不可变属性的存取器将总是具有值引用，因而调用的任何一方都不需要防御性复制，这使得它们效率更高。
Accessors for immutable properties will always have value semantics and do not need the defensive copying on either side of the call, making them more efficient.
另一方面，紧密耦合的调用和被调用系统会假定双方都已经拥有了让对方信任所需要的事项。
On the other hand, tightly calling and called coupled systems assume that both have the knowledge of what each requires to trust one another.
在另一方面，调用类可能需要临时许可权来完成另一个拥有那些许可权的类中的操作。
On the other hand, a calling class may need temporary permissions to complete an operation in another class that has those permissions.
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