C#集合类(HashTable, Dictionary, ArrayList)与HashTable线程安全

HashTable中的key/value均为object类型，由包含集合元素的存储桶组成。存储桶是 HashTable中各元素的虚拟子组，与大多数集合中进行的搜索和检索相比，存储桶可令搜索和检索更为便捷。每一存储桶都与一个哈希代码关联，该哈希代码是使用哈希函数生成的并基于该元素的键。HashTable的优点就在于其索引的方式，速度非常快。如果以任意类型键值访问其中元素会快于其他集合，特别是当数据量特别大的时候，效率差别尤其大。

HashTable的应用场合有：做对象缓存，树递归算法的替代，和各种需提升效率的场合。

//Hashtable sample
    System.Collections.Hashtable ht = new System.Collections.Hashtable();
    //--Be careful: Keys can't be duplicated, and can't be null----
    ht.Add(1, "apple");
    ht.Add(2, "banana");
    ht.Add(3, "orange");
    //Modify item value:
    if(ht.ContainsKey(1))
        ht[1] = "appleBad";
    //The following code will return null oValue, no exception
    object oValue = ht[5];
    //traversal 1:
    foreach (DictionaryEntry de in ht)
    {
        Console.WriteLine(de.Key);
        Console.WriteLine(de.Value);
    }
    //traversal 2:
    System.Collections.IDictionaryEnumerator d = ht.GetEnumerator();
    while (d.MoveNext())
    {
        Console.WriteLine("key：{0} value：{1}", d.Entry.Key, d.Entry.Value);
    }
    //Clear items
    ht.Clear();

//Hashtable sample System.Collections.Hashtable ht = new System.Collections.Hashtable(); //--Be careful: Keys can't be duplicated, and can't be null---- ht.Add(1, "apple"); ht.Add(2, "banana"); ht.Add(3, "orange"); //Modify item value: if(ht.ContainsKey(1)) ht[1] = "appleBad"; //The following code will return null oValue, no exception object oValue = ht[5]; //traversal 1: foreach (DictionaryEntry de in ht) { Console.WriteLine(de.Key); Console.WriteLine(de.Value); } //traversal 2: System.Collections.IDictionaryEnumerator d = ht.GetEnumerator(); while (d.MoveNext()) { Console.WriteLine("key：{0} value：{1}", d.Entry.Key, d.Entry.Value); } //Clear items ht.Clear();

Dictionary和HashTable内部实现差不多，但前者无需装箱拆箱操作，效率略高一点。

//Dictionary sample
   System.Collections.Generic.Dictionary<int, string> fruits =
       new System.Collections.Generic.Dictionary<int, string>();
   fruits.Add(1, "apple");
   fruits.Add(2, "banana");
   fruits.Add(3, "orange");
   foreach (int i in fruits.Keys)
   {
       Console.WriteLine("key：{0} value：{1}", i, fruits);
   }
   if (fruits.ContainsKey(1))
   {
       Console.WriteLine("contain this key.");
   }

//Dictionary sample System.Collections.Generic.Dictionary<int, string> fruits = new System.Collections.Generic.Dictionary<int, string>(); fruits.Add(1, "apple"); fruits.Add(2, "banana"); fruits.Add(3, "orange"); foreach (int i in fruits.Keys) { Console.WriteLine("key：{0} value：{1}", i, fruits); } if (fruits.ContainsKey(1)) { Console.WriteLine("contain this key."); }

ArrayList是一维变长数组，内部值为object类型，效率一般：

//ArrayList
    System.Collections.ArrayList list = new System.Collections.ArrayList();
    list.Add(1);//object type
    list.Add(2);
    for (int i = 0; i < list.Count; i++)
    {
        Console.WriteLine(list[i]);
    }

//ArrayList System.Collections.ArrayList list = new System.Collections.ArrayList(); list.Add(1);//object type list.Add(2); for (int i = 0; i < list.Count; i++) { Console.WriteLine(list[i]); }

HashTable是经过优化的，访问下标的对象先散列过，所以内部是无序散列的，保证了高效率，也就是说，其输出不是按照开始加入的顺序，而Dictionary遍历输出的顺序，就是加入的顺序，这点与Hashtable不同。如果一定要排序HashTable输出，只能自己实现：

//Hashtable sorting
System.Collections.ArrayList akeys = new System.Collections.ArrayList(ht.Keys); //from Hashtable
akeys.Sort(); //Sort by leading letter
foreach (string skey in akeys)
{
    Console.Write(skey + ":");
    Console.WriteLine(ht[skey]);
}

//Hashtable sorting System.Collections.ArrayList akeys = new System.Collections.ArrayList(ht.Keys); //from Hashtable akeys.Sort(); //Sort by leading letter foreach (string skey in akeys) { Console.Write(skey + ":"); Console.WriteLine(ht[skey]); }

HashTable与线程安全：

为了保证在多线程的情况下的线程同步访问安全，微软提供了自动线程同步的HashTable:

如果 HashTable要允许并发读但只能一个线程写, 要这么创建 HashTable实例:

//Thread safe HashTable
System.Collections.Hashtable htSyn = System.Collections.Hashtable.Synchronized(new System.Collections.Hashtable());
这样, 如果有多个线程并发的企图写HashTable里面的 item, 则同一时刻只能有一个线程写, 其余阻塞; 对读的线程则不受影响。

另外一种方法就是使用lock语句，但要lock的不是HashTable，而是其SyncRoot；虽然不推荐这种方法，但效果一样的，因为源代码就是这样实现的:

//Thread safe
private static Hashtable htCache = new Hashtable();
public static void AccessCache()
{
    lock (htCache.SyncRoot)
    {
        //Do something
    }
}

//Thread safe private static Hashtable htCache = new Hashtable(); public static void AccessCache() { lock (htCache.SyncRoot) { //Do something } }