Coroutine and Actor model

Unity3D中C#实现协程需要注意一下几点：

# An iterator member's signature cannot contain any out or ref parameters.

# An iterator cannot contain a return statement .

# An iterator may not contain any unsafe code.

# A finally block may not contain a yield return or a yield break statement.

# A yield return statement may not appear in a try block that has a catch block.

# A yield return statement may not appear in a catch block (yield break is OK)

解答这点的原因：

Iterator Blocks, Part One

Iterator Blocks, Part Two: Why no ref or out parameters?

Iterator Blocks, Part Three: Why no yield in finally?

Iterator Blocks, Part Four: Why no yield in catch?

Iterator Blocks, Part Five: Push vs Pull

Iterator Blocks, Part Six: Why no unsafe code?

Iterator Blocks, Part Seven: Why no anonymous iterators?

参考文献：

Coroutine: http://en.wikipedia.org/wiki/Coroutine

Actor model: http://en.wikipedia.org/wiki/Actor_model

Microsoft Asynchronous Agents Library: http://msdn.microsoft.com/en-us/library/dd492627.aspx