The declaration equivalent to C# declaration show would be
static string^ helper(Device^ dev);
However, that, apparently would not be enough for you. You need to learn how .NET references are work in general, and how are they used with C++/CLI. For that purpose, start reading some C++/CLI and .NET manual, pretty much from the very beginning — the format of this forums is not designed to explain it all in one Quick Answer. You should never use trial and error method of writing code. Instead, you should clearly understand the meaning of every line you write. And every character, if this is something like '*', '&', or '^'. :-)
—SA