global.asax

Other things about csharp. you could learn from this

<%@ Import Namespace="System.Diagnostics" %>

<script language="C#" runat="server">

  public void Application_Error(Object sender, EventArgs e)
  {
    string LogName = "Web_Errors";
    string Message;
    Message = "Url: " + Request.Path;
    Message = Message + " Error: " + Server.GetLastError().ToString();

    // Create Event Log if It Doesn't Exist
    if (EventLog.SourceExists(LogName))
      EventLog.CreateEventSource(LogName, LogName);

    // Fire off to Event Log
    EventLog Log = new EventLog();

    Log.Source = LogName;
    Log.WriteEntry(Message, EventLogEntryType.Error);

  }
</script>