C#.Example - Multiple indexers within a class

MultipleIndexers.cs

using System;
using System.Collections.Generic;

/// <summary>
/// Example of multiple indexers within a single class.
/// 
/// Why you might want/need to do this, who knows?
/// </summary>

namespace CSharp_Examples {
   /// <summary>
   /// Test object.
   /// </summary>
   public class Person {
      public int EmployeeID { get; set; }
      public int CitizenID { get; set; }
      public string Name { get; set; }

      public Person(int empID, int citID, string name) {
         EmployeeID = empID;
         CitizenID = citID;
         Name = name;
      }
   }

   /// <summary>
   /// Interface for explicit indexer usage.
   /// </summary>
   public interface ICitizen {
      Person this[int index] { get; }
   }

   /// <summary>
   /// Interface for alternative explicit indexer.
   /// </summary>
   public interface IEmployee {
      Person this[int index] { get; }
   }

   /// <summary>
   /// Object with a list of Person objects.
   /// </summary>
   public class Indexers : ICitizen, IEmployee {
      private List<Person> people = new List<Person>();

      /// <summary>
      /// Standard indexer.
      /// </summary>
      /// <param name="index">Index in array of Person objects.</param>
      /// <returns></returns>
      public Person this[int index] {
         get {
            return people[index];
         }
      }

      /// <summary>
      /// Add a Person object to the array.
      /// </summary>
      /// <param name="person"></param>
      /// <returns></returns>
      public Person Add(Person person) {
         int idx;

         lock (people) {
            idx = people.FindIndex(p => p.EmployeeID == person.EmployeeID || p.CitizenID == person.CitizenID);
            if (idx >= 0) {
               throw new ArgumentException("Duplicate employee ID or citizen ID", "person");
            }

            people.Add(person);
         }

         return person;
      }

      /// <summary>
      /// Indexer for the IEmployee interface. Gets an employee based on employee ID.
      /// </summary>
      /// <param name="employeeID"></param>
      /// <returns></returns>
      Person IEmployee.this[int employeeID] {
         get {
            return people.Find(p => p.EmployeeID == employeeID);
         }
      }

      /// <summary>
      /// Indexer for ICitizen interface. Gets an employee based on citizen ID.
      /// </summary>
      /// <param name="citizenID"></param>
      /// <returns></returns>
      Person ICitizen.this[int citizenID] {
         get {
            return people.Find(p => p.CitizenID == citizenID);
         }
      }
   }

   public class TestIndexers {
      public void Test() {
         Indexers persons = new Indexers();

         persons.Add(new Person(1, 301938912, "Bob Smith"));
         persons.Add(new Person(3, 920001233, "Jane Doe"));

         Person p;


         // Use ICitizen indexer to get person based on citizen ID.
         ICitizen c = persons;
         p = c[920001233];

         // Use IEmployee indexer to get person based on employee ID.
         IEmployee e = persons;
         p = e[1];

         // Returns null.
         p = ((IEmployee)persons)[2];
      }
   }
}

noice