如何创建从通用类扩展的抽象类

Question

There is a class defined in the following way.

class MvxViewController<TViewModel> where TViewModel : class, IMvxViewModel

I use this class by simply extending, for example:

class MyController : MvxViewController<MyViewModel>

As I have repeating tasks in every controller I would like to create an abstract class that extends from MvxViewController which in turn is extended by every of my controllers (where needed). However, I'm not able to create this abstract class because the syntax is always wrong. Some things I tried so far:

a) abstract class BaseViewController<T> : MvxViewController<T>
b) abstract class BaseViewController<T> where T : MvxViewController<T>
c) abstract class BaseViewController<T> : MvxViewController<T> where T : class

and others... but I'm still not able to get the syntax correct. What is wrong here?

Answer 1

The constraint on the abstract classes needs to be where T : class, IMvxViewModel to match the base. For example:

abstract class BaseViewController1<T> : MvxViewController<T> where T : class, IMvxViewModel

Answer 2

Because you have constrained the generic parameter for MvxViewController to both class and IMvxViewModel , you must also extend those same constraints to your abstract class' generic parameter.

abstract class BaseViewController<T> : MvxViewController<T> where T : class, IMvxViewModel

Constraints are NOT inherited and must be explicitly typed.

Answer 3

You need to put the constraints the MvxViewController<TViewModel> places on TViewModel on the type argument of your derived class, as follows:

abstract class BaseViewController<T> : MvxViewController<T> where T : class, IMvxViewModel {
    ...
}

This is required because C# compiler must check that TViewModel argument meets its constraints in the scenario when you derive another class from it.