What is the purpose of Yield and how does it work?

Question

While perusing Twitter I spotted a tweet by a Game Developer I follow that just said;

^{@ChevyRay 2:44 AM - 5 Jul 2016}
i give you: the stupidest 8 lines of code i've ever written that is actually used by my game's code

 static IEnumerable<int> RightAndleft { get { yield return 1; yield return -1; } } 

Immediately I looked at it and thought to myself what does this do? I'm sure the post was meant as an in-joke but I just didn't get it. So with that I went and researched yield but that didn't really answer my question.

I wondered if anyone here could shed some light on firstly what it's doing and also why.

Answer 1

The compiler turns this code into an enumerator. You can use this enumerator to iterate over the sequence {1, -1} :

foreach(var i in ClassName.RightAndLeft)
    Console.WriteLine(i);

results in

1
-1

Note that this property does not have two return statements and return type int . It returns an IEnumerable<int> , a sequence containing 1 and -1 .

See yield keyword for more information.

One important part to note is that the second line yield return -1; is executed after the first value has been printed out by Console.WriteLine in this example.

---

Since the poster is a game developer, he probably uses these two values as direction indicators and this enumeration to, well, enumerate all possible directions or something like that.

Answer 2

It returns IEnumerable<int> object that contains 1 and -1 .

See https://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx