In the following code,

// class overrides SPItemEventreceiver    
public override void ItemAdding(SPItemEventProperties properties)
{    
  using (var site = new SPSite(properties.SiteId)) //SiteId is integer
  {
      ...
  }
}

The following exception is thrown:

System.IO.FileNotFoundException: The Web application at http://URL could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application.

One way to work around this is to hard-code (or configure) the URL specified in Alternate Access Mappings. Putting the correct URL in Alternate Access Mappings is ultimately the correct solution, but if possible, I need a work-around that doesn't require configuration.