Hi guys, I'm having real trouble marshalling Pinvoke data types. I am trying to access an API, and am attempting to convert the C header data into code pivoke
can use. I managed to get it to compile, but am a getting runtime error (see below). Any idea what I am doing wrong?
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Test.HubAPI.APIWrap.MsgConnect(Int32 lMsgAPIVersion, String lpcszTopic, OPERATOR lpOperator, IntPtr lphContext)
   at Test.HubAPI.MainEntryPoint.Main() in c:\documents and settings\z76546\my documents\hubapi\class1.cs:line 190

This is the C header info:

#define APIFUNCTION int __stdcall


typedef void *MSGHANDLE;

typedef struct /* logon information structure */
        {
        char szOprId[OPERATORLEN + 1];
        char szOprPswd[OPERATORLEN + 1];
        } OPERATOR;
typedef OPERATOR *LPOPERATOR;


MSGAPIFUNCTION MsgConnect(long lMsgAPIVersion, const char *lpcszTopic, LPOPERATOR lpOperator, MSGHANDLE *lphContext);


This is my C# code:

public class APIWrap
{
...
        public const int APIVERSION = 1;
        public IntPtr hCtx;
...
        [StructLayout(LayoutKind.Sequential,Pack=4)]
            public struct OPERATOR
        {
            [ MarshalAs( UnmanagedType.ByValTStr, SizeConst=OPERATORLEN + 1)]
            public string szOprId;
            [ MarshalAs( UnmanagedType.ByValTStr, SizeConst=OPERATORLEN + 1)]
            public string szOprPswd;
        };

[DllImport("C:\\dev\\bin\\msg.dll")]
        public static extern int MsgConnect (int lMsgAPIVersion, string lpcszTopic, OPERATOR lpOperator, IntPtr lphContext);
}


class MainEntryPoint
{
     static void Main()
        {
            int Result;
            string IPAddress = "10.199.156.100:8020";
            APIWrap currentEmployee = new APIWrap();
            APIWrap.OPERATOR opr = new APIWrap.OPERATOR();
            opr.szOprId = "logon";
            opr.szOprPswd = "password";
            Result = APIWrap.MsgConnect(APIWrap.MSG_APIVERSION, IPAddress, opr, currentEmployee.hCtx);
        }
}

I think you're just missing the 'out' modifier on the operator parameter.

In your C header the parameter is an LPOPERATOR (which I think is a pointer, my C is a bit flaky). Where as in you C# version it is just declared OPERATOR which is a value type.

Try...
You could also try specifying more details for DllImport, like whether the strings are UniCode or ASCII and whether or not SetLastError is called by the external DLL.

The only other thing that I can think which would cause the problem would be declaring your structure with byte arrays rather than strings.

Your advice is correct, I added a ref to the opr object and also to hCtx which is a handle. It was defined in the C header file as:

typedef void *PSMSGHANDLE;

So the following code works:

Result = APIWrap.MsgConnect(APIWrap.MSG_APIVERSION, IPAddress, ref opr, ref currentEmployee.hCtx);

Thanks for your help!