>The developer suggested the following code to use
>
>

>  [DllImport("ww_local.dll", CallingConvention = CallingConvention.Cdecl)]
>        static extern string Invoke1(string str);
>

>
>so I used it as it. I hope I can involve him directly because I really don't want to go deep into VC++ code or even download it.
>
>Also, when I tried to open this dll with dependency walker, I got an error.
>

Yeah, the calling convention is important if they're using something instead of stdcall.

I wouldn't worry too much about dependency walker errors. It's usually enough to just look through the list of dependencies and see if there are dependencies outside of normal WinAPI ones that might hint at another third party library (It's also handy to see which version of the MSVC* libraries are required since that trips up a lot of things assuming the correct runtimes aren't available).

I usually also just double-check to make sure the function name casing is correct (if C++ you may have to hit F10 to unmangle them).