Cannot apply indexing with to an expression of type System Collections Generic IEnumerable

Question

Is there any specific reason why indexing is not allowed in IEnumerable   I found a workaround for the problem I had  but just curious to know why it does not allow indexing   Thanks

User · Answer

You can use ToList to convert to a list  For example   SomeItems ToList   1

User · Answer

you can use indexing if your enumerable type is string like below    string   MyEnumerableStringList  0

User · Answer

One reason can be that the IEnumerable may contain an unknown number of items  Some implementations produce the list of items as you iterate over it  see yield for samples   That does not work very well with accessing items using an index  which would require you to know that there are at least that many items in the list

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I had a column that did not allow nulls and I was inserting a null value

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The idea of interfaces is generally to expose a sort of base line contract by which code that performs work on an object can be guaranteed of certain functionality provided by that object  In the case of IEnumerable lt T gt   that contract happens to be  you can access all of my elements one by one    The kinds of methods that can be written based on this contract alone are many  See the Enumerable class for tons of examples   But to zero in on just one concrete one  think about Sum  In order to sum up a bunch of items  what do you need  What contract would you require  The answer is quite simple  just a way to see every item  no more  Random access isn t necessary  Even a total count of all the items is not necessary   To have added an indexer to the IEnumerable lt T gt  interface would have been detrimental in two ways    Code that requires the contract described above  access to a sequence of elements   if it required the IEnumerable lt T gt  interface  would be artificially restrictive as it could not deal with any type that did not implement an indexer  even though to deal with such a type should really be well within the capabilities of the code  Any type that wanted to expose a sequence of elements but was not appropriately equipped to provide random access by index  e g   LinkedList lt T gt   Dictionary lt TKey  TValue gt   would now have to either provide some inefficient means of simulating indexing  or else abandon the IEnumerable lt T gt  interface    All this being said  considering that the purpose of an interface is to provide a guarantee of the minimum required functionality in a given scenario  I really think that the IList lt T gt  interface is poorly designed  Or rather  the lack of an interface  between  IEnumerable lt T gt  and IList lt T gt   random access  but no modification  is an unfortunate oversight in the BCL  in my opinion

User · Answer

Because it s not   Indexing is covered by IList   IEnumerable means  I have some of the powers of IList  but not all of them    Some collections  like a linked list   cannot be indexed in a practical way   But they can be accessed item-by-item   IEnumerable is intended for collections like that    Note that a collection can implement both IList  amp  IEnumerable  and many others    You generally only find IEnumerable as a function parameter  meaning the function can accept any kind of collection  because all it needs is the simplest access mode

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The   -operator is resolved to the access property this sometype index   with implementation depending upon the Element-Collection   An Enumerable-Interface declares a blueprint of what a Collection should look like in the first place   Take this example to demonstrate the usefulness of clean Interface separation   var ienu    13 37  Split      Select int Parse     provides an WhereSelectArrayIterator var inta    13 37  Split      Select int Parse  ToArray   0      gt 13   inta GetType    System Int32   Also look at the syntax of the   -operator        example public class SomeCollection  public SomeCollection       private bool   bools     public bool this int index         get           if   index  lt  0    index  gt   bools Length                     Out of range index Exception                   return bools index               set           bools index    value

User · Answer

The IEnumerable lt T gt  interface does not include an indexer  you re probably confusing it with IList lt T gt   If the object really is an IList lt T gt   e g  List lt T gt  or an array T     try making the reference to it of type IList lt T gt  too   Otherwise  you can use myEnumerable ElementAt index   which uses the Enumerable ElementAt extension method  This should work for all IEnumerable lt T gt s   Note that unless the  run-time  object implements IList lt T gt   this will cause all of the first index   1 items to be enumerated  with all but the last being discarded    EDIT  As an explanation  IEnumerable lt T gt  is simply an interface that represents  that which exposes an enumerator   A concrete implementation may well be some sort of in-memory list that does allow fast-access by index  or it may not  For instance  it could be a collection that cannot efficiently satisfy such a query  such as a linked-list  as mentioned by James Curran   It may even be no sort of in-memory data-structure at all  such as an iterator  where items are generated   yielded   on demand  or by an enumerator that fetches the items from some remote data-source  Because IEnumerable lt T gt  must support all these cases  indexers are excluded from its definition

[c#] Cannot apply indexing with [] to an expression of type 'System.Collections.Generic.IEnumerable<>

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