//Join
topicString = String.Join(", ", user.Topics)
(note that Topics.ToString returns a 3 character ID)

//StringBuilder
StringBuilder stringBuilder = new StringBuilder(3*user.Topics.Count() + 2*user.Topics.Count());
foreach(var topic in user.Topics) {
   stringBuilder.Append(topic.Code);
   stringBuilder.Append(", ");
}
codes = stringBuilder.ToString();
codes = codes.Remove(codes.Length-2);


//Classic concatenation
foreach(var topic in user.Topics) {
    codes += topic.Code;
    codes += ", ";
}
codes = codes.Remove(codes.Length-2);