Hi Imar,

Could you please explain why omitting the Single() function from the query

var review22 = (from r in myDataContext.Reviews
                where r.Id == 22
                select r).Single();

would result in multiple returns of the same element?

Also, what factor would determine the number of such returns?

Thank you, as always, for your trouble.

With best regards,

Roman

The difference between this:
and this
is that the second example is returning a collection of items while the first returns a single instance. You could certainly use the first, but you'll need to ask for the first item from the collection.

-Peter
compiledthoughts.com

As Peter pointed out, this:

var review22 = from r in myDataContext.Reviews
               where r.Id == 22
               select r;

could return multiple records. You and I (and the database) know that the Id is unique so it can only return one item. However, LINQ doesn't know that. The "where" criteria could easily be something that returns multiple records (like where myClient.City = "Amsterdam" for example).

By using Single() you indicate you know only one item will be returned, so LINQ gives you exactly one item, and not a collection with only one item in it.

Finally, SingleOrDefault is useful if you're not sure if your query returns a record. When there isn't an item with an Id of 22, Single() will crash whereas SingleOrDefault will return null when the item could not be found.

Hope this helps,

Imar


---------------------------------------
Imar Spaanjaars
http://Imar.Spaanjaars.Com
Everyone is unique, except for me.
Author of Beginning ASP.NET 3.5 : in C# and VB, ASP.NET 2.0 Instant Results and Dreamweaver MX 2004
Want to be my colleague? Then check out this post.

Thank you, Imar and Peter. Very helpful. ~Roman