The goal is to avoid copying the string data when I need a const wchar_t*.
The answer seems to be yes, but the function PtrToStringChars doesn't have its own MSDN entry (it's only mentioned in the KB and blogs as a trick). That made me suspicious and I want to check with you guys. Is it safe to use that function?
Yes, no problem. It is actually somewhat documented but hard to find. The MSDN docs for the C++ libraries aren't great. It returns an interior pointer, that's not suitable for conversion to a const wchar_t* yet. You have to pin the pointer so the garbage collector cannot move the string. Use pin_ptr<> to do that.
You can use Marshal::StringToHGlobalUni() to create a copy of the string. Use that instead if the wchar_t* needs to stay valid for an extended period of time. Pinning objects too long isn't very healthy for the garbage collector.