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Does Linq's IEnumerable.Select return a reference to the original IEnumerable?

I was trying to clone an List in my code, because I needed to output that List to some other code, but the original reference was going to be cleared later on. So I had the idea of using the Select extension method to create a new reference to an IEnumerable of the same elements, for example:
List<int> ogList = new List<int> {1, 2, 3};
IEnumerable<int> enumerable = ogList.Select(s => s);
Now after doing ogList.Clear(), I was surprised to see that my new enumerable was also empty.
So I started fiddling around in LINQPad, and saw that even if my Select returned different objects entirely, the behaviour was the same.
List<int> ogList = new List<int> {1, 2, 3};
IEnumerable<int> enumerable = ogList.Select(s => 5); // Doesn't return the original int
enumerable.Count().Dump(); // Count is 3
ogList.Clear();
enumerable.Count().Dump(); // Count is 0!
Note that in LINQPad, the Dump()s are equivalent to Console.WriteLine().
Now probably my need to clone the list in the first place was due to bad design, and even if I didn't want to rethink the design I could easily clone it properly. But this got me thinking about what the Select extension method actually does.
According to the documentation for Select:
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.
So then I tried adding this code before clearing:
foreach (int i in enumerable)
{
i.Dump();
}
The result was still the same.
Finally, I tried one last thing to figure out if the reference in my new enumerable was the same as the old one. Instead of clearing the original List, I did:
ogList.Add(4);
Then I printed out the contents of my enumerable (the "cloned" one), expecting to see '4' appended to the end of it. Instead, I got:
5
5
5
5 // Huh?
Now I have no choice but to admit that I have no idea how the Select extension method works behind the scenes. What's going on?

List/List<T> are for all intents and purposes fancy resizable arrays. They own and hold the data for value types such as your ints or references to the data for reference types in memory and they always know how many items they have.
IEnumerable/IEnumerable<T> are different beasts. They provide a different service/contract. An IEnumerable is fictional, it does not exist. It can create data out of thin air, with no physical backing. Their only promise is that they have a public method called GetEnumerator() that returns an IEnumerator/IEnumerator<T>. The promise that an IEnumerator makes is simple:
some item could be available or not at a time when you decide you need it. This is achieved through a simple method that the IEnumerator interface has: bool MoveNext() - which returns false when the enumeration is completed or true if there was in fact a new item that needed to be returned. You can read the data through a property that the IEnumerator interface has, conveniently called Current.
To get back to your observations/question: as far as the IEnumerable in your example is concerned, it does not even think about the data unless your code tells it to fetch some data.
When you are writing:
List<int> ogList = new List<int> {1, 2, 3};
IEnumerable<int> enumerable = ogList.Select(s => s);
You are saying: Listen here IEnumerable, I might come to you asking for some items at some point in the future. I'll tell you when I will need them, for now sit still and do nothing. With Select(s => s) you are conceptually defining an identity projection of int to int.
A very rough simplified, non-real-life implementation of the select you've written is:
IEnumerable<T> Select(this IEnumerable<int> source, Func<int,T> transformer) something like
{
foreach (var i in source) //create an enumerator for source and starts enumeration
{
yield return transformer(i); //yield here == return an item and wait for orders
}
}
(this explains why you got a 5 when expecting a for, your transform was s => 5)
For value types, such as the ints in your case: If you want to clone the list, clone the whole list or part of it for future enumeration by using the result of an enumeration materialized through a List. This way you create a list that is a clone of the original list, entirely detached from its original list:
IEnumerable<int> cloneOfEnumerable = ogList.Select(s => s).ToList();
Later edit: Of course ogList.Select(s => s) is equivalent to ogList. I'm leaving the projection here, as it was in the question.
What you are creating here is: a list from the result of an enumerable, further consumed through the IEnumerable<int> interface. Considering what I've said above about the nature of IList vs IEnumerable, I would prefer to write/read:
IList<int> cloneOfEnumerable = ogList.ToList();
CAUTION: Be careful with reference types. IList/List make no promise of keeping the objects "safe", they can mutate to null for all IList cares. Keyword if you ever need it: deep cloning.
CAUTION: Beware of infinite or non-rewindable IEnumerables

Provided answers explain why you are not obtaining a cloned list (due to deferred execution of some LINQ extension methods).
However, keep in mind that list.Select(e => e).ToList() will get a real clone only when dealing with value types such as int.
If you have a list of reference types you will receive a cloned list of references to existent objects. In this case you should consider one of the solutions provided here for deep-cloning or my favorite from here (which might be limited by object inner structure).

