C#中迭代器的用法

1、注意：使用foreach对collection中的元素只能进行Readonly访问，不能用foreach来修改 集合（不包扩数组）中的元素

//msdn中的说法

foreach (DictionaryEntry myDE in myDictionary) {...}

[Visual Basic]
Dim myDE As DictionaryEntry
For Each myDE In myDictionary
...
Next myDE

[Visual Basic, C#] foreach
语句是对枚举数的包装，它只允许从集合中读取，不允许写入集合。
×枚举数即Enumerator
2、集合的访问三种方法：
    // Uses the enumerator.
    public static void PrintKeysAndValues(ShortStringDictionary myCol)
    {
        DictionaryEntry myDE;
        System.Collections.IEnumerator myEnumerator = myCol.GetEnumerator();
        while (myEnumerator.MoveNext())
            if (myEnumerator.Current != null)
            {
                myDE = (DictionaryEntry)myEnumerator.Current;
                Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value);
            }
        Console.WriteLine();
    }

    // Uses the foreach statement which hides the complexity of the enumerator.
    public static void PrintKeysAndValues2(ShortStringDictionary myCol)
    {
        foreach (DictionaryEntry myDE in myCol)
            Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value);
        Console.WriteLine();
    }

    // Uses the Keys property and the Item property.
    public static void PrintKeysAndValues3(ShortStringDictionary myCol)
    {
        ICollection myKeys = myCol.Keys;
        foreach (String k in myKeys)
            Console.WriteLine("   {0,-5} : {1}", k, myCol[k]);
        Console.WriteLine();
    }