How to get the index of an element in an IEnumerable

Question

I wrote this   public static class EnumerableExtensions       public static int IndexOf lt T gt  this IEnumerable lt T gt  obj  T value                return obj              Select  a  i    gt   a Equals value     i   -1               Max               public static int IndexOf lt T gt  this IEnumerable lt T gt  obj  T value              IEqualityComparer lt T gt  comparer                return obj              Select  a  i    gt   comparer Equals a  value     i   -1               Max              But I don t know if it already exists  does it

User · Answer

The best way to catch the position is by FindIndex This function is available only for List<>

Example

int id = listMyObject.FindIndex(x => x.Id == 15);

If you have enumerator or array use this way

int id = myEnumerator.ToList().FindIndex(x => x.Id == 15);

or

 int id = myArray.ToList().FindIndex(x => x.Id == 15);

User · Answer

An alternative to finding the index after the fact is to wrap the Enumerable  somewhat similar to using the Linq GroupBy   method   public static class IndexedEnumerable       public static IndexedEnumerable lt T gt  ToIndexed lt T gt  this IEnumerable lt T gt  items                return IndexedEnumerable lt T gt  Create items            public class IndexedEnumerable lt T gt    IEnumerable lt IndexedEnumerable lt T gt  IndexedItem gt        private readonly IEnumerable lt IndexedItem gt   items       public IndexedEnumerable IEnumerable lt IndexedItem gt  items                 items   items             public class IndexedItem               public IndexedItem int index  T value                        Index   index              Value   value                     public T Value   get  private set            public int Index   get  private set               public static IndexedEnumerable lt T gt  Create IEnumerable lt T gt  items                return new IndexedEnumerable lt T gt  items Select  item  index    gt  new IndexedItem index  item                public IEnumerator lt IndexedItem gt  GetEnumerator                 return  items GetEnumerator               IEnumerator IEnumerable GetEnumerator                 return GetEnumerator              Which gives a use case of   var items   new    1  2  3   var indexedItems   items ToIndexed    foreach  var item in indexedItems        Console WriteLine  items  0      1    item Index  item Value

User · Answer

I would implement it like this   public static class EnumerableExtensions       public static int IndexOf lt T gt  this IEnumerable lt T gt  obj  T value                return obj IndexOf value  null              public static int IndexOf lt T gt  this IEnumerable lt T gt  obj  T value  IEqualityComparer lt T gt  comparer                comparer   comparer    EqualityComparer lt T gt  Default          var found   obj              Select  a  i    gt  new   a  i                 FirstOrDefault x   gt  comparer Equals x a  value            return found    null   -1   found i

User · Answer

A few years later  but this uses Linq  returns -1 if not found  doesn t create extra objects  and should short-circuit when found  as opposed to iterating over the entire IEnumerable    public static int IndexOf lt T gt  this IEnumerable lt T gt  list  T item        return list Select  x  index    gt  EqualityComparer lt T gt  Default Equals item  x                                         index                                        -1                  FirstOr x   gt  x    -1  -1       Where  FirstOr  is   public static T FirstOr lt T gt  this IEnumerable lt T gt  source  T alternate        return source DefaultIfEmpty alternate                    First       public static T FirstOr lt T gt  this IEnumerable lt T gt  source  Func lt T  bool gt  predicate  T alternate        return source Where predicate                    FirstOr alternate

User · Answer

A bit late in the game  i know    but this is what i recently did  It is slightly different than yours  but allows the programmer to dictate what the equality operation needs to be  predicate   Which i find very useful when dealing with different types  since i then have a generic way of doing it regardless of object type and  lt T gt  built in equality operator   It also has a very very small memory footprint  and is very  very fast efficient    if you care about that   At worse  you ll just add this to your list of extensions   Anyway    here it is    public static int IndexOf lt T gt  this IEnumerable lt T gt  source  Func lt T  bool gt  predicate          int retval   -1       var enumerator   source GetEnumerator          while  enumerator MoveNext                    retval    1           if  predicate enumerator Current                           IDisposable disposable   enumerator as System IDisposable               if  disposable    null  disposable Dispose                 return retval                         IDisposable disposable   enumerator as System IDisposable       if  disposable    null  disposable Dispose         return -1       Hopefully this helps someone

