异常调用CertFreeCertificateContext在回调期间使用PInvoke访问ldap功能

当调用CertFreeCertificateContext方法时运行下面的代码时，我正在获得访问违反异常。

我想这是因为pServerCert参数在LdapServerCertDelegate上被mash化的方式，但一直无法找到解决方案。

using System;
using System.Runtime.InteropServices;
namespace ldaptest
{
    class Program
    {
        static void Main(string[] args)
        {
            new LdapAuthenticationProvider().AuthenticateUser("a.qas", "a", "administrator", "test123");
        }
    }
    public class LdapAuthenticationProvider
    {
        public void AuthenticateUser(string server, string domain, string username, string password)
        {
            IntPtr ld = ldap_sslinit(server, LDAP_SSL_PORT, 1);
            if (IntPtr.Zero == ld) throw new Exception("ldap_sslinit");
            var version = new IntPtr(LDAP_VERSION3);
            var ret = ldap_set_option(ld, LDAP_OPT_PROTOCOL_VERSION, version);
            if (ret != LDAP_SUCCESS) throw new Exception(string.Format("LDAP_OPT_PROTOCOL_VERSION 0x{0:X}", ret));
            var ldapOn = new IntPtr(LDAP_OPT_ON);
            ret = ldap_set_option(ld, LDAP_OPT_SSL, ldapOn);
            if (ret != LDAP_SUCCESS) throw new Exception(string.Format("LDAP_OPT_SSL 0x{0:X}", ret));
            // note the necessity to convert the delegate to a function pointer
            var callback = new LdapServerCertDelegate(AcceptAnySslCertificate);
            IntPtr pFn = Marshal.GetFunctionPointerForDelegate(callback);
            ret = ldap_set_option(ld, LDAP_OPT_SERVER_CERTIFICATE, pFn);
            if (ret != LDAP_SUCCESS) throw new Exception(string.Format("LDAP_OPT_SERVER_CERTIFICATE 0x{0:X}", ret));
            var tv = new l_timeval();
            ret = ldap_connect(ld, ref tv);
            if (ret != LDAP_SUCCESS) throw new Exception(string.Format("ldap_connect 0x{0:X}", ret));
            string login = string.Format(@"{0}'{1}", domain, username);
            ret = ldap_bind_s(ld, login, password, LDAP_AUTH_SIMPLE); // triggers the callback
            if (ret != LDAP_SUCCESS) throw new Exception(string.Format("ldap_bind_s 0x{0:X}", ret));
            ldap_unbind_s(ld);
            Console.WriteLine("Success");
            Console.Read();
        }
        private delegate bool LdapServerCertDelegate(IntPtr connection, IntPtr pServerCert);
        private bool AcceptAnySslCertificate(IntPtr connection, IntPtr pServerCert)
        {
            CertFreeCertificateContext(pServerCert); // << System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
            return true;
        }
        #region crypt32.dll functions
        [DllImport("crypt32.dll", CharSet = CharSet.Unicode)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool CertFreeCertificateContext(IntPtr pCertContext);
        #endregion
        #region Winldap.h definitions
        private const int LDAP_PORT = 389;
        private const uint LDAP_SSL_PORT = 636;
        private const int LDAP_VERSION3 = 3;
        private const int LDAP_OPT_PROTOCOL_VERSION = 17;
        private const int LDAP_OPT_SSL = 10;
        private const int LDAP_OPT_ON = 1;
        private const int LDAP_AUTH_SIMPLE = 128;
        private const int LDAP_OPT_SERVER_CERTIFICATE = 129;
        private const uint LDAP_SUCCESS = 0;
        [StructLayoutAttribute(LayoutKind.Sequential)]
        private struct l_timeval
        {
            private int tv_sec;
            private int tv_usec;
        }
        #endregion
        #region wldap32.dll functions
        /// http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ldap/ldap/ldap_sslinit.asp?frame=true
        [DllImport("wldap32.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "ldap_sslinitW",
            SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern IntPtr ldap_sslinit(string hostName, uint portNumber, int secure);
        [DllImport("wldap32.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "ldap_set_optionW",
            SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern uint ldap_set_option([In] IntPtr ldapHandle, int option, IntPtr invalue);
        [DllImport("wldap32.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "ldap_unbind_s",
            SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern uint ldap_unbind_s([In] IntPtr ldapHandle);
        [DllImport("wldap32.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "ldap_connect",
            SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern uint ldap_connect([In] IntPtr ld, [In] ref l_timeval timeout);
        [DllImport("wldap32.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "ldap_bind_sW",
            SetLastError = true, CharSet = CharSet.Unicode)]
        private static extern uint ldap_bind_s([In] IntPtr ld, string dn, string cred, uint method);
        #endregion
    }
}