Exception while accessing AD in Service Layer
private SearchResult GetUserById(string id)
    {
        SearchResult result = null;
        string targetEnviroment = this.serviceApplication.TargetEnvironment;
        try
        {
            DirectorySearcher ldapSearch = new DirectorySearcher();
            ldapSearch.SearchRoot = this.GetDirectoryEntry();
            if (targetEnviroment == "XYZ")
            {
                ldapSearch.Filter = "(&(objectClass=person)(userProperty=" + id + "))";
            }
            else
            {
                ldapSearch.Filter = "(&(objectClass=user)(samAccountName=" + id + "))";
            }
            ldapSearch.SearchScope = SearchScope.Subtree;
            result = ldapSearch.FindOne();
        }
        catch (Exception ex)
        {
            throw;
        }
        return result;
    }

public CorpDirUser CorpDir_GetUserById(string userId)
    {
        CorpDirUser result = null;
        string targetEnviroment = this.serviceApplication.TargetEnvironment;
        try
        {
            DirectoryEntry entry = this.GetUserById(userId).GetDirectoryEntry();
            if (entry != null)
            {
                if (targetEnviroment == "XYZ")
                {
                    result = MapToUserFromXYZLDAP(entry);
                }
                else
                {
                    result = MapToUserFromZYXLDAP(entry);
                }
            }
        }
        catch (Exception ex)
        {
            // logging
        }
        return result;
    }

private DirectoryEntry GetDirectoryEntry()
{
     string pathath = this.serviceApplication.CorpDirLDAPPath;
     string account = this.serviceApplication.CorpDirServiceAccountName;
     string password = this.serviceApplication.CorpDirServiceAccountPassword;
     return new DirectoryEntry("LDAP://" + path, account, password, AuthenticationTypes.None);
}