Can anyone explain IEnumerable and IEnumerator to me

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Can anyone explain IEnumerable and IEnumerator to me  For example  when to use it over foreach  what s the difference between IEnumerable and IEnumerator  Why do we need to use it

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Implementing IEnumerable essentially means that the object can be iterated over  This doesn t necessarily mean it is an array as there are certain lists that can t be indexed but you can enumerate them   IEnumerator is the actual object used to perform the iterations  It controls moving from one object to the next in the list   Most of the time  IEnumerable  amp  IEnumerator are used transparently as part of a foreach loop

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A Minor contribution   As many of them explain about  when to use  and  use with foreach   I thought of adding Another States Difference here as requested in question about the difference between both IEnumerable an IEnumerator   I created the below code sample based on the below discussion threads   IEnumerable   IEnumerator vs foreach  when to use what What is the difference between IEnumerator and IEnumerable   Enumerator preserves the state  iteration position  between function calls while iterations the other hand Enumerable does not    Here is the tested example with comments to understand    Experts please add correct me   static void EnumerableVsEnumeratorStateTest         IList lt int gt  numList   new List lt int gt          numList Add 1       numList Add 2       numList Add 3       numList Add 4       numList Add 5       numList Add 6        Console WriteLine  Using Enumerator - Remembers the state        IterateFrom1to3 numList GetEnumerator          Console WriteLine  Using Enumerable - Does not Remembers the state        IterateFrom1to3Eb numList        Console WriteLine  Using Enumerable - 2nd functions start from the item 1 in the collection       static void IterateFrom1to3 IEnumerator lt int gt  numColl        while  numColl MoveNext                  Console WriteLine numColl Current ToString              if  numColl Current  gt  3                           This method called 3 times for 3 items  4 5 6  in the collection                  It remembers the state and displays the continued values              IterateFrom3to6 numColl                      static void IterateFrom3to6 IEnumerator lt int gt  numColl        while  numColl MoveNext                  Console WriteLine numColl Current ToString              static void IterateFrom1to3Eb IEnumerable lt int gt  numColl        foreach  int num in numColl                Console WriteLine num ToString              if  num gt   5                           The below method invokes for the last 2 items                Since it doesnot persists the state it will displays entire collection 2 times              IterateFrom3to6Eb numColl                      static void IterateFrom3to6Eb IEnumerable lt int gt  numColl        Console WriteLine        foreach  int num in numColl                Console WriteLine num ToString

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Implementing IEnumerable means your class returns an IEnumerator object   public class People   IEnumerable       IEnumerator IEnumerable GetEnumerator                    return a PeopleEnumerator           Implementing IEnumerator means your class returns the methods and properties for iteration   public class PeopleEnumerator   IEnumerator       public void Reset           public bool MoveNext           public object Current        That s the difference anyway

