CAREFUL! IEnumerable can be inefficient and slow.

IEnumerableCanBeInefficient.cs

using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;

public class Program
{
    // CAREFUL! IEnumerable can be inefficient and slow.
    public static void Main()
    {
        var size = 10000;
        var numbers = new List<int>();
        for (int i = 0; i < size; i++)
        {
            numbers.Add(i);
        }

        // Make the three collections
        var enumNumbers = numbers.Where(n => true);
        var listNumbers = enumNumbers.ToList() as IEnumerable<int>;
        var arrayNumbers = enumNumbers.ToArray() as IEnumerable<int>;
        
        Console.WriteLine("==== ElementAt() ====");
        var sw = Stopwatch.StartNew();
        
        // IEnumerable
        for (int i = 0; i < size; i++)
        {
            enumNumbers.ElementAt(i);
        }

        Console.WriteLine("Enumerable: " + sw.ElapsedMilliseconds + " ms");
        sw.Restart();
        
        // List
        for (int i = 0; i < size; i++)
        {
            listNumbers.ElementAt(i);
        }

        Console.WriteLine("List: " + sw.ElapsedMilliseconds + " ms");
        sw.Restart();
        
        // Array
        for (int i = 0; i < size; i++)
        {
            arrayNumbers.ElementAt(i);
        }

        Console.WriteLine("Array: " + sw.ElapsedMilliseconds + " ms");
        
        Console.WriteLine();
        Console.WriteLine("==== Count() ====");
        sw.Restart();
        
        // IEnumerable
        for (int i = 0; i < size; i++)
        {
            enumNumbers.Count();
        }

        Console.WriteLine("Enumerable: " + sw.ElapsedMilliseconds + " ms");
        sw.Restart();
        
        // List
        for (int i = 0; i < size; i++)
        {
            listNumbers.Count();
        }

        Console.WriteLine("List: " + sw.ElapsedMilliseconds + " ms");
        sw.Restart();
        
        // Array
        for (int i = 0; i < size; i++)
        {
            arrayNumbers.Count();
        }

        Console.WriteLine("Array: " + sw.ElapsedMilliseconds + " ms");
        sw.Stop();
    }
}

/*
Results:

==== ElementAt() ====
Enumerable: 1244 ms
List: 0 ms
Array: 0 ms

==== Count() ====
Enumerable: 2993 ms
List: 0 ms
Array: 0 ms


Ref. source from IEnumerable<T>:

public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
        throw Error.ArgumentNull("source");
    ICollection<TSource> collectionoft = source as ICollection<TSource>;
    if (collectionoft != null)
        return collectionoft.Count;
    ICollection collection = source as ICollection;
    if (collection != null)
        return collection.Count;
    int count = 0;
    using (IEnumerator<TSource> e = source.GetEnumerator())
    {
        checked
        {
            while (e.MoveNext())
                count++;
        }
    }

    return count;
}

public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int index)
{
    if (source == null)
        throw Error.ArgumentNull("source");
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
        return list[index];
    if (index < 0)
        throw Error.ArgumentOutOfRange("index");
    using (IEnumerator<TSource> e = source.GetEnumerator())
    {
        while (true)
        {
            if (!e.MoveNext())
                throw Error.ArgumentOutOfRange("index");
            if (index == 0)
                return e.Current;
            index--;
        }
    }
}
*/