C# .First() vs [0]

Question

Interested, does approaches has any differences.
So, I created two snippets.

Snippet A 
List a = new List();
a.Add(4);
a.Add(6);
int         


        

Answer 1

You can check it by yourself :

    static void Main()
    {
        List resultsFirst = new List();
        List resultsIndex = new List();

        Stopwatch s = new Stopwatch();

        for (int z = 0; z < 100; z++)
        {
            List[] lists = new List[10000];

            int temp = 0;

            for (int i = 0; i < lists.Length; i++)
                lists[i] = new List() { 4, 6 };                

            s.Restart();

            for (int i = 0; i < lists.Length; i++)
                temp = lists[i].First();

            s.Stop();

            resultsFirst.Add(s.ElapsedTicks);

            s.Restart();

            for (int i = 0; i < lists.Length; i++)
                temp = lists[i][0];

            s.Stop();

            resultsIndex.Add(s.ElapsedTicks);
        }

        Console.WriteLine("LINQ First()  :   " + resultsFirst.Average());
        Console.WriteLine(Environment.NewLine);
        Console.WriteLine("By index      :   " + resultsIndex.Average());

        Console.ReadKey();
    }

Output in Release mode :

LINQ First() : 367

By index : 84

Output in debug mode :

LINQ First() : 401

By index : 177

P.S.

The source code for method First is:

public static TSource First(this IEnumerable source)
{
    IList list = source as IList;
    if (list != null)
    {
        if (list.Count > 0)
        {
            return list[0];
        }
    }
    else
    {
        using (IEnumerator enumerator = source.GetEnumerator())
        {
            if (enumerator.MoveNext())
            {
                return enumerator.Current;
            }
        }
    }
}

The casting operation source as IList or creating an Enumerator object is very likely the reason why First() is considerably slower.