ERROR: Object Reference Not Set to an Instance of an Object

[Michael Chong]

I have an (exe) executable program created in VB.NET 2003 that calls to a
MFC DLL written in VC++.NET 2003. I always get an error msg
"NullReferenceException: Object Reference Not Set to an Instance of an
Object" when my exe calls the following codes:

in VB.NET

Declare Function test Lib "C:\Cyob\IOComm\Debug\IOComm.dll" _
(ByVal a As Long, ByRef b As Integer) As Integer

Dim did As Integer
Dim ret As Integer

ret = test(PortHnd, did) //ERROR: Object Reference Not Set to an
Instance of an Object
in VC++.NET

extern "C" int APIENTRY test(HANDLE a, int *b)
{
*b=55; //ERROR: Object Reference Not Set to an Instance of an
Object
return 99;
}
Why is this happening? Any idea in solving such matter?
Thanks in Advance

Michael.