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：
