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Difference Between Select and SelectMany

I've been searching the difference between Select and SelectMany but I haven't been able to find a suitable answer. I need to learn the difference when using LINQ To SQL but all I've found are standard array examples.

Can someone provide a LINQ To SQL example?

you can look at the code for SelectMany with one function, or with two functions referencesource.microsoft.com/#System.Core/System/Linq/…
If you familiar with Kotlin it has quite similar implementations for collections as map aka C# Select and flatMap aka C# SelectMany. Basically Kotlin std library extension functions for collections has similarity to C# Linq library.
Would one agree that SelectFromMany would be a much more descriptive name than SelectMany?

G
Graham

SelectMany flattens queries that return lists of lists. For example

public class PhoneNumber
{
    public string Number { get; set; }
}

public class Person
{
    public IEnumerable<PhoneNumber> PhoneNumbers { get; set; }
    public string Name { 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);

// And to include data from the parent in the result: 
// pass an expression to the second parameter (resultSelector) in the overload:
var directory = people
   .SelectMany(p => p.PhoneNumbers,
               (parent, child) => new { parent.Name, child.Number });

Live Demo on .NET Fiddle


Related question on nesting SelectMany to flatten out a nested hierarchical structure.
To understand resultSelector more The below link helps blogs.interknowlogy.com/2008/10/10/…
One more demo with results from parent: dotnetfiddle.net/flcdCC
U
Uwe Keim

Select many is like cross join operation in SQL where it takes the cross product.
For example if we have

Set A={a,b,c}
Set B={x,y}

Select many can be used to get the following set

{ (x,a) , (x,b) , (x,c) , (y,a) , (y,b) , (y,c) }

Note that here we take the all the possible combinations that can be made from the elements of set A and set B.

Here is a LINQ example you can try

List<string> animals = new List<string>() { "cat", "dog", "donkey" };
List<int> number = new List<int>() { 10, 20 };

var mix = number.SelectMany(num => animals, (n, a) => new { n, a });

the mix will have following elements in flat structure like

{(10,cat), (10,dog), (10,donkey), (20,cat), (20,dog), (20,donkey)}

I know this is old, but I wanted to thank you for this, it saved me a lot! :) It can be useful to have a reference to those codes too: stackoverflow.com/questions/3479980/… Cheers!
SelectMany doesn't have to be used like that. It has an option to just take one function too.
I dunno if it's right to say that this is how SelectMany is. Rather, this is a way that SelectMany can be used, but is not actually the normal way of using it.
This was the simplest answer for me to understand.
@ChaimEliyah it may well be the answer that you understood most easily, but if this were the only answer you read then I wouldn't expect that you actually understood what SelectMany() is doing; this doesn't at all represent what the function does in the general case - it just provides a particular (unusual) thing that it is capable of achieving.
L
Lews Therin

https://i.stack.imgur.com/XWpx2.png

var players = db.SoccerTeams.Where(c => c.Country == "Spain")
                            .SelectMany(c => c.players);

foreach(var player in players)
{
    Console.WriteLine(player.LastName);
}

De Gea Alba Costa Villa Busquets

...


great example data
could you add an example for select to complete this answer :)
@Harry: from a previous sample, but developed a bit: dotnetfiddle.net/Ku6kLR
dude forgot all the legends... Xavi, Iniesta, Puyol :(
S
Sameer Alibhai

SelectMany() lets you collapse a multidimensional sequence in a way that would otherwise require a second Select() or loop.

More details at this blog post.


But the first one return Enumerables type of Children the second example return type of Parents ? Actually I am little bit confused,would you open it up little bit more ?
Other way around, actually. The second will completely flatten the hierarchy of enumerables, so that you get Children back. Try the article at the link I added, see if that helps.
The first one does not appear to be legal. I think the poster got confused himself. The second one would return an enumerable of parents.
Thanks, well actually yeah the examples were kinda confusing tho :) but thanks again for trying to help me.
8
8 revs, 4 users 72%

There are several overloads to SelectMany. One of them allows you to keep trace of any relationship between parent and children while traversing the hierarchy.

