如何返回正确与错误信息

class Program
{
static void Main(string[] args)
{
string err="";
Program p=new Program();
List<string> s = p.GetXXX(ref err);
System.Console.WriteLine(err);
foreach(string x in s){
System.Console.WriteLine(x);
}
}
public List<string> GetXXX(ref string err){
err = "-1";
List<string> s = new List<string>();
s.Add("XXX");
s.Add("MMM");
return s;
}
}