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Explanation via Analogy   Code Walkthrough First an explanation without code  then I ll add it in later  Let s say you are running an airline company  And in each plane you want to know information about the passengers flying in the plane  Basically you want to be able to  quot traverse quot  the plane  In other words  you want to be able to start at the front seat  and to then work your way towards the back of the plane  asking passengers some information  who they are  where they are from etc  An aeroplane can only do this  if it is   countable  and if it has a counter   Why these requirements  Because that s what the interface requires  If this is information overload  all you need to know is that you want to be able to ask each passenger in the plane some questions  starting from the first and making your way to the last  What does countable mean  If an airline is  quot countable quot   this means that there MUST be a flight attendant present on the plane  whose sole job is to count - and this flight attendant MUST count in a very specific manner   The counter flight attendant MUST start before the first passenger  at the front of everyone where they demo safety  how to put the life jacket on etc   He she  i e  the flight attendant  MUST  quot move next quot  up the aisle to the first seat  He she is to then record   i  who the person is in the seat  and  ii  their current location in the aisle   Counting Procedures The captain of the airline wants a report on every passenger as and when they are investigated or counted  So after speaking to the person in the first seat  the flight-attendant counter then reports to the captain  and when the report is given  the counter remembers his her exact position in the aisle and continues counting right where he she left off  In this manner the captain is always able to have information on the current person being investigated  That way  if he finds out that this individual likes Manchester City  then he can give that passenger preferential treatment etc   The counter keeps going till he reaches the end of the plane   Let s tie this with the IEnumerables  An enumerable is just a collection of passengers on a plane  The Civil Aviation Law - these are basically the rules which all IEnumerables must follow  Everytime the airline attendant goes to the captain with the passeger information  we are basically  yielding  the passenger to the captain  The captain can basically do whatever he wants with the passenger - except rearranging the passengers on the plane  In this case  they are given preferential treatment if they follow Manchester City  ugh    foreach  Passenger passenger in Plane      the airline hostess is now at the front of the plane     and slowly making her way towards the back     when she get to a particular passenger she gets some information     about the passenger and then immediately heads to the cabin     to let the captain decide what to do with it        lt ---------- Note the curly bracket that is here          we are now cockpit of the plane with the captain          the captain wants to give the passenger free          champaign if they support manchester city      if  passenger supports mancestercity                    passenger getFreeChampaign           else                    you get nothing  GOOD DAY SIR                 lt ---- Note the curly bracket that is here         the hostess has delivered the information         to the captain and goes to the next person        on the plane  if she has not reached the         end of the plane     Summary In other words  something is countable if it has a counter  And counter must  basically    i  remember its place  state    ii  be able to move next   iii  and know about the current person he is dealing with  Enumerable is just a fancy word for  quot countable quot   In other words  an enumerable allows you to  enumerate   i e  count

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IEnumerable is a box that contains Ienumerator  IEnumerable is base interface for all the collections  foreach loop can operate if the collection implements IEnumerable  In the below code it explains the step of having our own Enumerator  Lets first define our Class of which we are going to make the collection   public class Customer       public String Name   get  set        public String City   get  set        public long Mobile   get  set        public double Amount   get  set        Now we will define the Class which will act as a collection for our class Customer  Notice that it is implementing the interface IEnumerable  So that we have to implement the method GetEnumerator  This will return our custom Enumerator   public class CustomerList   IEnumerable       Customer   customers   new Customer 4       public CustomerList                 customers 0    new Customer   Name    Bijay Thapa   City    LA   Mobile   9841639665  Amount   89 45            customers 1    new Customer   Name    Jack   City    NYC   Mobile   9175869002  Amount   426 00            customers 2    new Customer   Name    Anil min   City    Kathmandu   Mobile   9173694005  Amount   5896 20            customers 3    new Customer   Name    Jim sin   City    Delhi   Mobile   64214556002  Amount   596 20               public int Count                 return customers Count              public Customer this int index                get                       return customers index                       public IEnumerator GetEnumerator                 return customers GetEnumerator       we can do this but we are going to make our own Enumerator         return new CustomerEnumerator this             Now we are going to create our own custom Enumerator as follow  So  we have to implement method MoveNext    public class CustomerEnumerator   IEnumerator               CustomerList coll          Customer CurrentCustomer          int currentIndex          public CustomerEnumerator CustomerList customerList                        coll   customerList              currentIndex   -1                     public object Current   gt  CurrentCustomer           public bool MoveNext                         if   currentIndex     gt   coll Count   - 1                  return false              else                 CurrentCustomer   coll currentIndex               return true                     public void Reset                            we dont have to implement this method                    Now we can use foreach loop over our collection like below       class EnumeratorExample               static void Main String   args                         CustomerList custList   new CustomerList                foreach  Customer cust in custList                                Console WriteLine  Customer Name   cust Name     City Name     cust City     Mobile Number     cust Amount                             Console Read

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IEnumerable and IEnumerator both are interfaces in C    IEnumerable is an interface defining a single method GetEnumerator   that returns an IEnumerator interface   This works for read-only access to a collection that implements that IEnumerable can be used with a foreach statement   IEnumerator has two methods  MoveNext and Reset  It also has a property called Current   The following shows the implementation of IEnumerable and IEnumerator

