A generic delegate that is called back when an object should publish an instance of a service.
在对象应发布服务实例时回调的泛型委托。
We simply delegate to these generic providers.
我们只需委托这些通用提供程序就可以了。
A contravariant delegate can be assigned another delegate of the same type, but with a less derived generic type parameter.
可以向逆变委托分配同一类型的其他委托，但需使用派生程度较小的泛型类型参数。
A covariant delegate can be assigned another delegate of the same type, but with a more derived generic type parameter.
可以向协变委托分配同一类型的其他委托，但需使用派生程度较大的泛型类型参数。
A type can be declared contravariant in a generic interface or delegate if it is used only as a type of method arguments and not used as a method return type.
在泛型接口或委托中，如果类型形参仅用作方法返回类型，而不用于方法实参，则可声明为协变的。
A type can be declared contravariant in a generic interface or delegate if it is used only as a type of method arguments and not used as a method return type.
在泛型接口或委托中，如果类型形参仅用作方法返回类型，而不用于方法实参，则可声明为协变的。
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