int dll_registerAccount(char* username, char* password);

[DllImport("pjsipDlld")]
static extern int dll_registerAccount(IntPtr username, IntPtr password);

[DllImport("pjsipDlld")]
static extern int dll_registerAccount(
    [MarshalAs(UnmanagedType.LPStr)]string username,
    [MarshalAs(UnmanagedType.LPStr)]string password);

public partial class NativeMethods {

    /// Return Type: int
    ///username: char*
    ///password: char*
    [System.Runtime.InteropServices.DllImportAttribute("<Unknown>", EntryPoint="dll_registerAccount")]
public static extern  int dll_registerAccount(System.IntPtr username, System.IntPtr password) ;

}

[DllImport("pjsipDlld")]
static extern int dll_registerAccount(string username, string password);

[DllImport("pjsipDlld")]
static extern int dll_registerAccount(StringBuilder username, StringBuilder password);

[DllImport("pjsipDlld", CharSet = CharSet.Ansi)]
    static extern int dll_registerAccount(string username, string password);

[DllImport(DllName, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
// bool MyFunction(char* fileName) <-- From the DLL
static extern bool MyFunction(string fileName);