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Differences between IEnumerable and IEnumerator     IEnumerable uses IEnumerator internally  IEnumerable doesn t know which item object is executing   Whenever we pass IEnumerator to another function  it knows the current position of item object  Whenever we pass an IEnumerable collection to another function  it doesn t know the current position of item object  doesn t know which item its executing   IEnumerable have one method GetEnumerator      public interface IEnumerable lt out T gt    IEnumerable       IEnumerator lt T gt  GetEnumerator         IEnumerator has one property called Current and two methods  Reset   and MoveNext    which is useful for knowing the current position of an item in a list    public interface IEnumerator        object Current   get         bool MoveNext         void Reset

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IEnumerable and IEnumerator  and their generic counterparts IEnumerable lt T  and IEnumerator lt T   are base interfaces of iterator implementations in  Net Framework Class Libray collections   IEnumerable is the most common interface you would see in the majority of the code out there  It enables the foreach loop  generators  think yield  and because of its tiny interface  it s used to create tight abstractions  IEnumerable depends on IEnumerator   IEnumerator  on the other hand  provides a slightly lower level iteration interface  It s referred to as the explicit iterator which gives the programmer more control over the iteration cycle   IEnumerable  IEnumerable is a standard interface that enables iterating over collections that supports it  in fact  all collection types I can think of today implements IEnumerable   Compiler support allows language features like foreach  In general terms  it enables this implicit iterator implementation   foreach Loop  foreach  var value in list    Console WriteLine value     I think foreach loop is one of the main reasons for using IEnumerable interfaces  foreach has a very succinct syntax and very easy to understand compared to classic C style for loops where you need to check the various variables to see what it was doing   yield Keyword  Probably a lesser known feature is that IEnumerable also enables generators in C  with the use of yield return and yield break statements   IEnumerable lt Thing gt  GetThings        if  isNotReady  yield break     while  thereIsMore       yield return GetOneMoreThing        Abstractions  Another common scenario in practice is using IEnumerable to provide minimalistic abstractions  Because it is a minuscule and read-only interface  you are encouraged to expose your collections as IEnumerable  rather than List for example   That way you are free to change your implementation without breaking your client s code  change List to a LinkedList for instance    Gotcha  One behaviour to be aware of is that in streaming implementations  e g  retrieving data row by row from a database  instead of loading all the results in memory first  you cannot iterate over the collection more than once  This is in contrast to in-memory collections like List  where you can iterate multiple times without problems  ReSharper  for example  has a code inspection for Possible multiple enumeration of IEnumerable    IEnumerator  IEnumerator  on the other hand  is the behind the scenes interface which makes IEnumerble-foreach-magic work  Strictly speaking  it enables explicit iterators   var iter   list GetEnumerator    while  iter MoveNext        Console WriteLine iter Current     In my experience IEnumerator is rarely used in common scenarios due to its more verbose syntax and slightly confusing semantics  at least to me  e g  MoveNext   returns a value as well  which the name doesn t suggest at all    Use case for IEnumerator  I only used IEnumerator in particular  slightly lower level  libraries and frameworks where I was providing IEnumerable interfaces  One example is a data stream processing library which provided series of objects in a foreach loop even though behind the scenes data was collected using various file streams and serialisations   Client code  foreach var item in feed GetItems        Console WriteLine item     Library  IEnumerable GetItems         return new FeedIterator  fileNames     class FeedIterator  IEnumerable       IEnumerator GetEnumerator             return new FeedExplicitIterator  stream            class FeedExplicitIterator  IEnumerator       DataItem  current       bool MoveNext              current   ReadMoreFromStream            return  current    null                        DataItem Current             return  current

