marshal_as, strings and fields vs. properties

Question

include "stdafx.h"

#include <string>
#include <msclr/marshal_cppstd.h>

ref class Test {
    System::String^ text;
    void Method() {
        std::string f = msclr::interop::marshal_as<std::string>(text); // line 8
    }
};

This code when compiled with VS2008 gives:

.\test.cpp(8) : error C2665: 'msclr::interop::marshal_as' : none of the 3 overloads could convert all the argument types
        f:\programy\vs9\vc\include\msclr\marshal.h(153): could be '_To_Type msclr::interop::marshal_as<std::string>(const char [])'
        with
        [
            _To_Type=std::string
        ]
        f:\programy\vs9\vc\include\msclr\marshal.h(160): or       '_To_Type msclr::interop::marshal_as<std::string>(const wchar_t [])'
        with
        [
            _To_Type=std::string
        ]
        f:\Programy\VS9\VC\include\msclr/marshal_cppstd.h(35): or       'std::string msclr::interop::marshal_as<std::string,System::String^>(System::String ^const &)'
        while trying to match the argument list '(System::String ^)'

But when I change the field into property:

    property System::String^ text;

then this code compiles without errors. Why?

Answer 1

A workaround is to make a copy like this:

ref class Test {
    System::String^ text;
    void Method() {
        System::String^ workaround = text;
        std::string f = msclr::interop::marshal_as<std::string>(workaround);
    }
};

Answer 2

Bug, fixed in VS2010. Feedback item is here.

Answer 3

I'm using this snippet a lot, and it's too much clutter to declare a new variable. However this works too:

msclr::interop::marshal_as<std::string>(gcnew String(string_to_be_converted))

Another option that works for me is this little template. Not only does it solve the bug discussed here, it also fixes another thing that's broken with marshal_as, namely that it does not work for nullptr input. But actually a good c++ translation for a nullptr System::String would be an .empty() std::string(). Here is the template:

template<typename ToType, typename FromType>
inline ToType frum_cast(FromType s)
{
    if (s == nullptr)
        return ToType();
    return msclr::interop::marshal_as<ToType>(s);
}