对于c# IEnumerator、IEnumerable、yield的理解

先给出这两个接口的定义

public interface IEnumerable
{
	IEnumerator GetEnumerator();
}
public interface IEnumerator
{
	object Current{get;}
	bool MoveNext();
	void Reset();
}

下面给出使用这两个接口，使得一个类可以使用foreach遍历的例子

public class IterationSample : IEnumerable
{
    object[] values;
    int startingPoint;

    public IterationSample(object[] values, int startingPoint)
    {
        this.values = values;
        this.startingPoint = startingPoint;
    }

    public IEnumerator GetEnumerator()
    {
        //return new IterationSampleIterator(this);
        for (int index = 0; index < values.Length; index++)
        {
            yield return values[(index + startingPoint) % values.Length];
        }
    }

    class IterationSampleIterator : IEnumerator
    {
        IterationSample parent;
        int position;
        internal IterationSampleIterator(IterationSample parent)
        {
            this.parent = parent;
            this.position = -1;
        }

        public object Current
        {
            get
            {
                if (position == -1 || position == parent.values.Length)
                {
                    throw new InvalidOperationException();
                }
                int index = (position + parent.startingPoint);
                index = index % parent.values.Length;
                return parent.values[index];
            }
        }

        public bool MoveNext()
        {
            if (position != parent.values.Length)
            {
                position++;
            }
            return position < parent.values.Length;
        }

        public void Reset()
        {
            position = -1;
        }
    }

}

void Start()
{
	string[] strArr = { "1", "2" };
    IterationSample sample = new IterationSample(strArr, 0);
    var iterator = sample.GetEnumerator();
    while (iterator.MoveNext())
    {
        object item = iterator.Current;
        Debug.Log(item);
    }

    foreach (var item in sample)
    {
        Debug.Log(item);
    }
}

现在给出使用yield关键字语法糖简化后的代码

public class IterationSample : IEnumerable
{
	object[] values;
	int startingPoint;
	
	public IterationSample(object[] values, int startingPoint)
	{
		this.values = values;
        this.startingPoint = startingPoint;
	}

	public IEnumerator GetEnumerator()
    {
        for (int index = 0; index < values.Length; index++)
        {
            yield return values[(index + startingPoint) % values.Length];
        }
    }
}

从上面两个例子可以看出，yield return语法实际上在背后帮助我们定义了一个IEnumerator

我们再看一个例子

public static IEnumerable<int> EnumerableFunc()//这个函数会返回一个可枚举的容器对象，这个对象中包含若干用来枚举的项
{
    yield return 1;
    yield return 2;
    yield return 3;
}

void Start()
{
	IEnumerable<int> objIEnumerable = EnumerableFunc();
    foreach (var item in objIEnumerable)
    {
        print("for each:" + item);
    }
    IEnumerator<int> objIEnumerator = objIEnumerable.GetEnumerator();
    while (objIEnumerator.MoveNext())
    {
        print("while:" + objIEnumerator.Current);
    }
}