Help calling a C++ external dll

Hey,

I'm trying to call a external dll from a c# but can't get it to work properly.

The dll exports this signature: FiSH_encrypt_msg(HWND mWnd, HWND aWnd, char *theData, char *params, BOOL show, BOOL nopause)

In c# I  added the following:
[DllImport("FiSH.DLL")]
public static extern String FiSH_encrypt_msg(This is where i need help);

Anyone who can help filling out the params ?

Thanks