为什么不能从Value查到Key值呢？
using System;
using System.Collections.Generic;
using System.Text;
using System.Collections;

namespace _7
{
class Program
{
static void Main(string[] args)
{
Hashtable hashtable = new Hashtable();//实例化Hashtable对象
hashtable.Clear();
hashtable.Add("id", "BH0001");//向Hashtable哈希表中添加元素
hashtable.Add("name", "TM");
hashtable.Add("sex", "Man");
Console.WriteLine(hashtable.ContainsValue("BH0001"));//判断Hashtable哈希表中是否包含指定的键值
Console.WriteLine(hashtable.ContainsKey("id"));//判断Hashtable哈希表中是否包含指定的键值

Console.WriteLine();
foreach (DictionaryEntry de in hashtable)
{
Console.WriteLine("Key = {0}, Value = {1}", de.Key, de.Value);
}

// To get the values alone, use the Values property.
ICollection valueColl = hashtable.Values;

// The elements of the ValueCollection are strongly typed
// with the type that was specified for hash table values.
Console.WriteLine();
foreach (string s in valueColl)
{
Console.WriteLine("Value = {0}", s);
}

// To get the keys alone, use the Keys property.
ICollection keyColl = hashtable.Keys;

// The elements of the KeyCollection are strongly typed
// with the type that was specified for hash table keys.
Console.WriteLine();
foreach (string s in keyColl)
{
Console.WriteLine("Key = {0}", s);
}

// The Item property is the default property, so you
// can omit its name when accessing elements.
Console.WriteLine("For key = \"id\", value = {0}.", hashtable["id"]);
//this is correct! it can get corresponding Value from Key!
Console.WriteLine("For value = \"TM\", key = {0}.", hashtable["TM"]);
//why can't get the correct result! from Value obtain its corresponding Key

}
}
}


哈希表从Key可以查到Value！
Console.WriteLine("For key = \"id\", value = {0}.", hashtable["id"]); OK！
为什么不能从Value查到Key值呢？
Console.WriteLine("For value = \"TM\", key = {0}.", hashtable["TM"]); key没有返回值！

[解决办法]