Recently I worked on a project where there was a requirement to query an Oracle Internet Directory (OID) server with a user's certificate subject to verifiy if the user existed in the OID directory and return the username to be used as the credential.  This seemed at first like it would be a simple task using the System.DirectoryServices namespace.  Knowing that Oracle OID was LDAP v3 compliant, I developed a prototype on my local machine that queried an Active Directory domain controller.  Everything worked out nicely.  I then moved the code to my development server and pointed to the Oracle LDAP server.  I kept getting an invalid username/password error.  This was interesting because the LDAP server allowed anonymous browsing and did not need credentials.  I scoured the Internet for answers and found many forum posts of people asking how to do this scenario, but no answers.  I wanted to blog about this in case someone else finds themselves in my situation.  The answer turned out to be pretty simple.  The System.DirectoryServices.Protocols namespace had all the classes I needed to talk to a non-AD generic LDAP server.  Here is a snippet of my final code.

string server = "SERVER";
string dn = "DN";
string searchAttribute = "SA";
string usernameAttribute = "UA";

LdapConnection lcon = new LdapConnection(server);
lcon.AuthType = AuthType.Anonymous;
string[] att = new string[1];
att[0] = usernameAttribute;
string filter = "(" + searchAttribute +"=" + criteria.Cert + ")";
SearchRequest sr = new SearchRequest(dn, filter, SearchScope.Subtree,att);
SearchResponse res = (SearchResponse)lcon.SendRequest(sr);
if (res.Entries != null  && res.Entries.Count > 0)
{
DirectoryAttribute da = res.Entries[0].Attributes[usernameAttribute];
this._name = da.GetValues(typeof(string)).GetValue(0).ToString();
if (_name != string.Empty && _name != "")
{
  _isAuthenticated = true;
}
else
{
  _isAuthenticated = false;
}
}
else
{
_isAuthenticated = false;
}