有时候您会想要在呼叫动作方法之前或是动作方法执行之后执行逻辑。
Sometimes you want to perform logic either before an action method is called or after an action method runs.
如果在呼叫项目上找不到资源，则会改以搜寻逻辑树状结构内的父项目，接著搜寻应用程式、布景主题，最后则会搜寻系统资源。
If the resource is not found on the calling element, the parent element in the logical tree is searched next, then the application, then themes, and finally system resources.
复杂度较低的提供者可能只会在代表查询的运算式树状架构中检查一个方法呼叫运算式，并让查询的其馀逻辑在其他地方处理。
A less complex provider might examine only one method call expression in the expression tree that represents the query, and let the remaining logic of the query be handled elsewhere.
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