XmlDocument xmlVerisi = new XmlDocument();
xmlVerisi.Load("http://www.tcmb.gov.tr/kurlar/today.xml");
decimal dolar = Convert.ToDecimal(xmlVerisi.SelectSingleNode(string.Format("Tarih_Date/Currency[@Kod='{0}']/ForexSelling", "USD")).InnerText.Replace('.', ','));
decimal euro = Convert.ToDecimal(xmlVerisi.SelectSingleNode(string.Format("Tarih_Date/Currency[@Kod='{0}']/ForexSelling", "EUR")).InnerText.Replace('.', ','));
decimal sterlin = Convert.ToDecimal(xmlVerisi.SelectSingleNode(string.Format("Tarih_Date/Currency[@Kod='{0}']/ForexSelling", "GBP")).InnerText.Replace('.', ','));