C# Form Authentication with LDAP AD

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Hi All,

I planned the Form authenticaon with ldap i assigned to code and works fine,

but i want to Filter security group member, only sign in "Messenger" group
in my web site, i used msdn library "GetGroups" otpion for "memberOf" object.
but it's not security group filter. all users login my web site this my code
below;

using System;
using System.Text;
using System.Collections;
using System.DirectoryServices;

namespace FormsAuth
{
public class LdapAuthentication
{
private String _path;
private String _filterAttribute;

public LdapAuthentication(String path)
{
_path = path;
}

public bool IsAuthenticated(String domain, String username, String
pwd)
{
String domainAndUsername = domain + @"\" + username;
DirectoryEntry entry = new DirectoryEntry(_path,
domainAndUsername, pwd);
try
{ //Bind to the native AdsObject to force authentication.
Object obj = entry.NativeObject;

DirectorySearcher search = new DirectorySearcher(entry);

search.Filter = "(SAMAccountName=" + username + ")";
search.PropertiesToLoad.Add("cn");
SearchResult result = search.FindOne();

if (null == result)
{
return false;
}

//Update the new path to the user in the directory.
_path = result.Path;
_filterAttribute = (String)result.Properties["cn"][0];
}
catch (Exception ex)
{
throw new Exception("Error authenticating user. " +
ex.Message);
}
return true;
}

public String GetGroups()
{
DirectoryEntry entr = new
DirectoryEntry("GC://gmdsidc1.msft.local/dc=msft,dc=Local", "msft.local" +
@"\" + "mutluozel", "1q2w3e4r");

DirectorySearcher search = new DirectorySearcher(entr);
search.Filter = "(cn=" + _filterAttribute + ")";
search.PropertiesToLoad.Add("memberOf");
StringBuilder groupNames = new StringBuilder();

try
{
SearchResult result = search.FindOne();

int propertyCount = result.Properties["memberOf"].Count;

String dn;
int equalsIndex, commaIndex;

for (int propertyCounter = 0; propertyCounter <
propertyCount; propertyCounter++)
{
dn =
(String)result.Properties["memberOf"][propertyCounter];

equalsIndex = dn.IndexOf("=", 1);
commaIndex = dn.IndexOf(",", 1);
if (-1 == equalsIndex)
{
return null;
}

groupNames.Append(dn.Substring((equalsIndex + 1),
(commaIndex - equalsIndex) - 1));
groupNames.Append("Messenger");
}
}
catch (Exception ex)
{
throw new Exception("Error obtaining group names. " +
ex.Message);
}
return groupNames.ToString();
}

}
}