.NET C/C++ P/INVOKE exception

Runnnig this code getting this error, could any one help figure out what is going on here?

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Runtime.InteropServices;namespace LSATest{class Program{    public static List<string> listBox1 = new List<string>();     static void Main(string[] args)    {        DateTime systime = new DateTime(1601, 1, 1, 0, 0, 0, 0); //win32 systemdate        UInt64 count;        IntPtr luidPtr = IntPtr.Zero;        LSAClass.LsaEnumerateLogonSessions(out count, out luidPtr);  //gets an array of pointers to LUIDs        IntPtr iter = luidPtr;      //set the pointer to the start of the array        for (ulong i = 0; i < count; i++)   //for each pointer in the array        {            IntPtr sessionData;            LSAClass.LsaGetLogonSessionData(iter, out sessionData);            LSAClass.SECURITY_LOGON_SESSION_DATA data = (LSAClass.SECURITY_LOGON_SESSION_DATA)Marshal.PtrToStructure(sessionData, typeof(LSAClass.SECURITY_LOGON_SESSION_DATA));            //if we have a valid logon            if (data.PSiD != IntPtr.Zero)            {                //get the security identifier for further use                System.Security.Principal.SecurityIdentifier sid = new System.Security.Principal.SecurityIdentifier(data.PSiD);                //extract some useful information from the session data struct                string username = Marshal.PtrToStringUni(data.Username.buffer).Trim();      //get the account username                string domain = Marshal.PtrToStringUni(data.LoginDomain.buffer).Trim();    //domain for this account                  string authpackage = Marshal.PtrToStringUni(data.AuthenticationPackage.buffer).Trim();    //authentication package                LSAClass.SECURITY_LOGON_TYPE secType = (LSAClass.SECURITY_LOGON_TYPE)data.LogonType;                DateTime time = systime.AddTicks((long)data.LoginTime);                  //get the datetime the session was logged in                //do something with the extracted data, ie, add to a display control....                listBox1.Add("User: "+ username +" *** Domain: "+ domain +" *** Login Type: ("+ data.LogonType +") "+ secType.ToString() +" *** Login Time: "+ time.ToLocalTime().ToString());            }            iter = (IntPtr)((int)iter + Marshal.SizeOf(typeof(LSAClass.LUID)));  //move the pointer forward            LSAClass.LsaFreeReturnBuffer(sessionData);   //free the SECURITY_LOGON_SESSION_DATA memory in the struct        }        LSAClass.LsaFreeReturnBuffer(luidPtr);   //free the array of LUIDs    }}

}