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using System  using System Collections  using System Collections Generic  using System Linq  using System Text  using System Threading Tasks   namespace Enudemo        class Person               string name               int roll           public Person string name  int roll                        this name   name              this roll   roll                     public override string ToString                         return string Format  Name       name     t Roll       roll                          class Demo   IEnumerable               ArrayList list1   new ArrayList             public Demo                         list1 Add new Person  Shahriar   332                list1 Add new Person  Sujon   333                list1 Add new Person  Sumona   334                list1 Add new Person  Shakil   335                list1 Add new Person  Shruti   336                       IEnumerator IEnumerable GetEnumerator                        return list1 GetEnumerator                           class Program               static void Main string   args                        Demo d   new Demo        Notice here  it is simple object but for                                    IEnumerator you can get the collection data              foreach  Person X in d                                Console WriteLine X                              Console ReadKey                         Output     Name   Shahriar  Roll   332 Name   Sujon     Roll   333 Name   Sumona    Roll   334 Name   Shakil    Roll   335 Name   Shruti    Roll   336

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An understanding of the Iterator pattern will be helpful for you  I recommend reading the same   Iterator Pattern  At a high level the iterator pattern can be used to provide a standard way of iterating through collections of any type   We have 3 participants in the iterator pattern  the actual collection  client   the aggregator and the iterator  The aggregate is an interface abstract class that has a method that returns an iterator  Iterator is an interface abstract class that has methods allowing us to iterate through a collection    In order to implement the pattern we first need to implement an iterator to produce a concrete that can iterate over the concerned collection  client  Then the collection  client  implements the aggregator to return an instance of the above iterator   Here is the UML diagram   So basically in c   IEnumerable is the abstract aggregate and IEnumerator is the abstract Iterator  IEnumerable has a single method GetEnumerator that is responsible for creating an instance of IEnumerator of the desired type  Collections like Lists implement the IEnumerable    Example  Lets suppose that we have a method getPermutations inputString  that returns all the permutations of a string and that the method returns an instance of IEnumerable lt string gt   In order to count the number of permutations we could do something like the below    int count   0          var permutations   perm getPermutations inputString           foreach  string permutation in permutations                        count                The c  compiler more or less converts the above to  using  var permutationIterator   perm getPermutations input  GetEnumerator                          while  permutationIterator MoveNext                                  count                              If you have any questions please don t hesitate to ask

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IEnumerable implements GetEnumerator  When called  that method will return an IEnumerator which implements MoveNext  Reset and Current   Thus when your class implements IEnumerable  you are saying that you can call a method  GetEnumerator  and get a new object returned  an IEnumerator  you can use in a loop such as foreach

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An object implementing IEnumerable allows others to visit each of its items  by an enumerator   An object implementing IEnumerator is the doing the iteration  It s looping over an enumerable object      Think of enumerable objects as of lists  stacks  trees

