Bagaimana untuk Mendapatkan Berbilang Kunci yang Dikaitkan dengan Nilai Khusus dalam Kamus Generik?

<code class="language-csharp">Dictionary<int, string> greek = new Dictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");

int[] betaKeys = greek.WhatDoIPutHere("Beta"); // 预期结果为单个 2</code>

<code class="language-csharp">using System;
using System.Collections.Generic;
using System.Linq;

class BiDictionary<TFirst, TSecond>
{
    private IDictionary<TFirst, IList<TSecond>> firstToSecond = new Dictionary<TFirst, IList<TSecond>>();
    private IDictionary<TSecond, IList<TFirst>> secondToFirst = new Dictionary<TSecond, IList<TFirst>>();

    public void Add(TFirst first, TSecond second)
    {
        IList<TSecond> seconds;
        IList<TFirst> firsts;

        if (!firstToSecond.TryGetValue(first, out seconds))
        {
            seconds = new List<TSecond>();
            firstToSecond[first] = seconds;
        }

        if (!secondToFirst.TryGetValue(second, out firsts))
        {
            firsts = new List<TFirst>();
            secondToFirst[second] = firsts;
        }

        seconds.Add(second);
        firsts.Add(first);
    }

    public IEnumerable<TSecond> GetByFirst(TFirst first)
    {
        IList<TSecond> list;
        return firstToSecond.TryGetValue(first, out list) ? list : Enumerable.Empty<TSecond>();
    }

    public IEnumerable<TFirst> GetBySecond(TSecond second)
    {
        IList<TFirst> list;
        return secondToFirst.TryGetValue(second, out list) ? list : Enumerable.Empty<TFirst>();
    }
}</code>

<code class="language-csharp">BiDictionary<int, string> greek = new BiDictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");
greek.Add(5, "Beta");

IEnumerable<int> betaKeys = greek.GetBySecond("Beta");

foreach (int key in betaKeys)
{
    Console.WriteLine(key); // 2, 5
}</code>