A while ago, I posted an entry about marshalling a managed System.String into an unmanaged C string, specifically a plain char*. The solution I suggested, back in 2007, involved calling Marshal::StringToHGlobalAuto method to allocate memory and copy the string data into it, and then cast the HGlobal pointer into a char*.

It seems that in Visual Studio 2008 a new way of doing it was added, as part of the new Marshalling Library. This library provides a whole set of conversions between System.String and popular unmanaged string representations, like char*, wchar_t*, BSTR, CStringT<wchar_t> and others I am even less familiar with.

The smarter string representations, like std::string, have their own destructors so I can carelessly let them drop out of scope without worrying about leaks. The more primitive ones, like char*, need to be explicitly released, so that’s why the Marshalling Library contains a new (managed) class called marshal_context which gives me exactly this explicit release.

Let’s compare my old code with the new:

   1:  const char* unmanagedString = NULL;

   2:  try

   3:  {

   4:      String^ managedString = gcnew String("managed string");

   5:      // Note the double cast.

   6:      unmanagedString = (char*)(void*)Marshal::StringToHGlobalAnsi(managedString);

   7:  }

   8:  finally

   9:  {

  10:      // Don't forget to release. Note the ugly casts again.

  11:      Marshal::FreeHGlobal((IntPtr)(void*)unmanagedString);

  12:  }

And the new:

   1:  marshal_context^ context = gcnew marshal_context();

   2:  String^ managedString = gcnew String("managed string");

   3:  const char* unmanagedString = context->marshal_as<const char*>( managedString );

Much shorter, I’m sure you’ll agree. And neater – no need for all the icky, icky casting between different pointer types. And most importantly, I don’t have to explicitly release the char* – the marshal_context class keeps a reference to all the strings that were marshalled through it, and when it goes out of scope its destructor makes sure to release them all. Very efficient, all in all.

6 Comments

• IT consulting services said Tuesday, March 10, 2009 9:28:25 AM

  Yes.. I absolutely agree to you. I liked the way you explained with both new and old codes...

• jbaggs said Friday, March 27, 2009 8:59:42 PM

  Thanks you for your clear presentation. The MS information on this problem were not to the point.

• Justin Bailey said Thursday, July 23, 2009 10:26:41 PM

  Thanks. I agree with jbaggs. Microsoft's help on this wasn't written for the ease of the reader.

• weblogs.asp.net said Monday, May 9, 2011 10:26:56 AM

  Tales from the unmanaged side system string gt char pt 2.. Reposted it :)

• weblogs.asp.net said Thursday, June 23, 2011 3:57:31 AM

  Tales from the unmanaged side system string gt char pt 2.. Very nice :)

• serak said Monday, February 18, 2013 9:46:11 AM

  what about marshal_as() method it can literally convert anything.