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The IEnumerable and IEnumerator Interfaces  To begin examining the process of implementing existing  NET interfaces  let   s first look at the role of IEnumerable and IEnumerator  Recall that C  supports a keyword named foreach that allows you to iterate over the contents of any array type      Iterate over an array of items  int   myArrayOfInts    10  20  30  40   foreach int i in myArrayOfInts       Console WriteLine i       While it might seem that only array types can make use of this construct  the truth of the matter is any type supporting a method named GetEnumerator   can be evaluated by the foreach construct To illustrate  follow me   Suppose we have a Garage class      Garage contains a set of Car objects  public class Garage      private Car   carArray   new Car 4         Fill with some Car objects upon startup     public Garage              carArray 0    new Car  Rusty   30         carArray 1    new Car  Clunker   55         carArray 2    new Car  Zippy   30         carArray 3    new Car  Fred   30            Ideally  it would be convenient to iterate over the Garage object   s subitems using the foreach construct  just like an array of data values      This seems reasonable     public class Program      static void Main string   args             Console WriteLine        Fun with IEnumerable   IEnumerator       n          Garage carLot   new Garage             Hand over each car in the collection        foreach  Car c in carLot                   Console WriteLine   0  is going  1  MPH            c PetName  c CurrentSpeed                 Console ReadLine             Sadly  the compiler informs you that the Garage class does not implement a method named GetEnumerator    This method is formalized by the IEnumerable interface  which is found lurking within the System Collections namespace  Classes or structures that support this behavior advertise that they are able to expose contained subitems to the caller  in this example  the foreach keyword itself   Here is the definition of this standard  NET interface      This interface informs the caller    that the object s subitems can be enumerated  public interface IEnumerable      IEnumerator GetEnumerator        As you can see  the GetEnumerator   method returns a reference to yet another interface named System Collections IEnumerator  This interface provides the infrastructure to allow the caller to traverse the internal objects contained by the IEnumerable-compatible container      This interface allows the caller to    obtain a container s subitems  public interface IEnumerator      bool MoveNext        Advance the internal position of the cursor     object Current   get      Get the current item  read-only property      void Reset        Reset the cursor before the first member      If you want to update the Garage type to support these interfaces  you could take the long road and implement each method manually  While you are certainly free to provide customized versions of GetEnumerator    MoveNext    Current  and Reset    there is a simpler way  As the System Array type  as well as many other collection classes  already implements IEnumerable and IEnumerator  you can simply delegate the request to the System Array as follows   using System Collections      public class Garage   IEnumerable         System Array already implements IEnumerator     private Car   carArray   new Car 4      public Garage              carArray 0    new Car  FeeFee   200         carArray 1    new Car  Clunker   90         carArray 2    new Car  Zippy   30         carArray 3    new Car  Fred   30           public IEnumerator GetEnumerator                 Return the array object s IEnumerator        return carArray GetEnumerator             After you have updated your Garage type  you can safely use the type within the C  foreach construct  Furthermore  given that the GetEnumerator   method has been defined publicly  the object user could also interact with the IEnumerator type      Manually work with IEnumerator  IEnumerator i   carLot GetEnumerator    i MoveNext    Car myCar    Car i Current  Console WriteLine   0  is going  1  MPH   myCar PetName  myCar CurrentSpeed     However  if you prefer to hide the functionality of IEnumerable from the object level  simply make use of explicit interface implementation   IEnumerator IEnumerable GetEnumerator          Return the array object s IEnumerator    return carArray GetEnumerator        By doing so  the casual object user will not find the Garage   s GetEnumerator   method  while the foreach construct will obtain the interface in the background when necessary   Adapted from the Pro C  5 0 and the  NET 4 5 Framework

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for example  when to use it over foreach    You don t use IEnumerable  over  foreach   Implementing IEnumerable makes using foreach possible   When you write code like   foreach  Foo bar in baz               it s functionally equivalent to writing   IEnumerator bat   baz GetEnumerator    while  bat MoveNext         bar    Foo bat Current            By  functionally equivalent   I mean that s actually what the compiler turns the code into   You can t use foreach on baz in this example unless baz implements IEnumerable     IEnumerable means that baz implements the method  IEnumerator GetEnumerator     The IEnumerator object that this method returns must implement the methods  bool MoveNext     and  Object Current     The first method advances to the next object in the IEnumerable object that created the enumerator  returning false if it s done  and the second returns the current object   Anything in  Net that you can iterate over implements IEnumerable   If you re building your own class  and it doesn t already inherit from a class that implements IEnumerable  you can make your class usable in foreach statements by implementing IEnumerable  and by creating an enumerator class that its new GetEnumerator method will return

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I have noticed these differences   A  We iterate the list in different way  foreach can be used for IEnumerable and while loop for IEnumerator   B  IEnumerator can remember the current index when we pass from one method to another  it start working with current index  but IEnumerable  can t remember the index and it reset the index to beginning  More in this video https   www youtube com watch v jd3yUjGc9M0

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Implementing IEnumerable enables you to get an IEnumerator for a list   IEnumerator allows foreach style sequential access to the items in the list  using the yield keyword   Before foreach implementation  in Java 1 4  for example   the way to iterate a list was to get an enumerator from the list  then ask it for the  next  item in the list  for as long as the value returned as the next item is not null   Foreach simply does that implicitly as a  language feature  in the same way that lock   implements the Monitor class behind the scenes   I expect foreach works on lists because they implement IEnumerable

[c#] Can anyone explain IEnumerable and IEnumerator to me?

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