这是在一个组件，不引用的定义。
ValueType' is defined in an assembly that is not referenced.
应用之处：您可以在一个组件或者导入的3个阶段应用该限定符。
Applied on: You can apply the qualifier at three levels of a component or import.
这种技术有助于敏捷开发——不需要创建许多只在一个组件中使用的具体类。
This technique helps in agile development, without the need to create many concrete classes that will be used only in one component.
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