Why System.NullReferenceException?

Question

using System; using System.Runtime.InteropServices; using NETENG; public class MainClass { [DllImport("neteng.dll")] public static extern bool NE_IsNetwork(string name); [DllImport("neteng.dll")] public static extern long NE_OpenNetwork(string name, NETENG.AccessModeEnum mode); [DllImport("neteng.dll")] public static extern bool NE_QueryElementByID(long net, NETENG.ElementTypeEnum eltype, NETENG.IDAttRec id, NETENG.HandleAttRec h); [DllImport("user32.dll")] public static extern int MessageBox(int hWnd, string pText, string pCaption, int uType); public static void Main(string[] args) { try { string name = @"D:\tracking\shapes\roadcenter_mkt_only.nws"; bool isItNet = NE_IsNetwork(name); string pCaption = "Example by Norman"; string pText = ""; if (isItNet == false) { pText = name + " is not a network."; } else { pText = name + " is a network."; } MessageBox(0, pText, pCaption, 0); long net = NE_OpenNetwork(name, NETENG.AccessModeEnum.NE_READONLY); if (net == 0) { MessageBox(0, "Network " + name + " not found.", "Not Found", 0); } else { MessageBox(0, "Network " + name + " found.", "Found", 0); } NETENG.HandleAttRec h1 = new NETENG.HandleAttRec();//A STRUCT h1.Handle = 0; NETENG.IDAttRec id1 = new NETENG.IDAttRec();//A STRUCT id1.UserID = 100; id1.LayerKey = 1; bool boolElement = NE_QueryElementByID(net, NETENG.ElementTypeEnum.NE_EDGE, id1, h1); MessageBox(0, Convert.ToString(boolElement),"Test", 0); }catch (Exception ex) { MessageBox(0, Convert.ToString(ex),"Error", 0); } } } ...when the code passes thru bool boolElement = NE_QueryElementByID(net, NETENG.ElementTypeEnum.NE_EDGE, id1, h1); ...ive passed proper parameter types and values already but it returns a System.NullReferenceException. WHY?