该域是由已声明的成员可访问性和直接包含类型的可访问域这二者共同确定的。
Which is determined by both the declared accessibility of the member and the accessibility domain of the immediately containing type.
如果在成员声明中未指定访问修饰符，则使用默认的可访问性。
If no access modifier is specified in a member declaration, a default accessibility is used.
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