User · Answer

This can get really cool with an extension  functioning as a proxy   for example   collection SelectWithIndex        vs   collection Select  item  index    gt  item     Which will automagically assign indexes to the collection accessible via this Index property   Interface   public interface IIndexable       int Index   get  set        Custom extension  probably most useful for working with EF and DbContext    public static class EnumerableXtensions       public static IEnumerable lt TModel gt  SelectWithIndex lt TModel gt           this IEnumerable lt TModel gt  collection  where TModel   class  IIndexable               return collection Select  item  index    gt                        item Index   index              return item                       public class SomeModelDTO   IIndexable       public Guid Id   get  set        public string Name   get  set        public decimal Price   get  set         public int Index   get  set          In a method var items   from a in db SomeTable             where a Id    someValue             select new SomeModelDTO                               Id   a Id                  Name   a Name                  Price   a Price                 return items SelectWithIndex                OrderBy m   gt  m Name               Skip pageStart               Take pageSize               ToList

User · Answer

Stumbled across this today in a search for answers and I thought I d add my version to the list  No pun intended   It utlises the null conditional operator of c 6 0   IEnumerable lt Item gt  collection   GetTheCollection     var index   collection  Select  item idx    gt  new   Item   item  Index   idx      or  FirstOrDefault     gt     Item Prop    something   FirstOrDefault     gt    Item    itemToFind   Index    -1    I ve done some  racing of the old horses   testing  and for large collections   100 000   worst case scenario that item you want is at the end  this is 2x faster than doing ToList   FindIndex    If the Item you want is in the middle its  4x faster   For smaller collections   10 000  it seems to be only marginally faster  Heres how I tested it https   gist github com insulind 16310945247fcf13ba186a45734f254e

User · Answer

The way I m currently doing this is a bit shorter than those already suggested and as far as I can tell gives the desired result    var index   haystack ToList   IndexOf needle     It s a bit clunky  but it does the job and is fairly concise

User · Answer

Using  Marc Gravell  s answer  I found a way to use the following method   source TakeWhile x   gt  x    value  Count      in order to get -1 when the item cannot be found   internal static class Utils        public static int IndexOf lt T gt  this IEnumerable lt T gt  enumerable  T item    gt  enumerable IndexOf item  EqualityComparer lt T gt  Default        public static int IndexOf lt T gt  this IEnumerable lt T gt  enumerable  T item  EqualityComparer lt T gt  comparer                int index   enumerable TakeWhile x   gt  comparer Equals x  item   Count            return index    enumerable Count     -1   index            I guess this way could be both the fastest and the simpler  However  I ve not tested performances yet

User · Answer

I d question the wisdom  but perhaps   source TakeWhile x   gt  x    value  Count       using EqualityComparer lt T gt  Default to emulate    if needed  - but you need to watch to return -1 if not found    so perhaps just do it the long way  public static int IndexOf lt T gt  this IEnumerable lt T gt  source  T value        int index   0      var comparer   EqualityComparer lt T gt  Default     or pass in as a parameter     foreach  T item in source                if  comparer Equals item  value   return index          index              return -1

User · Answer

I think the best option is to implement like this   public static int IndexOf lt T gt  this IEnumerable lt T gt  enumerable  T element  IEqualityComparer lt T gt  comparer   null        int i   0      comparer   comparer    EqualityComparer lt T gt  Default      foreach  var currentElement in enumerable                if  comparer Equals currentElement  element                         return i                     i               return -1      It will also not create the anonymous object

User · Answer

The whole point of getting things out as IEnumerable is so you can lazily iterate over the contents  As such  there isn t really a concept of an index  What you are doing really doesn t make a lot of sense for an IEnumerable  If you need something that supports access by index  put it in an actual list or collection

[c#] How to get the index of an element in an IEnumerable?

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