yield是一个语句不对应IL指定,编译后可以看到yield会变成一个IEnumerator(IEnumerable)实现.
参考:
http://www.cnblogs.com/montaque/archive/2005/04/21/142844.html
http://www.microsoft.com/china/msdn/library/langtool/vcsharp/CreElegCodAnymMeth.mspx?mfr=true
http://www.cnblogs.com/sugarcane/archive/2009/09/12/1565453.html
