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