IEnumerator和IEnumerable详解

发布日期:2018-04-09    浏览次数:260

IEnumerator和IEnumerable

从名字常来看,IEnumerator是枚举器的意思,IEnumerable是可枚举的意思。
了解了两个接口代表的含义后,接着看源码:
IEnumerator:

public interface IEnumerator
    {
        // Interfaces are not serializable
        // Advances the enumerator to the next element of the enumeration and
        // returns a boolean indicating whether an element is available. Upon
        // creation, an enumerator is conceptually positioned before the first
        // element of the enumeration, and the first call to MoveNext 
        // brings the first element of the enumeration into view.
        // 
        bool MoveNext();
    
        // Returns the current element of the enumeration. The returned value is
        // undefined before the first call to MoveNext and following a
        // call to MoveNext that returned false. Multiple calls to
        // GetCurrent with no intervening calls to MoveNext 
        // will return the same object.
        // 
        Object Current {
            get; 
        }
    
        // Resets the enumerator to the beginning of the enumeration, starting over.
        // The preferred behavior for Reset is to return the exact same enumeration.
        // This means if you modify the underlying collection then call Reset, your
        // IEnumerator will be invalid, just as it would have been if you had called
        // MoveNext or Current.
        //
        void Reset();
    }

IEnumerable:

    public interface IEnumerable
    {
        // Interfaces are not serializable
        // Returns an IEnumerator for this enumerable Object.  The enumerator provides
        // a simple way to access all the contents of a collection.
        [Pure]
        [DispId(-4)]
        IEnumerator GetEnumerator();
    }

发现IEnumerable只有一个GetEnumerator函数,返回值是IEnumerator类型,从注释我们可以得知IEnumerable代表继承此接口的类可以获取一个IEnumerator来实现枚举这个类中包含的集合中的元素的功能(比如List<T>,ArrayList,Dictionary等继承了IEnumeratble接口的类)。

用foreach来了解IEnumerable,IEnumerator的工作原理

我们模仿ArrayList来实现一个简单的ConstArrayList,然后用foreach遍历。

//一个常量的数组,用于foreach遍历
class ConstArrayList : IEnumerable
{
    public int[] constItems = new int[] { 1, 2, 3, 4, 5 };
    public IEnumerator GetEnumerator()
    {
        return new ConstArrayListEnumeratorSimple(this);
    }
}
//这个常量数组的迭代器
class ConstArrayListEnumeratorSimple : IEnumerator
{
    ConstArrayList list;
    int index;
    int currentElement;
    public ConstArrayListEnumeratorSimple(ConstArrayList _list)
    {
        list = _list;
        index = -1;
    }

    public object Current
    {
        get
        {
            return currentElement;
        }
    }

    public bool MoveNext()
    {
        if(index < list.constItems.Length - 1)
        {
            currentElement = list.constItems[++index];
            return true;
        }
        else
        {
            currentElement = -1;
            return false;
        }
    }

    public void Reset()
    {
        index = -1;
    }
}
class Program
{    
    static void Main(string[] args)
    {
        ConstArrayList constArrayList = new ConstArrayList();
        foreach(int item in constArrayList)
        {
            WriteLine(item);
        }
        ReadKey();
    }
}

输出结果:
1
2
3
4
5

代码达到了遍历效果,但是在用foreach遍历时,IEnumerator和IEnumerable究竟是如何运行的,我们可以通过增加增加日志可以直观的看到原因。

//一个常量的数组,用于foreach遍历
class ConstArrayList : IEnumerable
{
    public int[] constItems = new int[] { 1, 2, 3, 4, 5 };
    public IEnumerator GetEnumerator()
    {
        WriteLine("GetIEnumerator");
        return new ConstArrayListEnumeratorSimple(this);
    }
}
//这个常量数组的迭代器
class ConstArrayListEnumeratorSimple : IEnumerator
{
    ConstArrayList list;
    int index;
    int currentElement;
    public ConstArrayListEnumeratorSimple(ConstArrayList _list)
    {
        list = _list;
        index = -1;
    }

    public object Current
    {
        get
        {
            WriteLine("Current");
            return currentElement;
        }
    }

    public bool MoveNext()
    {
        if(index < list.constItems.Length - 1)
        {
            WriteLine("MoveNext true");   
            currentElement = list.constItems[++index];
            return true;
        }
        else
        {
            WriteLine("MoveNext false");
            currentElement = -1;
            return false;
        }
    }

    public void Reset()
    {
        WriteLine("Reset");
        index = -1;
    }
}
class Program
{    
    static void Main(string[] args)
    {
        ConstArrayList constArrayList = new ConstArrayList();
        foreach(int item in constArrayList)
        {
            WriteLine(item);
        }
        ReadKey();
    }
}

输出结果:
GetIEnumerator
MoveNext true
Current
1
MoveNext true
Current
2
MoveNext true
Current
3
MoveNext true
Current
4
MoveNext true
Current
5
MoveNext false

通过输出结果,我们可以发现,foreach在运行时会先调用ConstArrayList的GetIEnumerator函数获取一个ConstArrayListEnumeratorSimple,之后通过循环调用ConstArrayListEnumeratorSimple的MoveNext函数,index后移,更新Current属性,然后返回Current属性,直到MoveNext返回false。

总结一下:
GetIEnumerator()负责获取枚举器。
MoveNext()负责让Current获取下一个值,并判断遍历是否结束。
Current负责返回当前指向的值。
Rest()负责重置枚举器的状态(在foreach中没有用到)
这些就是IEnumerable,IEnumerator的基本工作原理了。

其次我们发现:

ConstArrayList constArrayList = new ConstArrayList();
foreach(int item in constArrayList)
{
    writeLine(item);
}

其实就等价于:

ConstArrayList constArrayList = new ConstArrayList();
IEnumerator enumeratorSimple = constArrayList.GetEnumerator();
while (enumeratorSimple.MoveNext())
{
    int item = (int)enumeratorSimple.Current;
    WriteLine(item);
}

也就是说foreach其实是一种语法糖,用来简化对可枚举元素的遍历代码。而被遍历的类通过实现IEnumerable接口和一个相关的IEnumerator枚举器来实现遍历功能。

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