how to using 'count'

thx suthish for your link..

yeah, finally i've got the answer..

 protected String GetCountComment(string IdTopic)
    {
        string value = String.Empty;
        DMSDataAccess db = new DMSDataAccess();
        var comment = db.DMSDataContext.Comments.Where(p => Convert.ToString(p.IdTopic) == IdTopic);

        value = Convert.ToString(comment.Count());
        return value;
    }