CODE:

std::string mbStr = "マルチバイト文字列";
std::string ucStr = L"ユニコード文字列";

//char*をString^に変換
String^ ret = gcnew String(mbStr.data(), 0, mbStr.length);

//wchar_t*をString^に変換
String^ ret = gcnew String(ucStr,data(), 0, ucStr.length);

//String^をchar*に変換
String^ text = "ドットネット文字列";
IntPtr mptr = Marshal::StringToHGlobalAnsi(text);
std::string ret = static_cast(mptr.ToPointer());
Marshal::FreeHGlobal(mptr);

//String^をwchar_t*に変換
String^ text = "ドットネット文字列";
IntPtr mptr = Marshal::StringToHGlobalUni(text);
std::wstring ret = static_cast(mptr.ToPointer());
Marshal::FreeHGlobal(mptr);

CODE:

#using 

#include 
#include 
using namespace System;
using namespace System::Runtime::InteropServices;

public ref class PrintTest
{
public:
	static void ShowMsg(String^ msg)
	{
		IntPtr mptr = Mashal::StringToHGlobal(msg);
		std::string ret = static_cast(mptr.ToPointer());
		printf("%s\n", ret.c_str() );
                Marshal::FreeHGlobal(mptr);
	}
};

int main(array^ args)
{
	String^ text = gcnew String("こんにちは");
	PrintTest::ShowMsg(text);
	return 0;
}