Hashtable Class

1. 以雜湊表存放計算後的排序結果, 需多使用記憶體空間.
2. 元素為DictionaryEntry物件的key/value pair. Key不可為null, Value可以為null.
3. 預設以0為lower bound. 建立Array時可指定不同的lower bound.
4. 可建立為多階陣列, 最多32階.
5. 建立陣列時, 需指定固定的元素個數.
6. 同時實作了以下介面, 因此可以使用相同的屬性或方法:
   • System.Collections.Generic.IList
   • System.Collections.Generic.ICollection<T>
   • System.Collections.Generic.IEnumerable<T>

Example

using System;
using System.Collections;

class Example
{
public static void Main()
{
// Create a new hash table.
//
Hashtable openWith = new Hashtable();

// Add some elements to the hash table. There are no 
// duplicate keys, but some of the values are duplicates.
openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// The Add method throws an exception if the new key is 
// already in the hash table.
try
{
openWith.Add("txt", "winword.exe");
}
catch
{
Console.WriteLine("An element with Key = \"txt\" already exists.");
}

// The Item property is the default property, so you 
// can omit its name when accessing elements. 
Console.WriteLine("For key = \"rtf\", value = {0}.", openWith["rtf"]);

// The default Item property can be used to change the value
// associated with a key.
openWith["rtf"] = "winword.exe";
Console.WriteLine("For key = \"rtf\", value = {0}.", openWith["rtf"]);

// If a key does not exist, setting the default Item property
// for that key adds a new key/value pair.
openWith["doc"] = "winword.exe";

// ContainsKey can be used to test keys before inserting 
// them.
if (!openWith.ContainsKey("ht"))
{
openWith.Add("ht", "hypertrm.exe");
Console.WriteLine("Value added for key = \"ht\": {0}", openWith["ht"]);
}

// When you use foreach to enumerate hash table elements,
// the elements are retrieved as KeyValuePair objects.
Console.WriteLine();
foreach( DictionaryEntry de in openWith )
{
Console.WriteLine("Key = {0}, Value = {1}", de.Key, de.Value);
}

// To get the values alone, use the Values property.
ICollection valueColl = openWith.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 = openWith.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);
}

// Use the Remove method to remove a key/value pair.
Console.WriteLine("\nRemove(\"doc\")");
openWith.Remove("doc");

if (!openWith.ContainsKey("doc"))
{
Console.WriteLine("Key \"doc\" is not found.");
}
}
}

/* This code example produces the following output:

An element with Key = "txt" already exists.
For key = "rtf", value = wordpad.exe.
For key = "rtf", value = winword.exe.
Value added for key = "ht": hypertrm.exe

Key = dib, Value = paint.exe
Key = txt, Value = notepad.exe
Key = ht, Value = hypertrm.exe
Key = bmp, Value = paint.exe
Key = rtf, Value = winword.exe
Key = doc, Value = winword.exe

Value = paint.exe
Value = notepad.exe
Value = hypertrm.exe
Value = paint.exe
Value = winword.exe
Value = winword.exe

Key = dib
Key = txt
Key = ht
Key = bmp
Key = rtf
Key = doc

Remove("doc")
Key "doc" is not found.
*/