I think what you want to do is:
List<string> FirstList = new List<string>();
List<string> Ids = new List<string>();
List<string> ThirdList = new List<string>();
foreach (string id in Ids) {
foreach (string rec in FirstList) {
if (rec.StartsWith (id)) {
ThirdList.Add (rec);
}
}
}
/ravi