how to pass parameter "_bstr_t cons &" to invoke a dll written in C++?

[digonio]

Hi,
I am a complete newbie and these are the first lines of code that I write, so please be patient :(
I need to create a C# exe file that calls a c++ function

What I wrote always throws an error at runtime:

Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
   at DDecrypt.DefaultDecrypt(String s)
   at DDecrypt.Main()

The function it calls is in a C++ dll ()closed source code); exploring the exposed functions I found:
[indent]...

void CLSCrypt::`default constructor closure'(void)
CLSCrypt::CLSCrypt(bool,class _bstr_t const &)
class _bstr_t CLSCrypt::DecryptString(class _bstr_t const &,enum STRCODING,bool)
 class _bstr_t CLSCrypt::DefaultDecrypt(class _bstr_t const &)
class _bstr_t CLSCrypt::DefaultEncrypt(class _bstr_t const &)
void CLSCrypt::Delete(void)
class _bstr_t CLSCrypt::EncryptString(class _bstr_t const &,enum STRCODING)
class _bstr_t CLSCrypt::GetBase64(unsigned char *,unsigned long)
bool CLSCrypt::GetFromBase64(class _bstr_t const &,unsigned char * &,unsigned long &)
bool CLSCrypt::GetFromHEX(class _bstr_t const &,unsigned char * &,unsigned long &)
class _bstr_t CLSCrypt::GetHEX(unsigned char *,unsigned long)
class _bstr_t CLSCrypt::GetMD5String(unsigned char *,unsigned long,enum STRCODING)
void CLSCrypt::SetAccess(class _bstr_t const &,unsigned long)
CLSCrypt::~CLSCrypt(void)

the function I want to call is

class _bstr_t CLSCrypt::DefaultDecrypt(class _bstr_t const &)

]

and it's mangled code is

?DefaultDecrypt@CLSCrypt@@SA?AV_bstr_t@@ABV2@@Z

to test the import of the function I wrote:

using System;
using System.Runtime.InteropServices;
 
public class DDecrypt
{
    [DllImport("LSUtils.dll", EntryPoint = "?DefaultDecrypt@CLSCrypt@@SA?AV_bstr_t@@ABV2@@Z", ExactSpelling = true)]
    public static extern String DefaultDecrypt(String s);
 
    public static void Main()
    {            
        String s = "this is a test!";
        String y = DefaultDecrypt(s);
        Console.WriteLine(y);
    }
}

when I run it the error is the one above;

I suppose it is due to the data type that I want to pass (a String while the function wants something else)

I tried to import the function as

public static extern String DefaultDecrypt([MarshalAs(UnmanagedType.BStr)] String s);

or even to mark the code as 'unsafe' and use as input

IntPtr ptr = Marshal.PtrToStringBSTR(s);

but the error is always the same;

Could somebody help me with this?
Thanks

[tlhintoq]

TIP: When you are writing your question, there is a button on the tool bar that wraps the [code] tags around your copy/pasted code. It helps a bunch. Its the button with a '#' on it. More on tags. They're cool. Check'em out.

[digonio]

(thanks for the hint and the corrections to the format as well)