无法将通用类转换为通用接口，为什么?

问题描述

我有一个从IRepository<TObject>继承的IRepository接口.我也有一个从S QLRepository<TObject>继承的SqlRepository类，该类又实现了IRepository<TObject>.为什么不能将SqlRepository的实例实例化为IRepository?

I have an IRepository interface that inherits from IRepository<TObject>. I also have a SqlRepository class that inherits from SQLRepository<TObject>, which in turn implements IRepository<TObject>. Why can't I instantiate a instance of SqlRepository as an IRepository?

public class MyObject : IObject {
    ...
}

public interface IRepository<TObject> where TObject : IObject, new() {
    ...
}

public interface IRepository : IRepository<MyObject> { }

public class SqlRepository<TObject> : IRepository<TObject> where TObject : IObject, new() {
    ...
}

public class SqlRepository : SqlRepository<MyObject> { }

public class Controller {
    private IRepository _repository;

    public Controller() {
        _repository = new SqlRepository();
    }
}

上面的示例在尝试将新的SqlRepository分配给Controller类中的_repository时失败，并显示以下错误消息.

The example above fails when trying to assign a new SqlRepository to _repository in the Controller class, with the following error message.

Argument '1': cannot convert from 'SqlRepository' to 'IRepository'

我没有掌握继承的基本原理，请帮忙.

Which basic principle of inheritance have I failed to grasp, please help.

推荐答案

因为IRepository是IRepository<MyObject>，而不是相反.

基本上:

SqlRepository是SqlRepository<MyObject>
这是IRepository<MyObject>.
为此，您应该根据自己的意愿从IRepository继承IRepository<TObject>或使SqlRepository实现IRepository.

SqlRepository is a SqlRepository<MyObject>
which is an IRepository<MyObject>.
To make that work, you should either inherit IRepository<TObject> from IRepository or make SqlRepository implement IRepository depending in your intention.

此问题根本不是泛型特有的.假设:

This issue is not generics-specific at all. Assume:

interface IMovable { }
interface IDrivable : IMovable { }
class Ball : IMovable { }

很明显，Ball不是IDrivable而是IMovable.

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