You have to be aware that an object that implements IEnumerable does not have to be a collection itself. It is an object that makes it possible to get an object that implements IEnumerator. Once you have the enumerator you can ask for the first element and for the next element until there are no more next elements.
Every LINQ function that returns an IEnumerable is not the sequence itself, it only enables you to ask for the enumerator. If you want a sequence, you'll have to use ToList.
There are several other LINQ functions that do not return an IEnumerable, but for instance a Dictionary, or only one element (FirstOrDefault(), Max(), Single(), Any(). These functions will get the enumerator from the IEnumerable and start enumerating until they have the result. Any will only have to check if you can start enumerating. Max will enumerate over all elements and remember the largest one. etc.
You'll have to be aware: as long as your LINQ statement is an IEnumerable of something, your source sequence is not accessed yet. If you change your source sequence before you start enumerating, the enumeration is over your changed source sequence.
If you don't want this, you'll have to do the enumeration before you change your source. Usually this will be ToList, but this can be any of the non-deferred function: Max(), Any(), FirstOrDefault(), etc.
List<TSource> sourceItems = ...
var myEnumerable = sourceItems
.Where(sourceItem => ...)
.GroupBy(sourceItem => ...)
.Select(group => ...);
// note: myEnumerable is an IEnumerable, it is not a sequence yet.
var list1 = sourceItems.ToList(); // Enumerate over the sequence
var first = sourceItems.FirstOrDefault(); // Enumerate and stop after the first
// now change the source, and to the same things again
sourceItems.Clear();
var list1 = sourceItems.ToList(); // returns empty list
var first = sourceItems.FirstOrDefault(); // return null: there is no first element
So every LINQ function that does not return IEnumerable, will start enumerating over sourceItems as the sequence is at the moment that you start enumerating. The IEnumerable is not the sequence itself.

This is an enumerable.
var enumerable = ogList.Select(s => s);
If you iterate through this enumerable, LINQ will in turn iterate over the original resultset. Each and every time. If you do anything to the original enumerable, the results will also be reflected in your LINQ calls.
If you need to freeze the data, store it in a list instead:
var enumerable = ogList.Select(s => s).ToList();
Now you've made a copy. Iterating over this list will not touch the original enumerable.

Getting the results of distinct( IEqualityComparer comparer)

As a LINQ-beginner I wonder why nobody mentioned in Implementing IEqualityComparer<T> for comparing arbitrary properties of any class (including anonymous) that the query actually has to be carried out to get the results. In other words, just calling
IEnumerable<Person> people = ... ; // some database call here
var distinctPeople = people.Distinct(new PropertyComparer<Person>("FirstName"));
will not trigger the execution of the specific Equals(Tx, Ty) and GetHashCode(T obj) methods in the PropertyComparer.
The message "Expanding the Results View will enumerate the IEnumerable" (in the Debugger) gave me the hint. Now, could I proceed with something like foreach (var dp in distinctPeople) to get the results?

This has nothing at all to do with the IEqualityComparer. It is entirely based on the method you're providing it to, in this case, Distinct. Distinct, along with all of the LINQ methods that return an IEnumerable, defer execution as much as possible, only performing the work that they need to compute the results when they actually need to do so.

LINQ - IEnumerable.ToList() and Deferred Execution confusion

I have an IEnumerable variable named "query" which represents an entity framework query. If I do the following, will it enumerate the query more than once? My confusion is in "result" being an IEnumerable and not a List.
IEnumerable<T> result = query.ToList();
bool test = result.Any();
as apposed to this:
// Using List to only enumerate the query once
List<T> result = query.ToList();
bool test = result.Any();
Also, I know this is silly, but would it enumerate twice if I do the following, or would it somehow know that "query" was already enumerated even though the result of the enumeration is not being used?
List<T> result = query.ToList();
bool test = query.Any();
Thanks!

Once you're calling ToList or ToArray you will create an in-memory collection. From then on you're not dealing with the database anymore.
So even if you declare it as IEnumerable<T> it actually remains a List<T> after query.ToList().
Also, all related LINQ extension methods will check if the sequence can be casted to a collection type. You can see that for example in Enumerable.Count the Count property wil be used if possible:
public static int Count<TSource>(this IEnumerable<TSource> source) {
if (source == null) throw Error.ArgumentNull("source");
ICollection<TSource> collectionoft = source as ICollection<TSource>;
if (collectionoft != null) return collectionoft.Count;
ICollection collection = source as ICollection;
if (collection != null) return collection.Count;
int count = 0;
using (IEnumerator<TSource> e = source.GetEnumerator()) {
checked {
while (e.MoveNext()) count++;
}
}
return count;
}
According to your last question, if it makes a difference wether or not you use the list or again the query in this code snippet:
List<T> result = query.ToList();
bool test = query.Any();
Yes, in this case you are not using the list in memory but the query which will then ask the database again even if .Any is not as expensive as .ToList.

