缺少字典<TKey,TValue> .KeyCollection扩展方法.NET 3.5(C#)

using System;
using System.Collections.Generic;

public static class DictionaryExtensions
{
    public static List<TKey> GetKeys<TKey, TValue>(this Dictionary<TKey, TValue> dictionary)
    {
        List<TKey> keys = new List<TKey>();
        foreach (KeyValuePair<TKey, TValue> kvp in dictionary)
        {
            keys.Add(kvp.Key);
        }
        return keys;
    }
}

Dictionary<string, int> myDictionary = new Dictionary<string, int>();
myDictionary.Add("apple", 1);
myDictionary.Add("banana", 2);
myDictionary.Add("orange", 3);

List<string> keys = myDictionary.GetKeys();
foreach (string key in keys)
{
    Console.WriteLine(key);
}

apple
banana
orange