1. You've forgotten quotes
string a = "1,23,45; 6,7,8";
2. If you want split string by a semicolon as mentioned in the question caption you can use
String.Split[
^] as follows. The result is array of strings
var splitted = a.Split(';');
3. If you want output as in question you can use
String.Replace[
^]
var replaced = a.Replace(";", Environment.NewLine);