c# - ToLookup() 如何使用多个索引？

Question

考虑到下面的 C# 控制台应用程序代码，使用

我应该如何修改它以替换行:

foreach (Product product in productsByCategory[category])

通过代码行

foreach (Product product in productsByCategory[category][Id])

?

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

namespace myQuestion
{
  class Program
  {
    static void Main(string[] args)
    {
      var products = new List<Product>
      {
        new Product { Id = 1, Category = "Garden", Value = 15.0 },
        new Product { Id = 1, Category = "Garden", Value = 40.0 },
        new Product { Id = 3, Category = "Garden", Value = 210.3 },
        new Product { Id = 4, Category = "Pets", Value = 2.1 },
        new Product { Id = 5, Category = "Electronics", Value = 19.95 },
        new Product { Id = 6, Category = "Pets", Value = 21.25 },
        new Product { Id = 7, Category = "Pets", Value = 5.50 },
        new Product { Id = 8, Category = "Garden", Value = 13.0 },
        new Product { Id = 9, Category = "Automotive", Value = 10.0 },
        new Product { Id = 10, Category = "Electronics", Value = 250.0 }
      };


      ILookup<string, Product> productsByCategory = 
              products.ToLookup( p => p.Category);
      string category = "Garden";
      int Id = 1;
      foreach (Product product in productsByCategory[category])
      {
        Console.WriteLine("\t" + product);
      }

      Console.ReadLine();
    }
  }

  public sealed class Product
  {
    public int Id { get; set; }
    public string Category { get; set; }
    public double Value { get; set; }
    public override string ToString()
    {
      return string.Format("[{0}: {1} - {2}]", Id, Category, Value);
    }
  }
}

更新:
这是一个人为的示例，旨在学习 C# 的概念 ToLookup Method .

作为引用，我是在阅读the David Andres' answer to question "What is the point of Lookup?" 后才想到这个问题的。 :

"A Lookup will map to potentially several values.  

Lookup["Smith"]["John"] will be a collection of size one billion."   

我想重现。

还是我理解错了？

Answer 1

不确定我是否正确理解了您的需求，但您为什么不能这样做:

foreach (Product product in productsByCategory[category].Where(x=> x.Id == Id))

或者使用匿名对象:

var productsByCategory = products.ToLookup(p => new { p.Category, p.Id });
string category = "Groceries";
int Id = 1;
foreach (Product product in productsByCategory[new {Category = category, Id= Id}])
{
    Console.WriteLine("\t" + product);
}

这是一个非常相似的问题，由 Servy 提供额外的解决方案