The role of yield keyword is the current elements of the collection to return immediately, examples:
Breakpoints can be seen, the console displays a set of each element will process the query to fetch the collection elements.
In fact, yield return is "syntactic sugar", its essence is to generate a query of the class that implements the generic IEnumerable and IEnumerator, non-generic interface, so it can be iterative (looping through).
1. The return value must be IEnumerable, IEnumerable <T>, IEnumerator, IEnumerator <T>
2. anonymous method can not be used yield, method parameters can not be used ref, in or out
yield break Usage: termination acquiring the collection element
