MC++ & C++/CLI String: How to convert 'System::String' to 'char*'?
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    MC++ & C++/CLI String: How to convert 'System::String' to 'char*'?

    Q: How to convert 'System::String' to 'char*' in MC++?

    A: There are several methods (notice that the examples are only meant to show how to convert from 'System::String' to 'char*'):


    • Using 'PtrToStringChars()'

      You can access the wrapped buffer of a 'System::String' using this function, that yields a' __gc' pointer to the first character of a 'System::String' object. If it's passed to a function that expects an unmanaged string ('wchar_t'), it must me pinned first.


      • In VC++ 7.x:


        Code:
        #include <vcclr.h> // required header for PtrToStringChars
        
        size_t GetSize(System::String* str)
        {
          const wchar_t __pin* unmngStr = PtrToStringChars(str);
          return wcslen(unmngStr);
        }
        
        size_t size = GetString(S"Method 1");
      • In VC++ 8:


        Code:
        #include <vcclr.h> // required header for PtrToStringChars
        
        size_t GetSize(System::String^ str)
        {
          pin_ptr<const wchar_t> unmngStr = PtrToStringChars(str);
          return wcslen(unmngStr);
        }
        
        size_t size = GetString("Method 1");



    • Using 'StringToHGlobalAnsi()'

      Method 'StringToHGlobalAnsi()' of the Marshal class from the .NET Framework copies the contents of a managed string into unmanaged memory, converting into ANSI format as it copies. Notice, that you'll have to call the 'FreeHGlobal()' method to release the allocated memory.


      • In VC++ 7.x:


        Code:
        using namespace System::Runtime::InteropServices; // for class Marshal
        
        void PrintMessage(System::String* str)
        {
        	TCHAR* str2 = (TCHAR*)(void*)Marshal::StringToHGlobalAnsi(str);
        	printf(str2);
        	Marshal::FreeHGlobal(str2);
        }
        
        PrintMessage(S"Method 2");
      • In VC++ 8:


        Code:
        using namespace System::Runtime::InteropServices; // for class Marshal
        
        void PrintMessage(System::String^ str)
        {
        	const char* str2 = (char*)(void*)Marshal::StringToHGlobalAnsi(str);
        	printf(str2);
        	Marshal::FreeHGlobal((System::IntPtr)(void*)str2);
        }
        
        PrintMessage("Method 2");



    • Using 'CString' class.


      • In VC++ 7.x, class 'CString' has a special constructor that takes a 'System::String' object:


        Code:
        #ifdef _MANAGED
        
        	// This template will compile only for
        	// class SystemString == System::String
        
        	template<class SystemString>
        	CStringT( SystemString * pString ) :
        		CThisSimpleString( StringTraits::GetDefaultManager() )
        	{
        		const wchar_t __pin* psz = PtrToStringChars( pString );
        		*this = psz;
        	}
        #endif
        Using this constructor you can write:


        Code:
        #include <atlstr.h> // for class CString
        
        void Print(System::String* str)
        {
        	CString cstr(str);
        	printf(cstr);
        }
        
        Print(S"Method 3");
      • In VC++ 8 (2005) the 'CString' constructor looks like this:


        Code:
        #if defined(__cplusplus_cli)
        
        	template <class SystemString>
        	CStringT( SystemString^ pString ) :
        		CThisSimpleString( StringTraits::GetDefaultManager() )
        	{
        		cli::pin_ptr<const System::Char> pChar = PtrToStringChars( pString);
        		const wchar_t *psz = pChar;
        		*this = psz;
        	}
        
        #elif defined(_MANAGED)
        
        	template <class SystemString>
        	CStringT( SystemString __gc* pString ) :
        		CThisSimpleString( StringTraits::GetDefaultManager() )
        	{		
        		const wchar_t __pin* psz = PtrToStringChars( pString );
        		*this = psz;
        	}
        
        #endif
        As a result, the previous example changes to:


        Code:
        #include <atlstr.h> // for class CString
        
        void Print(System::String^ str)
        {
        	CString cstr(str);
        	wprintf(cstr);
        }
        
        Print("Method 3");



    Last edited by Andreas Masur; January 19th, 2006 at 04:02 PM.

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