Func<TItem, TKey>を使ったKeyedCollection<TKey, TItem>の実装

Program.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp {
	class DefaultKeyedCollection<TKey, TItem> : KeyedCollection<TKey, TItem> {
		private readonly Func<TItem, TKey> _func;

		public DefaultKeyedCollection(Func<TItem, TKey> func) {
			_func = func;
		}

		protected override TKey GetKeyForItem(TItem item) {
			return _func(item);
		}
	}

	class Product {
		public int Id { get; set; }
		public string Name { get; set; }
	}

	class Program {
		static void Main(string[] args) {
			var products = new DefaultKeyedCollection<int, Product>(product => product.Id) {
				new Product { Id = 1, Name = "Aaa" },
				new Product { Id = 2, Name = "Bbb" },
				new Product { Id = 3, Name = "Ccc" },
			};

			Console.WriteLine(products[1].Name);	// Aaa
			Console.WriteLine(products[3].Name);	// Ccc
		}
	}
}