LINQ to SQL execution

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Hi all,

Please look at this snippet:

C#

var query = from t in db.ThingTable
             where t.Id == id
             select t;

if (!query.Any())
    return null;

ThingTable result = _Query.First();

LINQ to SQL isn't really my thing, but I have a question. Because (in my understanding) the query gets turned into an expression tree to be executed by SQL Server, is there any danger that the query will get executed twice - once for the Any() and once for First()? Or is it smart enough to reuse the result set?

Thanks.

Posted 24-Sep-13 2:44am

Rob Philpott

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Eduard Keilholz · Answer 1 · 2013-09-24T04:16:00

Solution 2

This will work way faster!

var foundThingTable = db.ThingTable.FirstOrDefault(tt => tt.Id == id);
return foundThingTable;

Posted 24-Sep-13 4:16am

Eduard Keilholz

Comments

Rob Philpott 26-Sep-13 5:15am

Yes, agreed. What I'm interested in though is whether the code above goes to the DB twice. I'm analysing, not fixing!

Alexander Dymshyts · Answer 2 · 2013-09-24T04:13:00

Solution 1

I am not sure, if it is enough smart to reuse the result set, that's why I am using such querry:

C#

var query = (from t in db.ThingTable
             where t.Id == id
             select t).FirstOrDefault();
 
if (query == null)
    return null;
 
ThingTable result = query;

In this case you will get only one record that you need and no need to call First() to get first record.

Posted 24-Sep-13 4:13am

Alexander Dymshyts

Updated 24-Sep-13 4:18am

v3

Comments

Eduard Keilholz 24-Sep-13 10:21am

You're almost there but there's a bit of code not necessary here. In your code, you can skip everything from the if down and just return 'query'. Results in the same.

Alexander Dymshyts 24-Sep-13 10:25am

I just remade his querry. May be he wants to make changes in that record and save it back to db, than he does not need to do return. If he needs to return record, than your solution is better.

Eduard Keilholz 24-Sep-13 10:27am

But still, the if and the type conversion don't make sense.

Alexander Dymshyts 24-Sep-13 10:33am

I agree about conversion. But if he wants to make change in a field, and save it back, then there will be error during saving to bd, if field marked as Not Null.

Eduard Keilholz 24-Sep-13 10:42am

Alright, but why not
if (query != null){
query.property = "x";
db.SaveChanges();
}
using
if (blah)
return null;
creates an extra exit point for your method and therefore your method more complex without any reason

Alexander Dymshyts 24-Sep-13 11:05am

You are right