How to get the index of an element in an IEnumerable?

Question

I wrote this:

public static class EnumerableExtensions {     public static int IndexOf<T>(this IEnumerable<T> obj, T value)     {         return obj             .Select((a, i) => (a.Equals(value)) ? i : -1)             .Max();     }      public static int IndexOf<T>(this IEnumerable<T> obj, T value            , IEqualityComparer<T> comparer)     {         return obj             .Select((a, i) => (comparer.Equals(a, value)) ? i : -1)             .Max();     } } 

But I don't know if it already exists, does it?

Answer 1

I'd question the wisdom, but perhaps:

source.TakeWhile(x => x != value).Count(); 

(using EqualityComparer<T>.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<T>(this IEnumerable<T> source, T value) {     int index = 0;     var comparer = EqualityComparer<T>.Default; // or pass in as a parameter     foreach (T item in source)     {         if (comparer.Equals(item, value)) return index;         index++;     }     return -1; } 

Answer 2

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.