yield个人理解及简明示例

1.写法有2种：
yield return <expression>和yield break
yield用于在迭代中返回一个值，并将值带入下一次迭代中。yield break则意味着停止迭代。纯粹的文字描述，一千个人有一千个说法，还是用代码更容易说清楚。
2.官方示例（略带修改）：

 private void button1_Click(object sender, EventArgs e)
        {
            string s = string.Empty;
            foreach (int i in List.Power(2,8))
            {
                s += i.ToString() + ",";
            }
            MessageBox.Show(s);
        }
    public class List
    {
        //using System.Collections;
        public static IEnumerable Power(int number, int exponent)
        {
            int counter = 0;
            int result = 1;
            
            while (counter++ < exponent)
            {
                result = result * number;
                yield return result;
            }
        }
    }

运行示例，发现power方法中的while代码部分，每循环执行一次，即输出一个值，并将这个值带入下一次循环，而power函数并没有每次被调用。

为了验证，我们修改下官方示例代码，来看看我们的判断是否有误：

private void button1_Click(object sender, EventArgs e)
        {
            string s = string.Empty;
            foreach (int i in List.Power())
            {
                s += i.ToString() + ",";
            }
            MessageBox.Show(s);
        }
    public static IEnumerable Power()
        {
            int counter = 0;
            int result = 1;
            int number = 2, exponent = 8;
            while (counter++ < exponent)
            {
                result = result * number;
                yield return result;
            }
        }

 运行结果与官方示例相同，说明.net framework每次只把yield部分所在的部分代码进行了迭代返回处理。

3.官方的另一个示例为用yield作为属性（输出方式略有修改）。

 private void button2_Click(object sender, EventArgs e)
        {
            var theGalaxies = new Galaxies();
            string ps = string.Empty;
            foreach (Galaxy theGalaxy in theGalaxies.NextGalaxy)
            {
                ps += (theGalaxy.Name + " " + theGalaxy.MegaLightYears.ToString() + " >> ");
            }
            MessageBox.Show(ps);
        } 
public System.Collections.Generic.IEnumerable<Galaxy> NextGalaxy
        {
            get
            {
                yield return new Galaxy { Name = "Tadpole", MegaLightYears = 400 };
                yield return new Galaxy { Name = "Pinwheel", MegaLightYears = 25 };
                yield return new Galaxy { Name = "Milky Way", MegaLightYears = 0 };
                yield return new Galaxy { Name = "Andromeda", MegaLightYears = 3 };
            }

        }

 输出结果：

从这个例子可以看出yield其实就是临时中断执行，输出后继续执行而已。

可以修改下这个例子，看效果如何：

 public class Galaxies
    {
        List<Galaxy> ls = new List<Galaxy>();
        public System.Collections.Generic.IEnumerable<Galaxy> NextGalaxy
        {
            get
            {
                yield return new Galaxy { Name = "Tadpole", MegaLightYears = 400 };
                yield return new Galaxy { Name = "Pinwheel", MegaLightYears = 25 };
                yield return new Galaxy { Name = "Milky Way", MegaLightYears = 0 };
                yield return new Galaxy { Name = "Andromeda", MegaLightYears = 3 };
            }

        }
        public System.Collections.Generic.IEnumerable<Galaxy> NextGalaxy1
        {

            get
            {
                ls.Add(new Galaxy { Name = "Tadpole", MegaLightYears = 400 });
                ls.Add(new Galaxy { Name = "Pinwheel", MegaLightYears = 25 });
                ls.Add(new Galaxy { Name = "Milky Way", MegaLightYears = 0 });
                ls.Add(new Galaxy { Name = "Andromeda", MegaLightYears = 3 });
                int i = -1;
                while (i++ < ls.Count - 1)
                {
                    yield return ls[i];
                }
            }
        }
    }

 调用NextGalaxy1后，结果与官方示例结果相同，还可以进一步修改NextGalaxy1，使其更容易别理解：

 public System.Collections.Generic.IEnumerable<Galaxy> NextGalaxy1
        {

            get
            {
                ls.Add(new Galaxy { Name = "Tadpole", MegaLightYears = 400 });
                ls.Add(new Galaxy { Name = "Pinwheel", MegaLightYears = 25 });
                ls.Add(new Galaxy { Name = "Milky Way", MegaLightYears = 0 });
                ls.Add(new Galaxy { Name = "Andromeda", MegaLightYears = 3 });
                int i = 0;
                while (i < ls.Count)
                {
                    yield return ls[i];
                    i++;
                }
            }
        }

 这样看来就很容易理解其含义了，更进一步说就是一边给你输出结果，一边继续给你执行代码，一举两得！

如果想中断执行，则直接用yield break即可。

代码如下：

 public System.Collections.Generic.IEnumerable<Galaxy> NextGalaxy1
        {

            get
            {
                ls.Add(new Galaxy { Name = "Tadpole", MegaLightYears = 400 });
                ls.Add(new Galaxy { Name = "Pinwheel", MegaLightYears = 25 });
                ls.Add(new Galaxy { Name = "Milky Way", MegaLightYears = 0 });
                ls.Add(new Galaxy { Name = "Andromeda", MegaLightYears = 3 });
                int i = -1;
                while (i++ < ls.Count - 1)
                {
                    yield return ls[i];
                    if (ls[i].MegaLightYears == 0)
                    {
                        yield break;
                    }

                }
            }
        }

 输出结果为：

附官方链接：http://msdn.microsoft.com/zh-cn/library/9k7k7cf0.aspx