Dictionary用法详解

1  Dictionary

(1) 表示键和值的集合.

(2)类型参数：

TKey   字典中的键的类型

TValue 字典中的值的类型

(3)Dictionary 泛型类提供了从一组键到一组值的映射。字典中的每个添加项都由一个值及其相关联的键组成。通过键来检索值的速度是非常快的，接近于 O(1)，这是因为 Dictionary 类是作为一个哈希表来实现的。

(4)定义：

Dictionary<string, string> openWith = new Dictionary<string, string>();

(5)使用Add方法可以向其中添加值

openWith.Add("txt", "notepad.exe");

当添加重复的键时会引发ArgumentException类型的异常。

(6)使用["txt"]的方法可以找到相对应的值，并可以修改这个键对应的值。

如果键不存在，可以通过如下方法添加新的键/值对。

openWith["doc"] = "winword.exe";

如果取值的键不存在，会引发一个KeyNotFoundException类型的异常。

(7)使用TryGetValue方法来尝试取出一个键对应的值，如果这个值存在就取出返回，这样可以避免抛出KeyNotFoundException异常。返回值类型为布尔类型。

(8)使用ContainsKey方法来判断是否包含指定的键，返回值类型为布尔类型。

(9)使用foreach遍历：

foreach (KeyValuePair<string, string> kvp in openWith)
            {
                Console.WriteLine("Key = {0}, Value = {1}",
                    kvp.Key, kvp.Value);
            }

(10)使用Values属性，可以仅取出所有的值而不取出所有的键。

 Dictionary<string, string>.ValueCollection valuecol = openWith.Values;

foreach (string str in valuecol)
      Console.WriteLine(str);

(11)使用Keys属性，可以仅取出所有的键而不取出所有的值。

(12)使用Remove方法，可以根据键来移除集合中的键/值对。

openWith.Remove("jin");当移除值的时候同时移除键。

2  实例程序

using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        // Create a new dictionary of strings, with string keys.
        //
        Dictionary<string, string> openWith = 
            new Dictionary<string, string>();

        // Add some elements to the dictionary. 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 dictionary.
        try
        {
            openWith.Add("txt", "winword.exe");
        }
        catch (ArgumentException)
        {
            Console.WriteLine("An element with Key = \"txt\" already exists.");
        }

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

        // The indexer 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 indexer for that key
        // adds a new key/value pair.
        openWith["doc"] = "winword.exe";

        // The indexer throws an exception if the requested key is
        // not in the dictionary.
        try
        {
            Console.WriteLine("For key = \"tif\", value = {0}.", 
                openWith["tif"]);
        }
        catch (KeyNotFoundException)
        {
            Console.WriteLine("Key = \"tif\" is not found.");
        }

        // When a program often has to try keys that turn out not to
        // be in the dictionary, TryGetValue can be a more efficient 
        // way to retrieve values.
        string value = "";
        if (openWith.TryGetValue("tif", out value))
        {
            Console.WriteLine("For key = \"tif\", value = {0}.", value);
        }
        else
        {
            Console.WriteLine("Key = \"tif\" is not found.");
        }

        // 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 dictionary elements,
        // the elements are retrieved as KeyValuePair objects.
        Console.WriteLine();
        foreach( KeyValuePair<string, string> kvp in openWith )
        {
            Console.WriteLine("Key = {0}, Value = {1}", 
                kvp.Key, kvp.Value);
        }

        // To get the values alone, use the Values property.
        Dictionary<string, string>.ValueCollection valueColl =
            openWith.Values;

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

        // To get the keys alone, use the Keys property.
        Dictionary<string, string>.KeyCollection keyColl =
            openWith.Keys;

        // The elements of the KeyCollection are strongly typed
        // with the type that was specified for dictionary 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.");
        }
    }
}