When you call ToList on your query it will be transformed into a list. The query will be evaluated right then, the items will be pulled out, and the list will be populated. From then on that List has no knowledge of the original query. No amount of manipulation of that list can in any way affect or evaluate that query, as it knows nothing about it.
It doesn't matter what you call that List, or what type of variable you stick it in, the list itself simply doesn't know anything about the IQueryable anymore. Iterating the variable holding the list multiple times will simply iterate that list multiple times.
In just the same way that the list doesn't know a thing about the query, the query doesn't know a thing about the list. It doesn't remember that it's items were put into a list and continue to return those items. (You can actually write query objects like this, in theory. It's called memoization, for the query to cache it's results when iterated and continue to provide objects from that cache when iterated later. EF doesn't memoize its queries by default, nor does it provide a tool for memoization by default, although 3rd party tools provide such extensions.) This means that the 3rd code snippet that you have will actually execute two separate database queries, not just one.

No the enumeration occour only in ToList().
http://msdn.microsoft.com/it-it/library/bb342261(v=vs.110).aspx

c# Find an item in 2 / multiple lists

I have the presumably common problem of having elements that I wish to place in 2 (or more) lists. However sometimes I want to find an element that could be in one of the lists. Now there is more than one way of doing this eg using linq or appending, but all seem to involve the unnecessary creation of an extra list containing all the elements of the separate lists and hence waste processing time.
So I was considering creating my own generic FindinLists class which would take 2 lists as its constructor parameters would provide a Find() and an Exists() methods. The Find and Exists methods would only need to search the second or subsequent lists if the item was not found in the first list. The FindInLists class could be instantiated in the getter of a ( no setter)property. A second constructor for the FindInLists class could take an array of lists as its parameter.
Is this useful or is there already a way to search multiple lists without incurring the wasteful overhead of the creation of a super list?

You could use the LINQ Concat function.
var query = list1.Concat(list2).Where(x => x.Category=="my category");

Linq already has this functionality by virtue of the FirstOrDefault method. It uses deferred execution so will stream from any input and will short circuit the return when a matching element is found.
var matched = list1.Concat(list2).FirstOrDefault(e => element.Equals(e));
Update
BaseType matched = list1.Concat(list2).Concat(list3).FirstOrDefault(e => element.Equals(e));

I believe IEnumerable<T>.Concat() is what you need. It doesn't create an extra list, it only iterates through the given pair of collections when queried
Concat() uses deferred execution, so at the time it's called it only creates an iterator which stores the reference to both concatenated IEnumerables. At the time the resulting collection is enumerated, it iterates through first and then through the second.
Here's the decompiled code for the iterator - no rocket science going on there:
private static IEnumerable<TSource> ConcatIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second)
{
foreach (TSource iteratorVariable0 in first)
{
yield return iteratorVariable0;
}
foreach (TSource iteratorVariable1 in second)
{
yield return iteratorVariable1;
}
}
When looking to the docs for Concat(), I've stumbled across another alternative I didn't know - SelectMany. Given a collection of collections it allows you to work with the children of all parent collections at once like this:
IEnumerable<string> concatenated = new[] { firstColl, secondColl }
.SelectMany(item => item);

you can do something like this:
var list1 = new List<int>{1,2,3,4,5,6,7};
var list2 = new List<int>{0,-3,-4,2};
int elementToPush = 4;//value to find among available lists
var exist = list1.Exists(i=>i==elementToPush) || list2.Exists(j=>j==elementToPush);
If at least one collection required element exists, result is false, otherwise it's true.
One row and no external storage creation.
Hope this helps.

