ConcurrentDictionary tryAdd

public Stream ExportSheet(DocumentSearchCriteria criteria)
        {
            var sheets = SpreadsheetServiceFactory.CreateInstance();
            var sheetList = new Dictionary<string, IList<IDictionary<string, object>>>();

            IElapsedTimePagedList<Document> docList = Query(criteria);
            docList.ToList<Document>().ForEach(x =>
            {
                var docAttrs = new ConcurrentDictionary<string, object>();
                foreach (DocumentAttribute attr in x.Attributes)
                {
                    if (attr.DisplayName != "OwnerSubjectId")
                    {
                        if (docAttrs.Where(a => a.Key == attr.DisplayName).Count() == 0)
                        {
                            docAttrs.TryAdd(attr.DisplayName, attr.Value.FirstOrDefault());
                        }
                        else
                        {
                            docAttrs[docAttrs.Where(a => a.Key == attr.DisplayName).FirstOrDefault().Key] += "\r\n" + attr.Value.FirstOrDefault();
                        }
                    }
                }

                var selectSheet = sheetList.Where(s => s.Key == x.Form.DisplayName);
                if (selectSheet.Count() == 0)
                {
                    var list = new List<IDictionary<string, object>>
                    {
                        docAttrs
                    };
                    sheetList.Add(x.Form.DisplayName, list);
                }
                else
                {
                    sheetList.Where(s => s.Key == x.Form.DisplayName).FirstOrDefault().Value.Add(docAttrs);
                }
            });


            var stream = sheets.WriteMutipleSheet(sheetList);

            return stream;
        }