c# LINQ: how to retrieve a single result

Question

Kind of new to linq,

whats the simplest way to retrieve a single result using linq?

example, my query

var query =
     from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select c.co2Target;

it should only return a single field with a double value. how do i pull it out of query? In the past i had used ExecuteScalar. How do i do it with linq? I would like to preserve its data type

UPDATE:

Here's where I am now. The problem is that the test query im running here is returning 4 instead of 3.75

var query =
                (from a in db.LUT_ProductInfos
                 where a.flavor == "Classic Coke" && a.Container == "Can"
                 select new { a.co2High }).Single();

            double MyVar = query.co2High.Value;

Tom Gullen · Accepted Answer · 2011-05-16T09:23:53.503

30

I think you mean return one value, not one record? You would need to do select new {} as follows:

var query =
     from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select new { c.co2Target };

Then if you only want to retrieve a single record as well as that:

var query =
     (from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select new { c.co2Target }).Single();

Retrieval would be done as follows:

var query =
         (from c in db.productInfo
         where c.flavor == "Classic Coke" && c.container == "Can"
         select new { c.co2Target }).Single();

double MyVar = query.co2Target;

edited May 16 '11 at 09:23

answered May 16 '11 at 09:04

Tom Gullen

61,249
84
283
456

1

This syntax wraps the result in a anonymous type, rather than returning just the value. I don't think this is what the OP intended. – Thorarin May 16 '11 at 09:11
thanks but hmm... the result should be 3.75 but its giving me 4. Tried formatting via String.Format(query) ... any ideas? EDIT: i see the guy above me caught it – Sinaesthetic May 16 '11 at 09:16
@Sinaes, what field type is co2Target in the database, and how are you retrieving it from query? – Tom Gullen May 16 '11 at 09:17
this particular one is a double. im trying to find the best way to retrieve it, that info is in the original post. – Sinaesthetic May 16 '11 at 09:19
@Sinaes I've updated the answer to show how to pull the value out – Tom Gullen May 16 '11 at 09:24
Cannot implicitly convert type 'double?' to 'double'. An explicit conversion exists (are you missing a cast?). I tried casting it, the number was still getting rounded up – Sinaesthetic May 16 '11 at 09:32
@Sinaes Do: `double MyVar = query.co2Target.Value;`. If the field doesn't accept nulls you do not need to specify `.Value`. – Tom Gullen May 16 '11 at 09:32
i actually was just in the middle of trying that. eliminated the error, but still rounding up – Sinaesthetic May 16 '11 at 09:34
im actually beginning to wonder if it has something to do with the fact that its bring the info in as a double when sql server has the datatype set as float. i just ran a query for that field unfiltered and all of the numbers are a bit off – Sinaesthetic May 16 '11 at 09:55
@Sinaes, yes that will matter, you said originally `this one is a double`, change it from a float to a double and it should be fine. – Tom Gullen May 16 '11 at 09:58
sql server doesn't have double. i just looked it up and its supposed to map it on its own. i ran a different query and was able to get a result back 11.06 ... odd... dunno whats goin on here – Sinaesthetic May 16 '11 at 09:59
oh good god im a moron. just realized that i was typing in co2High instead of co2Target... the result was right all along. so tired. lol... thanks for the help – Sinaesthetic May 16 '11 at 10:00
FirstOrDefault is safer no? – S.. Sep 04 '14 at 14:01

score 16 · Answer 2 · answered May 16 '11 at 09:02

16

Use the .Single() or .SingleOrDefault() extension methods.

var query =
     (from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select c.co2Target).Single();

answered May 16 '11 at 09:02

BenCr

5,991
5
44
68

score 12 · Answer 3 · answered May 16 '11 at 09:02

By using First() or FirstOrDefault()

var query =
    (from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select c.co2Target).FirstOrDefault();

Only use Single() or SingleOrDefault() if you know there is only one result, or if you want to fail if there are multiple results.

Thorarin · Answer 4 · 2011-05-16T09:39:40.160

You can use the Single extension method:

var result =
     (from c in db.productInfo
     where c.flavor == "Classic Coke" && c.container == "Can"
     select c.co2Target).Single();

Other related extension methods are SingleOrDefault, First and FirstOrDefault.

The difference between Single and First is that Single throws an exception if the query results in more than one result. The OrDefault variations will return null if no results were returned by the query, while Single and First throw an exception is no result exists.

If you're using Entity Framework 3.5, it does not support Single, so you will have to use First.

One other thing worth noting is that your original code resulted in an IQueryable<T>, which means it does not actually execute the query until you evaluate the result. Using any of these extension methods will force the query to run immediately.

score 4 · Answer 5 · answered May 16 '11 at 09:01

4

msdn : SingleOrDefault

Make use of Single() or SingleOrDefault() method to get result

Also check : Default Extension methods

answered May 16 '11 at 09:01

Pranay Rana

175,020
35
237
263

score 2 · Answer 6 · edited Nov 19 '12 at 19:06

2

string str = (
  from c in db.productInfo 
  where c.flavor == "Classic Coke" && c.container == "Can"
  select c.co2Target)
    .Single().columnName;

edited Nov 19 '12 at 19:06

Konrad Viltersten

36,151
76
250
438

answered Nov 19 '12 at 18:11

Asif Rehman

21
4

1

It would be helpful to add some additional information about why your answer solves the problem – jeremy Nov 19 '12 at 18:31
Please add some explanatory text along with the above code to make this a more useful answer. – Paul R Nov 19 '12 at 18:32

score 2 · Answer 7 · edited May 16 '11 at 09:31

use SingleOrDefault() if your query always returns only one element as result or exception will be thrown if the result of your query is more than one element.

(from c in db.productInfo
 where c.flavor == "Classic Coke" && c.container == "Can"
 select c.co2Target).SingleOrDefault();

use FirstOrDefualt() if your result more than one element and you need any one of then.

(from c in db.productInfo
 where c.flavor == "Classic Coke" && c.container == "Can"
 select c.co2Target).FirstOrDefault();

score -1 · Answer 8 · answered May 16 '11 at 09:17

-1

i prefer SingleOrDefault(), it does not throw an exception if the nothing is returned. MSDN reference

this way you can do a safe-guard condition check for such case.

var query = (from c in db.productInfo where c.flavor == "Classic Coke" && c.container == "Can"
                 select c.co2Target).SingleOrDefault();

answered May 16 '11 at 09:17

core_pro

161
1
1
7

10

I once had a guy go through all my code and change all my references from Single() to SingleOrDefault(). I chewed him out for an hour, because SingleOrDefault can introduce subtle bugs that you have to be aware of, while Single() will throw an exception if more than one record (or no records) is returned. Use single when there should be a single value, and single value only. It's better to throw an exception than let a sublte bug through. – Erik Funkenbusch May 16 '11 at 09:27

c# LINQ: how to retrieve a single result

8 Answers8

Linked