LINQ 微妙に拡張シリーズ

EnumerableExtensions.cs

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

public static class EnumerableExtensions
{
    private static int IndexOfByForEach<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
    {
        int index = 0;
        foreach (var item in source)
        {
            if (predicate(item))
            {
                return index;
            }
            index++;
        }
        return -1;
    }

    public static int IndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
    {
        var list = source as List<TSource>;
        if (list != null)
        {
            return list.FindIndex(x => predicate(x));
        }
        var array = source as TSource[];
        if (array != null)
        {
            return Array.FindIndex(array, x => predicate(x));
        }
        return source.IndexOfByForEach(predicate);
    }

    public static int LastIndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
    {
        var list = source as List<TSource>;
        if (list != null)
        {
            return list.FindLastIndex(x => predicate(x));
        }
        var array = source as TSource[];
        if (array != null)
        {
            return Array.FindLastIndex(array, x => predicate(x));
        }
        return source.Reverse().IndexOfByForEach(predicate);
    }
}