How can I add an item to a IEnumerable T collection

Question

My question as title above  For example    IEnumerable lt T gt  items   new T   new T  msg     items ToList   Add new T  msg2       but after all it only has 1 item inside   Can we have a method like items Add item    like the List lt T gt

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I just come here to say that  aside from Enumerable Concat extension method  there seems to be another method named Enumerable Append in  NET Core 1 1 1  The latter allows you to concatenate a single item to an existing sequence  So Aamol s answer can also be written as  IEnumerable lt T gt  items   new T   new T  msg     items   items Append new T  msg2       Still  please note that this function will not change the input sequence  it just return a wrapper that put the given sequence and the appended item together

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Sorry for reviving really old question but as it is listed among first google search results I assume that some people keep landing here  Among a lot of answers  some of them really valuable and well explained  I would like to add a different point of vue as  to me  the problem has not be well identified  You are declaring a variable which stores data  you need it to be able to change by adding items to it   So you shouldn t use declare it as IEnumerable  As proposed by  NightOwl888  For this example  just declare IList instead of IEnumerable  IList items   new T   new T  quot msg quot     items Add new T  quot msg2 quot      Trying to bypass the declared interface limitations only shows that you made the wrong choice  Beyond this  all methods that are proposed to implement things that already exists in other implementations should be deconsidered  Classes and interfaces that let you add items already exists  Why always recreate things that are already done elsewhere   This kind of consideration is a goal of abstracting variables capabilities within interfaces  TL DR   IMO these are cleanest ways to do what you need      1st choice   Changing declaration IList lt T gt  variable   new T        variable Add new T         2nd choice   Changing instantiation  letting the framework taking care of declaration var variable   new List lt T gt       variable Add new T      When you ll need to use variable as an IEnumerable  you ll be able to  When you ll need to use it as an array  you ll be able to call  ToArray     it really always should be that simple  No extension method needed  casts only when really needed  ability to use LinQ on your variable  etc     Stop doing weird and or complex things because you only made a mistake when declaring instantiating

User · Answer

Sure  you can  I am leaving your T-business aside    public IEnumerable lt string gt  tryAdd IEnumerable lt string gt  items        List lt string gt  list   items ToList        string obj           list Add obj        return list Select i   gt  i

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Not only can you not add items like you state  but if you add an item to a List lt T gt   or pretty much any other non-read only collection  that you have an existing enumerator for  the enumerator is invalidated  throws InvalidOperationException from then on    If you are aggregating results from some type of data query  you can use the Concat extension method   Edit  I originally used the Union extension in the example  which is not really correct  My application uses it extensively to make sure overlapping queries don t duplicate results   IEnumerable lt T gt  itemsA        IEnumerable lt T gt  itemsB        IEnumerable lt T gt  itemsC        return itemsA Concat itemsB  Concat itemsC

User · Answer

No  the IEnumerable doesn t support adding items to it  Your  alternative  is  var myList   new List items   myList Add otherItem

User · Answer

You cannot  because IEnumerable lt T gt  does not necessarily represent a collection to which items can be added  In fact  it does not necessarily represent a collection at all  For example   IEnumerable lt string gt  ReadLines          string s       do                  s   Console ReadLine              yield return s         while   string IsNullOrEmpty s       IEnumerable lt string gt  lines   ReadLines    lines Add  foo      so what is this supposed to do     What you can do  however  is create a new IEnumerable object  of unspecified type   which  when enumerated  will provide all items of the old one  plus some of your own  You use Enumerable Concat for that    items   items Concat new      foo        This will not change the array object  you cannot insert items into to arrays  anyway   But it will create a new object that will list all items in the array  and then  Foo   Furthermore  that new object will keep track of changes in the array  i e  whenever you enumerate it  you ll see the current values of items

User · Answer

In  net Core  there is a method Enumerable Append that does exactly that   The source code of the method is available on GitHub      The implementation  more sophisticated than the suggestions in other answers  is worth a look

User · Answer

Have you considered using ICollection lt T gt  or IList lt T gt  interfaces instead  they exist for the very reason that you want to have an Add method on an IEnumerable lt T gt    IEnumerable lt T gt  is used to  mark  a type as being   well  enumerable or just a sequence of items without necessarily making any guarantees of whether the real underlying object supports adding removing of items  Also remember that these interfaces implement IEnumerable lt T gt  so you get all the extensions methods that you get with IEnumerable lt T gt  as well

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Easyest way to do that is simply  IEnumerable lt T gt  items   new T   new T  msg     List lt string gt  itemsList   new List lt string gt     itemsList AddRange items Select y   gt  y ToString      itemsList Add  msg2      Then you can return list as IEnumerable also because it implements IEnumerable interface

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Others have already given great explanations regarding why you can not  and should not   be able to add items to an IEnumerable  I will only add that if you are looking to continue coding to an interface that represents a collection and want an add method  you should code to ICollection or IList  As an added bonanza  these interfaces implement IEnumerable

User · Answer

The type IEnumerable lt T gt  does not support such operations   The purpose of the IEnumerable lt T gt  interface is to allow a consumer to view the contents of a collection   Not to modify the values     When you do operations like  ToList   Add   you are creating a new List lt T gt  and adding a value to that list   It has no connection to the original list   What you can do is use the Add extension method to create a new IEnumerable lt T gt  with the added value   items   items Add  msg2      Even in this case it won t modify the original IEnumerable lt T gt  object   This can be verified by holding a reference to it   For example  var items   new string    foo    var temp   items  items   items Add  bar      After this set of operations the variable temp will still only reference an enumerable with a single element  foo  in the set of values while items will reference a different enumerable with values  foo  and  bar      EDIT  I contstantly forget that Add is not a typical extension method on IEnumerable lt T gt  because it s one of the first ones that I end up defining   Here it is  public static IEnumerable lt T gt  Add lt T gt  this IEnumerable lt T gt  e  T value      foreach   var cur in e        yield return cur        yield return value

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A couple short  sweet extension methods on IEnumerable and IEnumerable lt T gt  do it for me   public static IEnumerable Append this IEnumerable first  params object   second        return first OfType lt object gt    Concat second     public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  first  params T   second        return first Concat second        public static IEnumerable Prepend this IEnumerable first  params object   second        return second Concat first OfType lt object gt        public static IEnumerable lt T gt  Prepend lt T gt  this IEnumerable lt T gt  first  params T   second        return second Concat first       Elegant  well  except for the non-generic versions    Too bad these methods are not in the BCL

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The code below solved my problem  material MaterialCasualties   material MaterialCasualties Append materialCasualties  ToList     OR customCustomer CustomCustomerPayments   customCustomer CustomCustomerPayments Concat customCustomerPayment  ToList

User · Answer

you can do this     Create IEnumerable     IEnumerable lt T gt  items   new T   new T  msg        Convert to list  List lt T gt  list   items ToList       Add new item to list  list add new T  msg2        Cast list to IEnumerable items    IEnumerable lt T gt  items

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To add second message you need to -   IEnumerable lt T gt  items   new T   new T  msg     items   items Concat new    new T  msg2

[c#] How can I add an item to a IEnumerable<T> collection?

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