What does LINQ return when the results are empty

259

I have a question about LINQ query. Normally a query returns a IEnumerable<T> type. If the return is empty, not sure if it is null or not. I am not sure if the following ToList() will throw an exception or just a empty List<string> if nothing found in IEnumerable result?

   List<string> list = {"a"};
   // is the result null or something else?
   IEnumerable<string> ilist = from x in list where x == "ABC" select x;
   // Or directly to a list, exception thrown?
   List<string> list1 = (from x in list where x == "ABC" select x).ToList();

I know it is a very simple question, but I don't have VS available for the time being.

David.Chu.ca

출처 c# linq

7 답변

418

It will return an empty enumerable. It wont be null. You can sleep sound :)

30

You can also check the .Any() method: 

if (!YourResult.Any())

Just a note that .Any will still retrieve the records from the database; doing a .FirstOrDefault()/.Where() will be just as much overhead but you would then be able to catch the object(s) returned from the query

  • Where does the question mention a database? - cja
  • You'll have to ask the one who edited, I didn't mention any DB :) - Noich
  • Point editor is making is sound, though, DB or not. I believe they are saying .Any() is just going to tell you if you have any matching records, at all, where doing an actual query to find a specific value might be null when .Any() is not. - vapcguy
  • The edit might actually be wrong. If using linq to entities, the db might shortcut this and no data at all is being sent to the client except a true or false - Mafii
16
 

var lst = new List<int>() { 1, 2, 3 };
var ans = lst.Where( i => i > 3 );

(ans == null).Dump();  // False
(ans.Count() == 0 ).Dump();  // True

(Dump is from LinqPad)

  • Exactly! You get a better indication of the findings - netfed
  • TIL Count() is also a method, not just the property - heyNow
  • Shouldn't you use .Any() because count will enumerate through all items? - SHEePYTaGGeRNeP
11

.ToList returns an empty list. (same as new List() );

6

In Linq-to-SQL if you try to get the first element on a query with no results you will get sequence contains no elements error. I can assure you that the mentioned error is not equal to object reference not set to an instance of an object. in conclusion no, it won't return null since null can't say sequence contains no elements it will always say object reference not set to an instance of an object ;)

  • Oh, your explanation helps further understanding. Thank you ! - Kay Lee
5

Other posts here have made it clear that the result is an "empty" IQueryable, which ToList() will correctly change to be an empty list etc.

Do be careful with some of the operators, as they will throw if you send them an empty enumerable. This can happen when you chain them together.

  • "Do be careful with some of the operators, as they will throw if you send them an empty enumerable. This can happen when you chain them together." -- This is what got me. I had a null returned value that I then fed into another query. This caused the second query to throw no matter what I cast it to because there was no value being fed into the second query. - trevorc
4

It won't throw exception, you'll get an empty list.

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