validation
http://weblogs.asp.net/scottgu/archive/2010/01/15/asp-net-mvc-2-model-validation.aspx
http://blog.janjonas.net/2011-10-23/asp_net-mvc_3-strongly-typed-authorize-attribute-multiple-users-roles
http://www.cnblogs.com/jams742003/archive/2010/03/23/1692913.html