComBSTR atrBSTR = marshal_as<CComBSTR>(clrString);

You cannot directly use marshal_as for converting a String^ to a const char*, a const wchar_t* or a BSTR - because those conversions require the unmanaged resources to be freed after use. For those, you need to use a context object as shown below.

marshal_context context;
    
const char* pcszHello = context.marshal_as<const char*>(clrString);
const wchar_t* pcwszHello = context.marshal_as<const wchar_t*>(clrString);
BSTR bstrString = context.marshal_as<BSTR>(clrString);
    
Console::WriteLine(context._clean_up_list.Count);

The context object keeps track of the allocated objects and frees them in its destructor. Internally it maintains a linked list of objects that are allocated, and the output of the Console::WriteLine in the above code will be 3 (as we have allocated three objects using the context object). Initially I found this a little annoying to do, but I couldn't think of any alternate solution that was more elegant and where we could avoid using the context object. In fact, in my StringConvertor class I had internally used an std::vector to store all allocated objects so I could free them in the destructor.

Extending marshal_as functionality

While the built-in functionality only allows string conversions, it's also possible to extend marshal_as functionality to support other type conversions. As an example of doing this, I have written a sample extension that supports converting between the Windows Forms Rectangle structure and the Win32 RECT structure. For the sake of completion I have also added specializations for the MFC CRect wrapper (which is a thin wrapper around the RECT structure). Here's my extension (put into a separate header file) :-

namespace msclr
{
    namespace interop
    {
        template<> System::Drawing::Rectangle
            marshal_as<System::Drawing::Rectangle, RECT> (
            const RECT& from)
        {
            return System::Drawing::Rectangle(from.left, from.top,
                from.right - from.left, from.bottom - from.top);
        } 
    
        template<> System::Drawing::Rectangle marshal_as<
            System::Drawing::Rectangle, CRect> (
            const CRect& from)
        {
            return System::Drawing::Rectangle(from.left, from.top,
                from.Width(), from.Height());
        } 
    
        template<> RECT marshal_as<RECT, System::Drawing::Rectangle>(
            const System::Drawing::Rectangle& from)
        {
            System::Drawing::Rectangle rectangle = from; //remove const
            RECT rect = {rectangle.Left, rectangle.Top,
                rectangle.Right, rectangle.Bottom};
            return rect;
        }
    
        template<> CRect marshal_as<CRect, System::Drawing::Rectangle>(
            const System::Drawing::Rectangle& from)
        {
            System::Drawing::Rectangle rectangle = from; //remove const
            return CRect (rectangle.Left, rectangle.Top,
                rectangle.Right, rectangle.Bottom);
        }
    }
}

Effectively I've added four specializations for the four conversions that I need. Note how I have put the conversions into the msclr::interop namespace. This is to allow me (and anyone else) to use these additional conversions the same way I'd be using marshal_as for the regular conversions.

Now using these conversions would be quite trivial as show below.

//To Rectangle
    
RECT rect = {10, 10, 110, 110};
System::Drawing::Rectangle rectangle =
    marshal_as<System::Drawing::Rectangle>(rect);
    
CRect mfcRect(20, 20, 220, 220);
rectangle = marshal_as<System::Drawing::Rectangle>(mfcRect);
    
//From Rectangle
    
RECT rectBack = marshal_as<RECT>(rectangle);
    
CRect mfcRectBack = marshal_as<CRect>(rectangle);

One important thing to be aware of is that in all four conversions I've added there is no need to handle context as there is no explicit need for memory deallocation. This may not always be the case as we'll see in the next section.