原则：上下文属性访问语句必须出现在消息构造语句后。否则出现访问未构造消息属性的错误。
MyMessage.PartA = something;
MyMessage(SomeContextProeprty) = someValue;
MyMessage.PartB = somethingElse;
Error!
MyMessage.PartA = something;
MyMessage(SomeContextProeprty) = someValue;
MyMessage.PartB = somethingElse;
Error!
MyMessage.PartA = something;
MyMessage.PartB = somethingElse;
MyMessage(SomeContextProeprty) = someValue;
OK!
Original: yossidahan blog "Order of operations in an assign shape"
