using System;

using System.Collections.Generic;

using System.Linq;

​

public class Person {

    public string Name { get; set; }

    public Person(string name) { Name = name; }

}

​

public class Program {

    public static void Main() {

        var persons = new List<Person> {

            new Person("Mark Twain"),

            new Person("Alice Parker"),

            new Person("John Doe"),

            new Person("Bob Dole")

        };

        //Console.WriteLine("A) " + persons.IndexOf(p => p.Name == "John Doe"));

​

        Console.WriteLine("B) " + persons.FindIndex(p => p.Name == "John Doe"));

​

        Console.WriteLine("C) " + persons.Select((p, i) => new { p, i }).First(pi => pi.p.Name == "John Doe")?.i);

        Console.WriteLine("D) " + persons.TakeWhile(p => p.Name != "John Doe").Count());

    }

}