LINQ: Returning an empty string/null instead of an System.InvalidOperationException error

Question

I have this linq query:

string appuserid = (from c in db.AppUsers
                          where (c.AppUserID == AppUserId)
                          select c.AppUserID).Single().ToString();

Lets say testuser1 does not exist in the db

If I let AppUserId = testuser1 I get:

System.InvalidOperationException.

My question is, is there anyway to rewrite this query to avoid this error and perhaps return an empty string or let it be null? I need to return something that I can write some logic around.

Use `SingleOrDefault` it will return a null if it doesnt exist. — crthompson, Apr 07 '15 at 20:11
possible duplicate of [Returning a single value with Linq to SQL](http://stackoverflow.com/questions/1544322/returning-a-single-value-with-linq-to-sql) — crthompson, Apr 07 '15 at 20:12
@DimitarTsonev, both exist and are used for different reasons. — crthompson, Apr 07 '15 at 20:13
In that same link it explains `DefaultIfEmpty` as well which may be of use in your case. — crthompson, Apr 07 '15 at 20:16

kirkmcpherson · Answer 1 · 2015-04-07T20:39:58.780

2

string appuserid = (from c in db.AppUsers
                          where (c.AppUserID == AppUserId)
                          select c.AppUserID).SingleOrDefault() ?? String.Empty;

This will return an empty string.

edited Apr 07 '15 at 20:39

answered Apr 07 '15 at 20:24

kirkmcpherson

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Since the default of a string is null, this will throw a `NullReferenceException` – Steve Mitcham Apr 07 '15 at 20:34
You are correct. I fixed it to return an empty string if null; – kirkmcpherson Apr 07 '15 at 20:41
operator _??_ can't be applied for operans from _AppUsers_ and _string_ – Grundy Apr 07 '15 at 20:53
We are selecting AppUserID which I'm assuming is a String. We can use ?? on a String. – kirkmcpherson Apr 07 '15 at 20:56
@kirkmcpherson, Actually his code ends with `.ToString()` - so I doubt `AppUserID` itself is a `string`. – ebb Apr 08 '15 at 07:50

score 1 · Answer 2 · answered Apr 07 '15 at 20:20

1

Try this

string appuserid = (from c in db.AppUsers
                          where (c.AppUserID == AppUserId)
                          select c.AppUserID).SingleOrDefault();

This will return null, if no element found.

answered Apr 07 '15 at 20:20

Kishore Kumar

12,675
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Christos · Accepted Answer · 2015-04-07T20:28:07.027

You could use the method SingleOrDefault. Then you could check if that you get is not or not.

var appUser = (from c in db.AppUsers
                 where c.AppUserID == AppUserId
                 select c).SingleOrDefault();

if(appUser!=null)
    var appUserId = appUser.AppUserID.ToString();

Or more compact:

var appUser = db.AppUsers.SingleOrDefault(x=>x.AppUserID==AppUserId);

if(appUser!=null)
    var appUserId = appUser.AppUserID.ToString();

The method SingleOrDefault returns the single item from a sequence for which the predicate (the expression in the where clause) is true. If there are more than one items, for which the predicate is true, an exception will be thrown. Furthermore, if there isn't any item in the sequence for which the predicate is true, you will get the default value for the projected item.

LINQ: Returning an empty string/null instead of an System.InvalidOperationException error

3 Answers3