.Select Vs .Include Vs .Select Many

Question

I found the following example regarding the differences between .Select & .SelectMany

public class PhoneNumber { public string Number { get; set; } } public class Person { public IEnumerable<PhoneNumber> PhoneNumbers { get; set; } } IEnumerable<Person> people = new List<Person>(); // Select gets a list of lists of phone numbers IEnumerable<IEnumerable<PhoneNumber>> phoneLists = people.Select(p => p.PhoneNumbers); // SelectMany flattens it to just a list of phone numbers. IEnumerable<PhoneNumber> phoneNumbers = people.SelectMany(p => p.PhoneNumbers);

But i can not actually understnad exaclty what is the difference between (Select gets a list of lists of phone numbers , while, SelectMany flattens it to just a list of phone numbers.).

Seond question , what is the difference between writing:-

people.Select(p => p.PhoneNumbers);

&

people.Include(p => p.PhoneNumbers);

MarcinJuraszek · Accepted Answer · 2013-07-09 09:17:23Z

14

Select gets a list of lists of phone numbers , while, SelectMany flattens it to just a list of phone numbers.

That's exactly the difference. SelectMany takes a collection of collections and return all items as one collection.

people.Select(p => p.PhoneNumbers);

Select only the phone numbers.

people.Include(p => p.PhoneNumbers);

Select people entities with phone numbers loaded.

answered Jul 9, 2013 at 9:17

MarcinJuraszek

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3 Comments

John John Over a year ago

so what is ment by this "That's exactly the difference. SelectMany takes a collection of collections and return all items as one collection"

MarcinJuraszek Over a year ago

Items are the same, but they are flatten into one list. So you don't have List<List<A>> but List<A> made by concatenating items of all inner lists of source.

John John Over a year ago

and what si the idea of having these two different type of collections ?

Jonas W · Accepted Answer · 2013-07-09 09:19:06Z

You have already answered your first question. =)

The : people.Include(x=>x.PhoneNumbers)

Will load the phonenumbers in the peoples. If you are not doing this, the people.PhoneNumber will be null and not loaded.

So this statement will not execute since PhoneNumbers is null and not loaded :

var phonenumbers = db.peoples.First().PhoneNumbers; //phonenumbers = null

But if you do like this :

var phonenumbers = db.peoples.Include(x=>x.PhoneNumbers).First().PhoneNumbers; //phonenumbers will now be a IEnumerable with Phonenumbers

but i can not understnad what is the difference between the two "select gets a list of lists of phone numbers , while, SelectMany flattens it to just a list of phone numbers."
That's the point.. if you just want the phonenumbers and uses select.. You will recieve a IE<IE<PhoneNumber>>, with selectmany you get a IE<PhoneNumber> witch in some cases can be very handy.

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