Remove overlapping items between two List<string> collections

Question

I have two collections of List, let's call them allFieldNames (complete set) and excludedFieldNames (partial set). I need to derive a third List that gives me all non-excluded field names. In other words, the subset list of allFieldNames NOT found in excludedFieldNames. Here is my current code:

public List<string> ListFieldNames(List<string> allFieldNames, List<string> excludedFieldNames)
        {
            try
            {
                List<string> lst = new List<string>();

                foreach (string s in allFieldNames)
                {
                    if (!excludedFieldNames.Contains(s)) lst.Add(s);
                }
                return lst;
            }
            catch (Exception ex)
            {
                return null;
            }
        }

I know there has to be a more efficient way than manual iteration. Suggestions please.

Answer 1

You can use the Except method:

return allFieldNames.Except(excludedFieldNames).ToList();

(And if you're happy to return an IEnumerable<string> rather than a List<string> then you could omit the final ToList call as well.)