Would this SingleOrDefault() optimization be worthwhile or is it overkill / harmful?

Question

I was messing around with LinqToSQL and LINQPad and I noticed that SingleOrDefault() doesn't do any filtering or limiting in the generated SQL (I had almost expected the equivalent of Take(1)).

So Assuming you wanted to protect yourself from large quantities accidentally being returned, would the following snippet be useful or is it a bad idea?

// SingleType is my LinqToSQL generated type 
// Singles is the table that contains many SingleType's
// context is my datacontext
public SingleType getSingle(int id)
{
     var query = from s in context.Singles where s.ID == id select s;
     var result = query.Take(2).SingleOrDefault();
     return result;
}

As opposed to the normal way I would have done it (notice no .Take(2) )

public SingleType getSingle(int id)
{
     var query = from s in Singles where s.ID == id select s;
     var result = query.SingleOrDefault();
     return result;
}

I figured with the Take(2), I would still get the functionality of SingleOrDefault() with the added benefit of never having to worry about returning {n} rows accidentally, but I'm not sure if its even worth it unless i'm constantly expecting to accidentally return {n} rows with my query.

So, is this worthwhile? Is it harmful? Are there any pro's / con's that I'm not seeing?

Edit:

SQL Generated without the Take(2)

SELECT [t0].[blah], (...) 
FROM [dbo].[Single] AS [t0]
WHERE [t0].[ID] = @p0

SQL Generated with the Take(2)

SELECT TOP 2 [t0].[blah], (...) 
FROM [dbo].[Single] AS [t0]
WHERE [t0].[ID] = @p0

Also, when I speak of SingleOrDefault's functionality, I specifically desire to have it throw an exception if 2 or more are returned, which is why i'm doing a "Take(2)". The difference being, without the .Take(2), it will return {n} rows from the database, when it really only needs to return 2 (just enough to make it throw).

Answer 1

SingleOrDefault (and the IEnumerable<T>.SingleOrDefault() ) both raise an InvalidOperationException if the sequence has more than one element.

Your case above can never happen - it will throw an exception.

---

Edit:

My suggestion here would depend on your usage scenario. If you think there will be cases where you're going to regularly return more than a few rows from this query, then add the .Take(2) line. This will give you the same behavior, and the same exception, but eliminate the potential for returning many records from the DB.

However, your use of SingleOrDefault() is suggesting that there should never be >1 row returned. If that's really the case, I would leave this off and just treat this as an exception. In my opinion, you're reducing the readability of the code by suggesting that it would be normal to have >2 records when you add .Take(2), and in this case, I don't believe that's true. I'd take the perf. hit in the exceptional case for the simplicity of leaving it off.

Answer 2

Allen, is ID the primary key for the Singles table? If it is I don't fully understand your issue, as your second query will return one record or null. And the SQL will be where ID = ###... Using Take(2).SingleOrDefault() defeats the purpose of SingleOrDefault.

Answer 3

You already mentioned the point. If you exspect the query to return a huge number of rows quite often, it might give you a performance gain. But if this is an exceptional case - and SingleOrDefault() clearly indicates that - it is not worth the effort. It just pollutes your code and you should document it if you decide to let it in.

UPDATE

Just noticed that you are querying the id. I assume it is the primary key and you will get one or zero rows in consequence. So in theory you should not care much about using Single(), First(), Take(1) or whatever. But I would still consider it good design to use Single() to explicitly state that you exspect exactly one row. A cohort told me a few weeks ago that they even had a project, where something went terribly wrong and the primary key was no longer unique because of a mayor database malfunction. So better safe than sorry.

Answer 4

Single is more of a convenience method to get the single element of a query than a way to limit the number of results. By using Single you are, in effect, saying "I know this query can only have one item, so just give it to me," just like when doing someArray[0] when you know there will only be one element. SingleOrDefault adds the ability to return null rather than throwing an exception when dealing with sequences of length 0. You shouldn't be using Single or SingleOrDefault with queries that may return more than 1 result: an InvalidOperationException will be thrown.

If ID in your query is the primary key of the table, or a UNIQUE column, the database will ensure that the result set contains 1 row or none with no need for a TOP clause.

However, if you are selecting on a non-unique / non-key column and want the first result or last result (note that these have no meanings unless you also introduce an OrderBy) then you can use First or Last (which both have OrDefault counterparts) to get the SQL you desire:

var query = from s in context.Singles 
            where s.Id == id
            orderby s.someOtherColumn
            select s;

var item = query.FirstOrDefault();

On a side note, you can save yourself some typing if you are, indeed, doing a query for a single element:

var query = from s in context.Singles where s.Id == id select s;
var item = query.SingleOrDefault();

can become:

var item = context.Singles.SingleOrDefault(s => s.Id == id);