gistfile1.cs

    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<Customer>();
            list.Add(new Customer() { Name = "jordan", Age = 19 });
            list.Add(new Customer() { Name = "小章", Age = 20 });
            list.Add(new Customer() { Name = "盧克", Age = 34 });
            list.Add(new Customer() { Name = "9gy", Age = 29 });
            list.Add(new Customer() { Name = "小盧克", Age = 17 });

            var result = list.GetIndex(x => x.Age == 34);

            Console.WriteLine(result);
        }
    }

    public static class JoeyExtension
    {
        public static int GetIndex<TSource>(this IEnumerable<TSource> source, Predicate<TSource> predicate)
        {
            var result = source.ToList<TSource>().FindIndex(predicate);

            return result;
        }
    }
    internal class Customer
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }