C#: Realtime Currency Exchange Class

Soon, I’m facing a new project where the need for up-to-date currency exchanges is crucial.

This is not the first time I’ve done this, but I wanted to make a reusable, self-maintainable class so I don’t have to do it again.

The European Central Bank (ECB) provides the currency exchange rates on a daily basis in XML format, so all there needs to be done, is to wrap the XML file into a generic dictionary for easy use. With that in mind, here are the rules of the Currency class:

• Use XML file from ECB
• Update the currencies daily
• Must still function if the ECB has downtime
• Must be self maintained
• Must be plug ‘n play. Only CLR classes can be used.

In order to make the class self maintained it uses a System.Timers.Timer to check the ECB website once an hour for updates. As soon as it finds an update, the XML file is being downloaded and parsed. This is the Download method which runs once an hour.

/// <summary>
/// Downloads the latest exchange rates from ECB.
/// </summary>
private static void Download()
{
  try
  {
   _Timer.Stop();
   HttpWebRequest request = WebRequest.Create(_Url) as HttpWebRequest;
   using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
   {
    if (response.StatusCode == HttpStatusCode.OK && response.LastModified != _LastModified)
    {
     using (StreamWriter writer = new StreamWriter(_Filename, false))
     {
      StreamReader reader = new StreamReader(response.GetResponseStream());
      writer.Write(reader.ReadToEnd());
      reader.Close();
     }
    _LastModified = response.LastModified;
    }
   }

   _Timer.Interval = _UpdateInterval;
  }
   catch (Exception)
  {
   // If an error occurs, try again in 10 minuttes.
   _Timer.Interval = 1000 * 60 * 10;
  }
  finally
  {
   _Timer.Start();
  }
}

Examples of use

double fromUSDtoEUR = Currency.Exchange("USD", "EUR", 12.75);
double rateFromUSDtoNOK = Currency.GetRate("NOK", "USD");