IEnumerable vs List - What to Use How do they work

Question

I have some doubts over how Enumerators work  and LINQ  Consider these two simple selects   List lt Animal gt  sel    from animal in Animals                      join race in Species                     on animal SpeciesKey equals race SpeciesKey                     select animal  Distinct   ToList      or  IEnumerable lt Animal gt  sel    from animal in Animals                             join race in Species                            on animal SpeciesKey equals race SpeciesKey                            select animal  Distinct      I changed the names of my original objects so that this looks like a more generic example  The query itself is not that important  What I want to ask is this   foreach  Animal animal in sel      do stuff        I noticed that if I use IEnumerable  when I debug and inspect  sel   which in that case is the IEnumerable  it has some interesting members   inner    outer    innerKeySelector  and  outerKeySelector   these last 2 appear to be delegates  The  inner  member does not have  Animal  instances in it  but rather  Species  instances  which was very strange for me  The  outer  member does contain  Animal  instances  I presume that the two delegates determine which goes in and what goes out of it  I noticed that if I use  Distinct   the  inner  contains 6 items  this is incorrect as only 2 are Distinct   but the  outer  does contain the correct values  Again  probably the delegated methods determine this but this is a bit more than I know about IEnumerable  Most importantly  which of the two options is the best performance-wise    The evil List conversion via  ToList     Or maybe using the enumerator directly   If you can  please also explain a bit or throw some links that explain this use of IEnumerable

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Nobody mentioned one crucial difference  ironically answered on a question closed as a duplicated of this       IEnumerable is read-only and List is not    See Practical difference between List and IEnumerable

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There is a very good article written by  Claudio Bernasconi s TechBlog here  When to use IEnumerable  ICollection  IList and List  Here some basics points about scenarios and functions

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The advantage of IEnumerable is deferred execution  usually with databases   The query will not get executed until you actually loop through the data  It s a query waiting until it s needed  aka lazy loading    If you call ToList  the query will be executed  or  materialized  as I like to say   There are pros and cons to both  If you call ToList  you may remove some mystery as to when the query gets executed  If you stick to IEnumerable  you get the advantage that the program doesn t do any work until it s actually required

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IEnumerable describes behavior  while List is an implementation of that behavior  When you use IEnumerable  you give the compiler a chance to defer work until later  possibly optimizing along the way  If you use ToList   you force the compiler to reify the results right away   Whenever I m  stacking  LINQ expressions  I use IEnumerable  because by only specifying the behavior I give LINQ a chance to defer evaluation and possibly optimize the program  Remember how LINQ doesn t generate the SQL to query the database until you enumerate it  Consider this   public IEnumerable lt Animals gt  AllSpotted         return from a in Zoo Animals            where a coat HasSpots    true            select a     public IEnumerable lt Animals gt  Feline IEnumerable lt Animals gt  sample        return from a in sample            where a race Family     Felidae             select a     public IEnumerable lt Animals gt  Canine IEnumerable lt Animals gt  sample        return from a in sample            where a race Family     Canidae             select a      Now you have a method that selects an initial sample   AllSpotted    plus some filters  So now you can do this   var Leopards   Feline AllSpotted     var Hyenas   Canine AllSpotted       So is it faster to use List over IEnumerable  Only if you want to prevent a query from being executed more than once  But is it better overall  Well in the above  Leopards and Hyenas get converted into single SQL queries each  and the database only returns the rows that are relevant  But if we had returned a List from AllSpotted    then it may run slower because the database could return far more data than is actually needed  and we waste cycles doing the filtering in the client   In a program  it may be better to defer converting your query to a list until the very end  so if I m going to enumerate through Leopards and Hyenas more than once  I d do this   List lt Animals gt  Leopards   Feline AllSpotted    ToList    List lt Animals gt  Hyenas   Canine AllSpotted    ToList

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In addition to all the answers posted above  here is my two cents  There are many other types other than List that implements IEnumerable such ICollection  ArrayList etc  So if we have IEnumerable as parameter of any method  we can pass any collection types to the function  Ie we can have method to operate on abstraction not any specific implementation