Example: suppose you have the following structure: League -> Teams -> Player.

You can easily return a flat collection of players. However you may lose any reference to the team the player is part of.

Fortunately there is an overload for such purpose:

var teamsAndTheirLeagues = 
         from helper in leagues.SelectMany
               ( l => l.Teams
                 , ( league, team ) => new { league, team } )
                      where helper.team.Players.Count > 2 
                           && helper.league.Teams.Count < 10
                           select new 
                                  { LeagueID = helper.league.ID
                                    , Team = helper.team 
                                   };

The previous example is taken from Dan's IK blog. I strongly recommend you take a look at it.


S
Selim Yildiz

I understand SelectMany to work like a join shortcut.

So you can:

var orders = customers
             .Where(c => c.CustomerName == "Acme")
             .SelectMany(c => c.Orders);

The provided example works, but SelectMany does not exactly work like a join. A join allows to "use" any field of the original table plus any field of the joined table. But here you have to specify an object of a list attached to the original table. For example, .SelectMany(c => new {c.CompanyName, c.Orders.ShippedDate}); would not work. SelectMany is rather flattening the list of lists - and you can pick any (but only one at a time) of the contained lists for the result. For comparison: Inner join in Linq.
R
Reza Jenabi

The SelectMany() method is used to flatten a sequence in which each of the elements of the sequence is a separate.

I have class user same like this

class User
    {
        public string UserName { get; set; }
        public List<string> Roles { get; set; }
    }

main:

var users = new List<User>
            {
                new User { UserName = "Reza" , Roles = new List<string>{"Superadmin" } },
                new User { UserName = "Amin" , Roles = new List<string>{"Guest","Reseption" } },
                new User { UserName = "Nima" , Roles = new List<string>{"Nurse","Guest" } },
            };

var query = users.SelectMany(user => user.Roles, (user, role) => new { user.UserName, role });

foreach (var obj in query)
{
    Console.WriteLine(obj);
}
//output

//{ UserName = Reza, role = Superadmin }
//{ UserName = Amin, role = Guest }
//{ UserName = Amin, role = Reseption }
//{ UserName = Nima, role = Nurse }
//{ UserName = Nima, role = Guest }

You can use operations on any item of sequence

int[][] numbers = {
                new[] {1, 2, 3},
                new[] {4},
                new[] {5, 6 , 6 , 2 , 7, 8},
                new[] {12, 14}
            };

IEnumerable<int> result = numbers
                .SelectMany(array => array.Distinct())
                .OrderBy(x => x);

//output

//{ 1, 2 , 2 , 3, 4, 5, 6, 7, 8, 12, 14 }
 List<List<int>> numbers = new List<List<int>> {
                new List<int> {1, 2, 3},
                new List<int> {12},
                new List<int> {5, 6, 5, 7},
                new List<int> {10, 10, 10, 12}
            };

 IEnumerable<int> result = numbers
                .SelectMany(list => list)
                .Distinct()
                .OrderBy(x=>x);

//output

// { 1, 2, 3, 5, 6, 7, 10, 12 }

What if one or more of the lists happen to be null, can you still aggregate the others? I'm getting an error because of having a couple of null results.
A
Alexandr

Select is a simple one-to-one projection from source element to a result element. Select- Many is used when there are multiple from clauses in a query expression: each element in the original sequence is used to generate a new sequence.


R
Rm558

Some SelectMany may not be necessary. Below 2 queries give the same result.

Customers.Where(c=>c.Name=="Tom").SelectMany(c=>c.Orders)

Orders.Where(o=>o.Customer.Name=="Tom")

For 1-to-Many relationship,

if Start from "1", SelectMany is needed, it flattens the many. if Start from "Many", SelectMany is not needed. (still be able to filter from "1", also this is simpler than below standard join query)

from o in Orders
join c in Customers on o.CustomerID equals c.ID
where c.Name == "Tom"
select o

n
nzrytmn

The formal description for SelectMany() is:

Projects each element of a sequence to an IEnumerable and flattens the resulting sequences into one sequence.

