Linq query to select single string from multiple List<string>

Question

I am having a hard time understanding why I am getting the results that I am.

I have two lists of strings:

var list1 = new List<String> {"item 1", "item 2"};
var list2 = new List<String> { "item 3", "item 4" };

Version 1 - Output: "item 2"

var item =
            from x in (list1.Concat(list2))
            where x.EndsWith("2")
            select x;

        Console.WriteLine(item.First());

Version 2 - Output: "i"

var item =
            from x in (list1.Concat(list2))
            where x.EndsWith("2")
            select x.First();

        Console.WriteLine(item.First());

Version 3 - Output: "System.Linq.Enumerable+WhereEnumerableIterator`1[System.String]"

var item =
            from x in (list1.Concat(list2))
            where x.EndsWith("2")
            select x;

        Console.WriteLine(item);

Given that version 2 outputs "i", I would expect version 3 to output "item 2". Why is this behavior occurring?

Rex M · Accepted Answer · 2014-05-23 17:16:41Z

5

In version 3, select x is returning a sequence of strings that match your critera; it just happens to be a sequence with one item in it.

Console.WriteLine internally calls .ToString() on whatever you pass into it. Since there is no meaningful string representation for IEnumerable<T>, the default in .NET is to print the string name of the type.

Based on your wording, I think part of your confusion does come from a misunderstanding of why version 2 works the way it does. In version 2, select x.First() is actually a bit of a quirk/coincidence, because a string is also an IEnumerable<char>, so you can do LINQ operations on the string. .First() returns the first element of that char sequence, for each result that matches your criteria. So you're saying:

"For each element which matches my criteria, select the first character, and then return the sequence of all the first characters for the matches."

So in fact, item in version 2 is an IEnumerable<char> with one element in it. Calling Console.WriteLine() on an IEnumerable<char> will just print the chars in order. So you get "i".

(note: I see after I answered this, the question was edited to call .First() both inside the projection and on the result, so the bit about passing IEnumerable<char> to Console.WriteLine isn't totally relevant anymore)

Keep in mind LINQ is pretty much about working with sets until you explicitly reduce them down. For example, Select is simply a projection or transformation. It returns the same number of items that were passed to it, transformed. Where reduces down the set, but it's still a set.

edited May 23, 2014 at 17:16

answered May 23, 2014 at 17:02

Rex M

144k34 gold badges292 silver badges315 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

BradleyDotNET Over a year ago

+1 This is the only other answer I've seen that recognizes the question is about the 3rd case, and not the 2nd

Habib · Accepted Answer · 2014-05-23 17:08:00Z

2

Your Version 2 is selecting first item/char from the string x.First(), whereas your first version is selecting first item from the result set-> First string.

Version 1 is like - Select First Item from the Result Set

var item =  (from x in (list1.Concat(list2))
            where x.EndsWith("2")
            select x).First(); //First complete string will be selected

and version 2 is like- Select First Item from a string in result set

var item =  from x in (list1.Concat(list2))
            where x.EndsWith("2")
            select x.First(); //only first char will be selected for string

Your third case is selecting an IEnumerable<string>, so when you call Console.WriteLine, it calls the default implementation of ToString and thus you get

"System.Linq.Enumerable+WhereEnumerableIterator`1[System.String]"

edited May 23, 2014 at 17:08

answered May 23, 2014 at 17:02

Habib

224k30 gold badges420 silver badges446 bronze badges

3 Comments

Joe Bauer Over a year ago

Your examples are very helpful here. So, will a select without a .First() or .Single() always return a set as opposed to a single element?

Habib Over a year ago

@JoeBauer, yes. Exactly.

Habib Over a year ago

@JoeBauer, one more thing, Even if there are no rows matching the criteria select would return an empty collection/set.

Yuval Itzchakov · Accepted Answer · 2014-05-23 17:03:11Z

0

When using First() you are materializing the list, causing the iterator to execute. That is eager execution. Your third version uses select which does not materialize the list abd uses Defeered Execution which returns an iterator, hence calling ToString() on it returning the iterators name

answered May 23, 2014 at 17:03

Yuval Itzchakov

150k32 gold badges275 silver badges333 bronze badges

Comments

BradleyDotNET · Accepted Answer · 2014-05-23 17:03:52Z

0

Because Where returns an IEnumerable.

You have written the equivalent of:

var whoKnows = list1.Concat(list2).Where(x => x.EndsWith("2"));
Console.WriteLine(whoKnows);

The ToString of a collection just returns the class name.

answered May 23, 2014 at 17:03

BradleyDotNET

61.5k10 gold badges106 silver badges124 bronze badges

Comments

zs2020 · Accepted Answer · 2014-05-23 17:05:39Z

0

The type of the x in your Version 2 is String. The type of item in Version 1 is IEnumerable.

So your Version 2 return a list of characters which are the first characters of each string. In your Version 1, item.First() return the first element of the result set which is a string.

answered May 23, 2014 at 17:05

zs2020

54.7k30 gold badges157 silver badges223 bronze badges

Comments

FRL · Accepted Answer · 2014-05-23 19:12:02Z

0

//This raise a exception if no item found
var item=list1.Concat(list2).First(i => i.EndsWith("2"));
//this return default value (null) if no item found
var item2 = list1.Concat(list2).FirstOrDefault(i => i.EndsWith("2"));

answered May 23, 2014 at 19:12

FRL

7867 silver badges10 bronze badges

Collectives™ on Stack Overflow

Linq query to select single string from multiple List<string>

6 Answers 6

1 Comment

3 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

1 Comment

3 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related