C# Logger - Write strings to log file

public class Logger
{
 public static bool _doLog = true;

 public static void Log(String filename, String Message)
 {
  if (!_doLog)
   return;

  if (File.Exists(filename))
  {
   using (StreamWriter sw = File.AppendText(filename))
   {
    sw.WriteLine(String.Format("{0} : {1}", DateTime.Now, Message));
    sw.Close();
   }
  }
  else
  {
   using (StreamWriter sw = File.CreateText(filename))
   {
    sw.WriteLine(String.Format("{0} : {1}", DateTime.Now, Message));
    sw.Close();
   }
  }
 }
}