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Posted 12 Jul 2007

Using and extending the Orcas marshal_as library

, 12 Jul 2007 CPOL
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This article covers basic marshal_as usage, as well as how to extend marshal_as to support additional type conversions


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 <code lang=mc++>marshal_as template function. While <code lang=mc++>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 <code lang=mc++>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 <code lang=mc++>System::String

<pre lang=mc++>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.

<pre lang=mc++>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 <code lang=mc++>marshal_as for converting a <code lang=mc++>String^ to a <code lang=mc++>const char*, a <code lang=mc++>const wchar_t* or a <code lang=mc++>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.

<pre lang=mc++>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 <code lang=mc++>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 <code lang=mc++>StringConvertor class I had internally used an <code lang=mc++>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 <code lang=mc++>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 <code lang=mc++>Rectangle structure and the Win32 <code lang=mc++>RECT structure. For the sake of completion I have also added specializations for the MFC <code lang=mc++>CRect wrapper (which is a thin wrapper around the <code lang=mc++>RECT structure). Here's my extension (put into a separate header file) :-

<pre lang=mc++>namespace msclr { namespace interop { template<> System::Drawing::Rectangle marshal_as<System::Drawing::Rectangle, RECT> ( const RECT& from) { return System::Drawing::Rectangle(from.left,, from.right - from.left, from.bottom -; } template<> System::Drawing::Rectangle marshal_as< System::Drawing::Rectangle, CRect> ( const CRect& from) { return System::Drawing::Rectangle(from.left,, 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 <code lang=mc++>msclr::interop namespace. This is to allow me (and anyone else) to use these additional conversions the same way I'd be using <code lang=mc++>marshal_as for the regular conversions.

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

<pre lang=mc++>//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.

Extending marshal_as for objects needing context

When you have type conversions which require explicit resource deallocation, you have to handle it slightly differently. As an example I have written some code that'll extend <code lang=mc++>marshal_as to support conversions between .NET <code lang=mc++>Font objects and native Windows <code lang=mc++>HFONT structures<pre lang=mc++>namespace msclr { namespace interop { template<> ref class context_node<System::Drawing::Font^, HFONT> : public context_node_base { private: System::Drawing::Font^ _font; public: context_node(System::Drawing::Font^% to, HFONT from) { to = _font = System::Drawing::Font::FromHfont((IntPtr)from); } ~context_node() { this->!context_node(); } protected: !context_node() { delete _font; } }; template<> ref class context_node<HFONT, System::Drawing::Font^> : public context_node_base { private: HFONT _hFont; public: context_node(HFONT& to, System::Drawing::Font^ from) { to = _hFont = (HFONT)from->ToHfont().ToPointer(); } ~context_node() { this->!context_node(); } protected: !context_node() { DeleteObject(_hFont); } }; } }

I've added two specializations of the <code lang=mc++>context_node template class which is used by the <code lang=mc++>marshal_context class to handle conversions that need a context. In the constructors I create the object that's requested and in the destructor/finalizer I free the resource. In the case of <code lang=mc++>HFONT, I have called <code lang=mc++>DeleteObject whereas for <code lang=mc++>Font, I have called <code lang=mc++>delete (which calls <code lang=mc++>Dispose). I need not have done that for <code lang=mc++>Font as the <code lang=mc++>Font finalizer would have come into play eventually, but for GDI objects it's preferable to free them as soon as they are not required as they tend to be rather heavy on the memory side.

Using these conversions is pretty similar to how <code lang=mc++>marshal_as conversions are used for <code lang=mc++>const char* or <code lang=mc++>BSTR (where we use a context object too).

<pre lang=mc++>HFONT hFont = CreateSampleFont(); //... marshal_context context; System::Drawing::Font^ font = context.marshal_as<System::Drawing::Font^>(hFont); HFONT hFontCopy = context.marshal_as<HFONT>(font); //... DeleteObject(hFont);

That's it. Pretty straightforward to extend and to use.


  • July 12th 2007 - Article first published


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Nish Nishant
United States United States
Nish Nishant is the Principal Software Architect/Consultant for Ganymede Software Solutions LLC, and is based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored C++/CLI in Action for Manning Publications in 2005, and had previously co-authored Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on and another 250+ blog articles on his WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : If you are interested in hiring Nish as a consultant, you can reach him via his google email id voidnish.

Company Website :

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Comments and Discussions

Questionconst& warning Pin
Dina Goldshtein9-Feb-12 2:23
memberDina Goldshtein9-Feb-12 2:23 
AnswerRe: const& warning Pin
Nish Sivakumar9-Feb-12 6:35
mvpNish Sivakumar9-Feb-12 6:35 
GeneralBroken link Pin
Henry Minute23-Mar-11 4:00
mvpHenry Minute23-Mar-11 4:00 
GeneralRe: Broken link Pin
Nishant Sivakumar24-Mar-11 3:46
mvpNishant Sivakumar24-Mar-11 3:46 
GeneralNice Article Pin
Majid Shahabfar17-Jul-07 7:48
memberMajid Shahabfar17-Jul-07 7:48 
GeneralWell done! Pin
Bartosz Bien13-Jul-07 7:27
memberBartosz Bien13-Jul-07 7:27 
GeneralRe: Well done! Pin
Nishant Sivakumar14-Jul-07 7:53
mvpNishant Sivakumar14-Jul-07 7:53 
GeneralHai Pin
rilov13-Jul-07 4:48
memberrilov13-Jul-07 4:48 
GeneralRe: Hai Pin
Nishant Sivakumar14-Jul-07 7:54
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GeneralRe: Hai Pin
Mr.Prakash16-Jul-07 23:43
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