Changed the ValidateUser function

 public bool ValidateUser(string userName, string password)
        {
            bool validation;
            try
            {
                LdapConnection ldc = new LdapConnection(new LdapDirectoryIdentifier((string)null, false, false));
                NetworkCredential nc = new NetworkCredential(userName, password, "domainname.com");
                ldc.Credential = nc;
                ldc.AuthType = AuthType.Negotiate;
                ldc.Bind(nc); // user has authenticated at this point, as the credentials were used to login to the dc. 
                string myvar = ldc.SessionOptions.DomainName;
                validation = true;
            }
            catch (LdapException)
            {
                validation = false;
            }
            return validation;
        }