Linq-to-Sql: recursively get children

Question

I have a Comment table which has a CommentID and a ParentCommentID. I am trying to get a list of all children of the Comment. This is what I have so far, I haven't tested it yet.

private List<int> searchedCommentIDs = new List<int>();
// searchedCommentIDs is a list of already yielded comments stored
// so that malformed data does not result in an infinite loop.
public IEnumerable<Comment> GetReplies(int commentID) {
    var db = new DataClassesDataContext();
    var replies = db.Comments
        .Where(c => c.ParentCommentID == commentID 
            && !searchedCommentIDs.Contains(commentID));
    foreach (Comment reply in replies) {
        searchedCommentIDs.Add(CommentID);
        yield return reply;
        // yield return GetReplies(reply.CommentID)); // type mis-match.
        foreach (Comment replyReply in GetReplies(reply.CommentID)) {
            yield return replyReply;
        }
    }
}

2 questions:

1. Is there any obvious way to improve this? (Besides maybe creating a view in sql with a CTE.)
2. How come I can't yield a IEnumerable <Comment> to an IEnumerable <Comment>, only Comment itself?
3. Is there anyway to use SelectMany in this situation?

Answer 1

I'd probably use either a UDF/CTE, or (for very deep structures) a stored procedure that does the same manually.

Note that if you can change the schema, you can pre-index such recursive structures into an indexed/ranged tree that lets you do a single BETWEEN query - but the maintenance of the tree is expensive (i.e. query becomes cheap, but insert/update/delete become expensive, or you need a delayed scheduled task).

---

Re 2 - you can only yield the type specified in the enumeration (the T in IEnumerable<T> / IEnumerator<T>).

You could yield an IEnumerable<Comment> if the method returned IEnumerable<IEnumerable<Comment>> - does that make sense?

Improvements:

• perhaps a udf (to keep composability, rather than a stored procedure) that uses the CTE recursion approach
• use using, since DataContext is IDisposable...

so:

using(var db = new MyDataContext() ) { /* existing code */ }

• LoadWith is worth a try, but I'm not sure I'd be hopeful...
• the list of searched ids is risky as a field - I guess you're OK as long as you don't call it twice... personally, I'd use an argument on a private backing method... (i.e. pass the list between recursive calls, but not on the public API)