SelectMany() flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein.

class PetOwner
{
    public string Name { get; set; }
    public List<String> Pets { get; set; }
}

public static void SelectManyEx()
{
     PetOwner[] petOwners =
         { new PetOwner { Name="Higa, Sidney",
              Pets = new List<string>{ "Scruffy", "Sam" } },
           new PetOwner { Name="Ashkenazi, Ronen",
              Pets = new List<string>{ "Walker", "Sugar" } },
           new PetOwner { Name="Price, Vernette",
              Pets = new List<string>{ "Scratches", "Diesel" } } };

// Query using SelectMany().
IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

Console.WriteLine("Using SelectMany():");

// Only one foreach loop is required to iterate
// through the results since it is a
// one-dimensional collection.
foreach (string pet in query1)
{
    Console.WriteLine(pet);
}

// This code shows how to use Select()
// instead of SelectMany().
IEnumerable<List<String>> query2 =
    petOwners.Select(petOwner => petOwner.Pets);

Console.WriteLine("\nUsing Select():");

// Notice that two foreach loops are required to
// iterate through the results
// because the query returns a collection of arrays.
foreach (List<String> petList in query2)
{
    foreach (string pet in petList)
    {
        Console.WriteLine(pet);
    }
    Console.WriteLine();
}
}

/*
   This code produces the following output:

    Using SelectMany():
    Scruffy
    Sam
    Walker
    Sugar
    Scratches
    Diesel

   Using Select():
   Scruffy
   Sam

   Walker
   Sugar

   Scratches
   Diesel
  */

The main difference is the result of each method while SelectMany() returns a flattern results; the Select() returns a list of list instead of a flattern result set.

Therefor the result of SelectMany is a list like

{Scruffy, Sam , Walker, Sugar, Scratches , Diesel}

which you can iterate each item by just one foreach. But with the result of select you need an extra foreach loop to iterate through the results because the query returns a collection of arrays.


R
RickL

Without getting too technical - database with many Organizations, each with many Users:-

var orgId = "123456789";

var userList1 = db.Organizations
                   .Where(a => a.OrganizationId == orgId)
                   .SelectMany(a => a.Users)
                   .ToList();

var userList2 = db.Users
                   .Where(a => a.OrganizationId == orgId)
                   .ToList();

both return the same ApplicationUser list for the selected Organization.

The first "projects" from Organization to Users, the second queries the Users table directly.


M
Matt Klein

Just for an alternate view that may help some functional programmers out there:

Select is map

SelectMany is bind (or flatMap for your Scala/Kotlin people)


E
Eric Bole-Feysot

It's more clear when the query return a string (an array of char):

For example if the list 'Fruits' contains 'apple'

'Select' returns the string:

Fruits.Select(s=>s) 

[0]: "apple"

'SelectMany' flattens the string:

Fruits.SelectMany(s=>s)

[0]: 97  'a'
[1]: 112 'p'
[2]: 112 'p'
[3]: 108 'l'
[4]: 101 'e'

M
Mohammad Galouz Lee

Consider this example :

        var array = new string[2]
        {
            "I like what I like",
            "I like what you like"
        };
        //query1 returns two elements sth like this:
        //fisrt element would be array[5]  :[0] = "I" "like" "what" "I" "like"
        //second element would be array[5] :[1] = "I" "like" "what" "you" "like"
        IEnumerable<string[]> query1 = array.Select(s => s.Split(' ')).Distinct();

        //query2 return back flat result sth like this :
        // "I" "like" "what" "you"
        IEnumerable<string> query2 = array.SelectMany(s => s.Split(' ')).Distinct();

So as you see duplicate values like "I" or "like" have been removed from query2 because "SelectMany" flattens and projects across multiple sequences. But query1 returns sequence of string arrays. and since there are two different arrays in query1 (first and second element), nothing would be removed.


probably better to now include .Distinct() at end and state it outputs "I" "like" "what" "I" "like" "I" "like" "what" "you" "like"
s
snr

The SelectMany method knocks down an IEnumerable<IEnumerable<T>> into an IEnumerable<T>, like communism, every element is behaved in the same manner(a stupid guy has same rights of a genious one).

var words = new [] { "a,b,c", "d,e", "f" };
var splitAndCombine = words.SelectMany(x => x.Split(','));
// returns { "a", "b", "c", "d", "e", "f" }

K
KEMBL

One more example how SelectMany + Select can be used in order to accumulate sub array objects data.

