private bool checkCurrencyCode(string currencyCode)
        {
            // download known extisting listing, to check currency
            var          fileLink = new NBPXmlFile("c002z200103");
            StringReader tempXml  = fileLink.DownloadAsStringReader();
            bool         result   = false;

            foreach (string code in NBPXmlHelper.GetElementValues(tempXml, "kod_waluty"))
            {
                result = (code == currencyCode);
                if (result)
                {
                    break;
                }
            }
            return(result);
        }
        private async Task parseDataAsync()
        {
            Progress <CurrencyData> proggress = new Progress <CurrencyData>();

            proggress.ProgressChanged += ReportProgress;
            List <Task> tasks = new List <Task>();

            foreach (NBPXmlFile file in FileNames.GetFilesBetween(ListingsFrom, ListingsTo))
            {
                if (_rangeOfFiles == int.MinValue)
                {
                    _rangeOfFiles = FileNames.Range;
                }
                StringReader xmlFile = await file.DownloadAsStringReaderAsync();

                tasks.Add(Task.Factory.StartNew(() => NBPXmlHelper.GetGetCurrencyInformationFromXMLProgress(xmlFile, CurrencyCode, proggress)));
                Debug.WriteLine(counter++);
            }
            await Task.WhenAll(tasks);
        }