遅延実行で嵌った罠。いちいち新しいインスタンスが作成されてしまう。

lazytrap.cs

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

static class Program
{
    static void Main(string[] args)
    {
        string[] inner = new string[] { "a", "b", "c", "d", "e" };
        string[] outer = new string[] { "b", "d" };
        
        var linked = outer.Join(inner, p => p, q => q, (p, q) => new Foo(p));
        foreach (var item in linked)
        {
            item.Increment();
            Console.WriteLine("Name: {0}, Counter: {1}", item.Name, item.Counter);
        }
        
        foreach (var item in linked)
        {
            item.Increment();
            Console.WriteLine("Name: {0}, Counter: {1}", item.Name, item.Counter);
        }
        
    }
    
    class Foo
    {
        public int Counter { get; private set; }
        public string Name { get; private set; }
        
        public Foo(string name)
        {
            this.Counter = 0;
            this.Name = name;
        }
        
        public void Increment()
        {
            Counter++;
        }
    }
}