// awkward way
catch(Exception ex){
    if (ex is RudenSpecificException)
    {
        ((RudenSpecificException) ex).RudenMessage;
    }
}

// fast way
catch(Exception ex)
{
   var rudenSpecific = ex as RudenSpecificException;
   if (rudenSpecific != null) rudenSpecific.Message; // not a big win in lines in this case, but you can use fields directly via one AS command
}