You could probably just create a List of lists and then use linq on that list. It is still creating a new List but it is a list of references rather than duplicating the contents of all the lists.
List<string> a = new List<string>{"apple", "aardvark"};
List<string> b = new List<string>{"banana", "bananananana", "bat"};
List<string> c = new List<string>{"cat", "canary"};
List<string> d = new List<string>{"dog", "decision"};
List<List<string>> super = new List<List<string>> {a,b,c,d};
super.Any(x=>x.Contains("apple"));
the Any call should return after the first list returns true so as requested will not process later lists if it finds it in an earlier list.
Edit: Having written this I prefer the answers using Concat but I leave this here as an alternative if you want something that might be more aesthetically pleasing. ;-)

LINQ extension methods - Any() vs. Where() vs. Exists()

Unfortunately the names of these methods make terrible search terms, and I've been unable to find a good resource that explains the difference between these methods--as in when to use each.
Thanks.
Edit:
The sort of query that I'm trying to fully understand is something like this:
context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList();
And thanks to all who've answered.

Where returns a new sequence of items matching the predicate.
Any returns a Boolean value; there's a version with a predicate (in which case it returns whether or not any items match) and a version without (in which case it returns whether the query-so-far contains any items).
I'm not sure about Exists - it's not a LINQ standard query operator. If there's a version for the Entity Framework, perhaps it checks for existence based on a key - a sort of specialized form of Any? (There's an Exists method in List<T> which is similar to Any(predicate) but that predates LINQ.)

context.Authors.Where(a => a.Books.Any(b => b.BookID == bookID)).ToList();
a.Books is the list of books by that author. The property is automatically created by Linq-to-Sql, provided you have a foreign-key relationship set up.
So, a.Books.Any(b => b.BookID == bookID) translates to "Do any of the books by this author have an ID of bookID", which makes the complete expression "Who are the authors of the book with id bookID?"
That could also be written something like
from a in context.Authors
join b in context.Books on a.AuthorId equal b.AuthorID
where b.BookID == bookID
select a;
UPDATE:
Any() as far as I know, only returns a bool. Its effective implementation is:
public Any(this IEnumerable<T> coll, Func<T, bool> predicate)
{
foreach(T t in coll)
{
if (predicte(t))
return true;
}
return false;
}

Just so you can find it next time, here is how you search for the enumerable Linq extensions. The methods are static methods of Enumerable, thus Enumerable.Any, Enumerable.Where and Enumerable.Exists.
google.com/search?q=Enumerable.Any
google.com/search?q=Enumerable.Where
google.com/search?q=Enumerable.Exists
As the third returns no usable result, I found that you meant List.Exists, thus:
google.com/search?q=List.Exists
I also recommend hookedonlinq.com as this is has very comprehensive and clear guides, as well clear explanations of the behavior of Linq methods in relation to deferness and lazyness.

Any - boolean function that returns true when any of object in list satisfies condition set in function parameters. For example:
List<string> strings = LoadList();
boolean hasNonEmptyObject = strings.Any(s=>string.IsNullOrEmpty(s));
Where - function that returns list with all objects in list that satisfy condition set in function parameters. For example:
IEnumerable<string> nonEmptyStrings = strings.Where(s=> !string.IsNullOrEmpty(s));
Exists - basically the same as any but it's not generic - it's defined in List class, while Any is defined on IEnumerable interface.

Any() returns true if any of the elements in a collection meet your predicate's criteria. (Any() does not iterate through the entire collection, as it returns upon the first match.)
Where() returns an enumerable of all elements in a collection that meet your predicate's criteria.
Exists() does the same thing as Any() except it's just an older implementation that was there on the List back before LINQ.

IEnumerable introduces quite a number of extensions to it which helps you to pass your own delegate and invoking the resultant from the IEnumerable back. Most of them are by nature of type Func
The Func takes an argument T and returns TResult.
In case of
Where - Func : So it takes IEnumerable of T and Returns a bool. The where will ultimately returns the IEnumerable of T's for which Func returns true.
So if you have 1,5,3,6,7 as IEnumerable and you write .where(r => r<5) it will return a new IEnumerable of 1,3.
Any - Func basically is similar in signature but returns true only when any of the criteria returns true for the IEnumerable. In our case, it will return true as there are few elements present with r<5.
Exists - Predicate on the other hand will return true only when any one of the predicate returns true.
So in our case if you pass .Exists(r => 5) will return true as 5 is an element present in IEnumerable.

foreach (var item in model.Where(x => !model2.Any(y => y.ID == x.ID)).ToList())
{
enter code here
}
same work you also can do with Contains
secondly Where is give you new list of values.
thirdly using Exist is not a good practice, you can achieve your target from Any and contains like
EmployeeDetail _E = Db.EmployeeDetails.where(x=>x.Id==1).FirstOrDefault();
Hope this will clear your confusion.