C# 5.0 新特性——Async和Await使异步编程更简单

http://www.cnblogs.com/zhili/archive/2013/05/15/csharp5asyncandawait.html

http://blog.zhaojie.me/2012/04/exception-handling-in-csharp-async-await-2.html

https://msdn.microsoft.com/zh-cn/library/hh191443.aspx