public override IList <string> Validate(CommandParameters parameters, ValidationContext context)
        {
            var messages = base.Validate(parameters, context);

            string file = parameters.GetValue <string>("File");

            if (!System.IO.File.Exists(file))
            {
                messages.Add(string.Format("The file '{0}' does not exist", file));
            }

            return(messages);
        }