你能否用一个特定类型的类来继承一个泛型类？ [英] Can you subclass a generics class with a specific typed class?

问题描述

  public abstract class GenericBase< T>< p> 
其中T：Interface1 
 {
} 
  

子类泛型与特定实现：

  public class Customer：
 GenericBase< Type1> 
  

（ Type1 执行 Interface1 ）。

我有另一个抽象基类有一个引用：

 受保护的GenericBase< Interface1> genericInstance; 
  

最后，当我试图将genericInstance赋值给基类的一个实例时，它给了我一个编译器错误，说它不能隐式地将客户转换为GenericBase< Interface1>。
$ b $ pre $ base $ .genericInstance = new Customer（） ; //编译错误

我不明白为什么我会得到这个错误if Customer 是 GenericBase< Type1> 和 Type1 implements的子类型接口1 。不是 Customer 有效地是 GenericBase< Interface1> 类型，如果它是的子类， GenericBase< Type1> ？

我假设我在这里误解了泛型。有没有办法允许这种行为？

解决方案

在C＃中，协变（派生类型为基类型）不能应用于泛型类。因此，您需要使用输出参数修饰符来应用专门标记为协变的接口>在新的 IGenericBase 接口上。

  protected IGenericBase< Interface1> genericInstance = new Customer（）; 
 
 public interface IGenericBase< out T> {} 
 
公共抽象类GenericBase< T> ：IGenericBase< T> 
其中T：接口1 {} 
 
公共接口接口1 {} 
 $ b公共类类型1：接口1 {} 
 
公共类客户： GenericBase<类型1> {} 
  

I have a generics class with a subclass that provides specific types.

public abstract class GenericBase<T>
    where T:Interface1
{
}

I subclass the generics with specific implementations:

public class Customer:
    GenericBase<Type1>

(Type1 implements Interface1).

I have another abstract base class that has a reference to this:

protected GenericBase<Interface1> genericInstance;

Finally, when I attempt to assign the genericInstance to an instance of the base class, it gives me a compiler error, saying that it "cannot implicitly convert Customer to GenericBase<Interface1>".

base.genericInstance = new Customer(); //Compiler error

I don't understand why I would get this error if Customer is a subtype of GenericBase<Type1>, and Type1 implements Interface1. Isn't Customer effectively a type of GenericBase<Interface1>, if it's a subclass of GenericBase<Type1>?

I assume I'm misunderstanding something about generics here; is there a way to allow this behavior?

解决方案

In C#, covariance (assigning a derived type to a base type) cannot be applied to generic classes. As a result, you would need to apply an interface specifically marked as covariant, using the out parameter modifier on a new IGenericBase interface.

protected IGenericBase<Interface1> genericInstance = new Customer();

public interface IGenericBase<out T> {}

public abstract class GenericBase<T> : IGenericBase<T>
    where T:Interface1 {}

public interface Interface1 {}

public class Type1 : Interface1 {}

public class Customer: GenericBase<Type1> {}

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