marsha_as "Performs the marshaling on a specific data object to convert it between a managed and a native data type". Here there is the table for possible type conversion.
I use it this way:
marshal_as<std::wstring>(value)
TCHAR can be char or wchar_t, both of them present in marshal_as specialization, I suppose you need to point TCHAR* as template parameter:
TCHAR* result = marshal_as<TCHAR*>(value)
Actually MSDN says that you have to use it this way:
#include <msclr\marshal.h>
using namespace System;
using namespace msclr::interop;
int main(array<System::String ^> ^args)
{
System::String^ managedString = gcnew System::String("Hello World!!!");
marshal_context ^ context = gcnew marshal_context();
const wchar_t* nativeString = context->marshal_as<const wchar_t*>(managedString);
//use nativeString
delete context;
return 0;
}