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I will share one misused concept that I fell into one day   var names   new List lt string gt    mercedes    mazda    bmw    fiat    ferrari     var startingWith M   names Where x   gt  x StartsWith  m      var startingWith F   names Where x   gt  x StartsWith  f          updating existing list names 0     ford       Guess what should be printed before continuing print  startingWith M ToList      print  startingWith F ToList        Expected result     I was expecting     print  startingWith M ToList         mercedes  mazda print  startingWith F ToList         fiat  ferrari   Actual result     what printed actualy    print  startingWith M ToList         mazda print  startingWith F ToList         ford  fiat  ferrari   Explanation  As per other answers  the evaluation of the result was deferred until calling ToList or similar invocation methods for example ToArray   So I can rewrite the code in this case as   var names   new List lt string gt    mercedes    mazda    bmw    fiat    ferrari        updating existing list names 0     ford       before calling ToList directly var startingWith M   names Where x   gt  x StartsWith  m      var startingWith F   names Where x   gt  x StartsWith  f      print  startingWith M ToList      print  startingWith F ToList        Play arround  https   repl it E8Ki 0

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If all you want to do is enumerate them  use the IEnumerable   Beware  though  that changing the original collection being enumerated is a dangerous operation - in this case  you will want to ToList first  This will create a new list element for each element in memory  enumerating the IEnumerable and is thus less performant if you only enumerate once - but safer and sometimes the List methods are handy  for instance in random access

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A class that implement IEnumerable allows you to use the foreach syntax   Basically it has a method to get the next item in the collection  It doesn t need the whole collection to be in memory and doesn t know how many items are in it  foreach just keeps getting the next item until it runs out   This can be very useful in certain circumstances  for instance in a massive database table you don t want to copy the entire thing into memory before you start processing the rows   Now List implements IEnumerable  but represents the entire collection in memory  If you have an IEnumerable and you call  ToList   you create a new list with the contents of the enumeration in memory   Your linq expression returns an enumeration  and by default the expression executes when you iterate through using the foreach  An IEnumerable linq statement executes when you iterate the foreach  but you can force it to iterate sooner using  ToList     Here s what I mean   var things        from item in BigDatabaseCall       where          select item      this will iterate through the entire linq statement  int count   things Count        this will stop after iterating the first one  but will execute the linq again bool hasAnyRecs   things Any        this will execute the linq statement  again  foreach  var thing in things           this will copy the results to a list in memory var list   things ToList       this won t iterate through again  the list knows how many items are in it int count2   list Count        this won t execute the linq statement - we have it copied to the list foreach  var thing in list

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There are many cases  such as an infinite list or a very large list  where IEnumerable cannot be transformed to a List  The most obvious examples are all the prime numbers  all the users of facebook with their details  or all the items on ebay    The difference is that  List  objects are stored  right here and right now   whereas  IEnumerable  objects work  just one at a time   So if I am going through all the items on ebay  one at a time would be something even a small computer can handle  but   ToList    would surely run me out of memory  no matter how big my computer was  No computer can by itself contain and handle such a huge amount of data    Edit  - Needless to say - it s not  either this or that   often it would make good sense to use both a list and an IEnumerable in the same class  No computer in the world could list all prime numbers  because by definition this would require an infinite amount of memory  But you could easily think of a class PrimeContainer which contains an  IEnumerable lt long gt  primes  which for obvious reasons also contains a SortedList lt long gt   primes  all the primes calculated so far  the next prime to be checked would only be run against the existing primes  up to the square root   That way you gain both - primes one at a time  IEnumerable  and a good list of  primes so far   which is a pretty good approximation of the entire  infinite  list

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The most important thing to realize is that  using Linq  the query does not get evaluated immediately  It is only run as part of iterating through the resulting IEnumerable lt T gt  in a foreach - that s what all the weird delegates are doing   So  the first example evaluates the query immediately by calling ToList and putting the query results in a list  The second example returns an IEnumerable lt T gt  that contains all the information needed to run the query later on   In terms of performance  the answer is it depends  If you need the results to be evaluated at once  say  you re mutating the structures you re querying later on  or if you don t want the iteration over the IEnumerable lt T gt  to take a long time  use a list  Else use an IEnumerable lt T gt   The default should be to use the on-demand evaluation in the second example  as that generally uses less memory  unless there is a specific reason to store the results in a list

[c#] IEnumerable vs List - What to Use? How do they work?

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