C/C++ Wrapper help

[AaronS]

I'm working on a project that is using a 3rd party DLL, and I'm having trouble writing the wrapper for it in c#. I've searced for hours for the answer, and can just can't seem to find a solution for this one. I think a lot of it has to do with understanding and wrapping the pointers properly

Here is the part of the C header file I'm working with

typedef struct _HCLIEN

PVOID Client
auth *cl_auth
} HCLIENT, *PHCLIENT

#define CLIENT HCLIEN

extern "C" CLIENT
__stdcall
Open_Client(char *Server, char System)
Here is all I've been able to come up with as far as creating the wrapper

[StructLayout(LayoutKind.Sequential,Pack=4,CharSet= CharSet.Ansi)
public struct HCLIENT

public IntPtr Client
public auth cl_auth
}

[DllImport("mydll.dll")
static extern int Open_Client(string Server, string System)

public void blah(
{
int returnval = (int) Open_Client("param1","param2")

I'm hitting the DLL properly, but it always returns it's default error code, and never any other value. I know the dll works properly as I can run it fine with a test application written by the developers of the DLL

Can anyone help

Thanks in advance
Aaron