Suppose we have users with they phones:

class Phone { 
    public string BasePart = "555-xxx-xxx"; 
}

class User { 
    public string Name = "Xxxxx";
    public List<Phone> Phones; 
}

Now we need to select all phones' BaseParts of all users:

var usersArray = new List<User>(); // array of arrays
List<string> allBaseParts = usersArray.SelectMany(ua => ua.Phones).Select(p => p.BasePart).ToList();

Which do you think is better? Yours or usersArray.SelectMany(ua => ua.Phones.Select(p => p.BasePart))
S
Sharunas Bielskis

Here is a code example with an initialized small collection for testing:

class Program
{
    static void Main(string[] args)
    {
        List<Order> orders = new List<Order>
        {
            new Order
            {
                OrderID = "orderID1",
                OrderLines = new List<OrderLine>
                {
                    new OrderLine
                    {
                        ProductSKU = "SKU1",
                        Quantity = 1
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU2",
                        Quantity = 2
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU3",
                        Quantity = 3
                    }
                }
            },
            new Order
            {
                OrderID = "orderID2",
                OrderLines = new List<OrderLine>
                {
                    new OrderLine
                    {
                        ProductSKU = "SKU4",
                        Quantity = 4
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU5",
                        Quantity = 5
                    }
                }
            }
        };

        //required result is the list of all SKUs in orders
        List<string> allSKUs = new List<string>();

        //With Select case 2 foreach loops are required
        var flattenedOrdersLinesSelectCase = orders.Select(o => o.OrderLines);
        foreach (var flattenedOrderLine in flattenedOrdersLinesSelectCase)
        {
            foreach (OrderLine orderLine in flattenedOrderLine)
            {
                allSKUs.Add(orderLine.ProductSKU);
            }
        }

        //With SelectMany case only one foreach loop is required
        allSKUs = new List<string>();
        var flattenedOrdersLinesSelectManyCase = orders.SelectMany(o => o.OrderLines);
        foreach (var flattenedOrderLine in flattenedOrdersLinesSelectManyCase)
        {
            allSKUs.Add(flattenedOrderLine.ProductSKU);
        }

       //If the required result is flattened list which has OrderID, ProductSKU and Quantity,
       //SelectMany with selector is very helpful to get the required result
       //and allows avoiding own For loops what according to my experience do code faster when
       // hundreds of thousands of data rows must be operated
        List<OrderLineForReport> ordersLinesForReport = (List<OrderLineForReport>)orders.SelectMany(o => o.OrderLines,
            (o, ol) => new OrderLineForReport
            {
                OrderID = o.OrderID,
                ProductSKU = ol.ProductSKU,
                Quantity = ol.Quantity
            }).ToList();
    }
}
class Order
{
    public string OrderID { get; set; }
    public List<OrderLine> OrderLines { get; set; }
}
class OrderLine
{
    public string ProductSKU { get; set; }
    public int Quantity { get; set; }
}
class OrderLineForReport
{
    public string OrderID { get; set; }
    public string ProductSKU { get; set; }
    public int Quantity { get; set; }
}

u
user5966157

It is the best way to understand i think.

            var query =
            Enumerable
                .Range(1, 10)
                .SelectMany(ints => Enumerable.Range(1, 10), (a, b) => $"{a} * {b} = {a * b}")
                .ToArray();

        Console.WriteLine(string.Join(Environment.NewLine, query));

        Console.Read();

Multiplication Table example.


Only if meaning of "best" has changed dramatically.
so this is the best way u think ?? then what is the difficult way of thinking ??