使用microsoft dhcp服务器管理api查询dhcp保留时出现NullReferenceException

我想用c#中的microsoft dhcp服务器管理api枚举我们的microsoft dhcp服务器的所有dhcp保留条目。

这是我的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
using System.Net;
using System.Windows.Forms;
using System.Collections;
namespace Sequence
{
    [StructLayout(LayoutKind.Sequential)]
    public struct DHCP_CLIENT_INFO_ARRAY
    {
        public uint NumElements;
        public IntPtr Clients;
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct DHCP_CLIENT_UID
    {
        public uint DataLength;
        public IntPtr Data;
    }
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public struct DHCP_CLIENT_INFO
    {
        public uint ip;
        public uint subnet;
        public DHCP_CLIENT_UID mac;
        [MarshalAs(UnmanagedType.LPWStr)]
        public string ClientName;
        [MarshalAs(UnmanagedType.LPWStr)]
        public string ClientComment;
    }
    class CDHCP
    {
        public static ArrayList findDhcpClients(string server, string subnet)
        {
            ArrayList foundClients = new ArrayList();
            uint parsedMask = StringIPAddressToUInt32(subnet);
            uint resumeHandle = 0;
            uint numClientsRead = 0;
            uint totalClients = 0;
            IntPtr info_array_ptr;
            uint response = DhcpEnumSubnetClients(
                server,
                parsedMask,
                ref resumeHandle,
                65536,
                out info_array_ptr,
                ref numClientsRead,
                ref totalClients
                );
            DHCP_CLIENT_INFO_ARRAY rawClients = (DHCP_CLIENT_INFO_ARRAY)Marshal.PtrToStructure(info_array_ptr, typeof(DHCP_CLIENT_INFO_ARRAY));
            IntPtr current = rawClients.Clients;
            for (int i = 0; i < (int)rawClients.NumElements; i++)
            {
                DHCP_CLIENT_INFO rawMachine = (DHCP_CLIENT_INFO)Marshal.PtrToStructure(Marshal.ReadIntPtr(current), typeof(DHCP_CLIENT_INFO));
                CDHCPCLIENT thisClient = new CDHCPCLIENT();
                thisClient.ip = UInt32IPAddressToString(rawMachine.ip);
                thisClient.hostname = rawMachine.ClientName;
                thisClient.mac = string.Format("{0:x2}-{1:x2}-{2:x2}-{3:x2}-{4:x2}-{5:x2}",
                        Marshal.ReadByte(rawMachine.mac.Data, 0),
                        Marshal.ReadByte(rawMachine.mac.Data, 1),
                        Marshal.ReadByte(rawMachine.mac.Data, 2),
                        Marshal.ReadByte(rawMachine.mac.Data, 3),
                        Marshal.ReadByte(rawMachine.mac.Data, 4),
                        Marshal.ReadByte(rawMachine.mac.Data, 5));
                foundClients.Add(thisClient);
                current = (IntPtr)((int)current + (int)Marshal.SizeOf(typeof(IntPtr)));
            }
            return foundClients;
        }
        public static uint StringIPAddressToUInt32(string ip)
        {
            IPAddress i = IPAddress.Parse(ip);
            byte[] ipByteArray = i.GetAddressBytes();
            uint ipUint = (uint)ipByteArray[0] << 24;
            ipUint += (uint)ipByteArray[1] << 16;
            ipUint += (uint)ipByteArray[2] << 8;
            ipUint += (uint)ipByteArray[3];
            return ipUint;
        }
        public static string UInt32IPAddressToString(uint ip)
        {
            IPAddress i = new IPAddress(ip);
            string[] ipArray = i.ToString().Split('.');
            return ipArray[3] + "." + ipArray[2] + "." + ipArray[1] + "." + ipArray[0];
        }
        [DllImport("C:''Windows''System32''dhcpsapi.dll", SetLastError = true, CharSet = CharSet.Auto)]
        public static extern uint DhcpEnumSubnetClients(
            string ServerIpAddress,
            uint SubnetAddress,
            ref uint ResumeHanle,
            uint PreferredMaximum,
            out IntPtr ClientInfo,
            ref uint ElementsRead,
            ref uint ElementsTotal);
    }
}

不幸的是，我在这一行得到了一个NullReferenceException：

DHCP_CLIENT_INFO_ARRAY rawClients = (DHCP_CLIENT_INFO_ARRAY)Marshal.PtrToStructure(info_array_ptr, typeof(DHCP_CLIENT_INFO_ARRAY));

我这样调用函数：

public void GetDHCPReservation()
    {
        ArrayList clients = CDHCP.findDhcpClients("192.168.1.3", "192.168.1.5");
        foreach (CDHCPCLIENT c in clients)
        {
            MessageBox.Show(string.Format("(0,-35}{1,-15}{2,-15}", c.hostname, c.ip, c.mac));
        }
    }

有人能帮我吗？提前谢谢。