Select Query using LINQ

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Hello,

I am novice to LINQ.
I want the following SQL query to be in LINQ:-

C#

string strSql = "SELECT FULLNAME FROM Contact  WHERE ContactID IN (" + wherecondition + ")";

Please help.

Thanks

Posted 23-Jun-15 0:57am

rupai99

Updated 23-Jun-15 1:07am

Herman<T>.Instance

v3

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AnvilRanger · Answer 1 · 2015-06-23T01:51:00

Solution 1

Here is a great resource for getting started with linq

https://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b[^]

Posted 23-Jun-15 1:51am

AnvilRanger

Andy Lanng · Answer 2 · 2015-06-23T01:32:00

Solution 2

The issue is that you have to insist that there is only one result. You can use .Single() if you know there are no more and no less than 1 result:

C#

string name = db.Contacts.Single(c=>c.ContactID == contactId)

if there are 0 to 1 then use .FirstOrDefault which would return null if there are 0 results.

Otherwise you can use a where clause to select an enumeration, from which you can select out the field you want using .Select():

C#

string[] name = db.Contacts.Where(c=>contactIds.Contains(c.ContactID)).Select(c=>c.FullName).ToArray()

Hope that helps ^_^
Andy

Posted 23-Jun-15 1:32am

Andy Lanng

Comments

Nathan Minier 23-Jun-15 7:37am

I would highly suggest using FirstOrDefault() for both performance and resiliency purposes. Single (and SingleOrDefault) enumerates the entire collection so that if there are more than 1 matching entry, an exception is thrown.

Single is good for integrity checks, but bad for queries.

Richard Deeming 23-Jun-15 8:20am

Not quite. The overloads of Single and SingleOrDefault which don't take a predicate will only enumerate a maximum of two items in the collection - if there is a second item, they throw an exception immediately, rather than enumerating the rest of the collection.

You can see this in the source code[^].

When translated to SQL, it will issue a SELECT TOP 2 ... query.

Nathan Minier 23-Jun-15 8:56am

I'll happily concede that point, in that instance. I'm not sure that it makes it preferential to First(OrDefault), though. Is there a compelling reason that the First for query, Single for integrity rule is wrong?

Richard Deeming 23-Jun-15 9:03am

If you only ever expect your query to return a single item, then it's a good idea to validate that assumption; otherwise, your code might proceed to do the wrong thing, and you won't know until it's too late.

Rather than splitting into query / integrity, I'd say use First when you don't care if there's more than one match, and Single when you do care.

Andy Lanng 23-Jun-15 9:17am

I concede that rule-of-thumb

Andy Lanng 23-Jun-15 8:57am

If I was worried about the size of my selected item and I knew there could only be 1 result, then I would use First[OrDefault](), but only after I checked why the heck my select was so large.
If the select is large and I was unsure of how many results I would count first (I do this for my generic Paging and Single queries)
Otherwise, I use Single. My selects are never large (by design) so the extra effort of maybe getting a second row is negligible.

All points are valid for the right situation ^_^