Overview of Marshaling in C++

In mixed mode, you sometimes must marshal your data between native and managed types. Visual Studio 2008 introduced the marshaling library to help you marshal and convert data in a simple way. The marshaling library consists of a set of functions and a marshal_context class that perform marshaling for common types. The library is defined in these headers in the include/msclr directory for your Visual Studio edition:

Header Description
marshal.h marshal_context class and context-free marshaling functions
marshal_atl.h Functions for marshaling ATL types
marshal_cppstd.h Functions for marshaling standard C++ types
marshal_windows.h Functions for marshaling Windows types

The default path for msclr folder is something like this depending on which edition you have and the build number:

C:\\Program Files (x86)\\Microsoft Visual Studio\\Preview\\Enterprise\\VC\\Tools\\MSVC\\14.15.26528\\include\\msclr

You can use the marshaling library with or without a marshal_context Class. Some conversions require a context. Other conversions can be implemented using the marshal_as function. The following table lists the current conversions supported, whether they require a context, and what marshal file you have to include:

From type To type Marshal method Include file
System::String^ const char * marshal_context marshal.h
const char * System::String^ marshal_as marshal.h
char * System::String^ marshal_as marshal.h
System::String^ const wchar_t* marshal_context marshal.h
const wchar_t * System::String^ marshal_as marshal.h
wchar_t * System::String^ marshal_as marshal.h
System::IntPtr HANDLE marshal_as marshal_windows.h
HANDLE System::IntPtr marshal_as marshal_windows.h
System::String^ BSTR marshal_context marshal_windows.h
BSTR System::String^ marshal_as marshal.h
System::String^ bstr_t marshal_as marshal_windows.h
bstr_t System::String^ marshal_as marshal_windows.h
System::String^ std::string marshal_as marshal_cppstd.h
std::string System::String^ marshal_as marshal_cppstd.h
System::String^ std::wstring marshal_as marshal_cppstd.h
std::wstring System::String^ marshal_as marshal_cppstd.h
System::String^ CStringT<char> marshal_as marshal_atl.h
CStringT<char> System::String^ marshal_as marshal_atl.h
System::String^ CStringT<wchar_t> marshal_as marshal_atl.h
CStringT<wchar_t> System::String^ marshal_as marshal_atl.h
System::String^ CComBSTR marshal_as marshal_atl.h
CComBSTR System::String^ marshal_as marshal_atl.h

Marshaling requires a context only when you marshal from managed to native data types and the native type you are converting to does not have a destructor for automatic clean up. The marshaling context destroys the allocated native data type in its destructor. Therefore, conversions that require a context will be valid only until the context is deleted. To save any marshaled values, you must copy the values to your own variables.

Note

If you have embedded NULLs in your string, the result of marshaling the string is not guaranteed. The embedded NULLs can cause the string to be truncated or they might be preserved.

This example shows how to include the msclr directory in an include header declaration:

#include "msclr\marshal_cppstd.h"

The marshaling library is extensible so that you can add your own marshaling types. For more information about extending the marshaling library, see How to: Extend the Marshaling Library.

In earlier versions, you could marshal data by using Platform Invoke. For more information about PInvoke, see Calling Native Functions from Managed Code.

See Also

C++ Support Library
How to: Extend the Marshaling Library