如何解决这个问题?无效的方差:类型参数'T'必须是有效目不暇接 [英] How to fix this error? Invalid variance: The type parameter 'T' must be invariantly valid on
问题描述
我在编译时以下错误消息:
I'm having the below error message at compile time:
无效的差异:类型参数'T'必须是'ConsoleApplication1.IRepository目不暇接有效。 '。'GETALL()T'是协变的
"Invalid variance: The type parameter 'T' must be invariantly valid on 'ConsoleApplication1.IRepository.GetAll()'. 'T' is covariant."
和下面的是我的代码:
class Program
{
static void Main(string[] args)
{
IRepository<BaseClass> repository;
repository = new RepositoryDerived1<Derived1>();
Console.ReadLine();
}
}
public abstract class BaseClass
{
}
public class Derived1 : BaseClass
{
}
public interface IRepository<out T> where T: BaseClass, new()
{
IList<T> GetAll();
}
public class Derived2 : BaseClass
{
}
public abstract class RepositoryBase<T> : IRepository<T> where T: BaseClass, new()
{
public abstract IList<T> GetAll();
}
public class RepositoryDerived1<T> : RepositoryBase<T> where T: BaseClass, new()
{
public override IList<T> GetAll()
{
throw new NotImplementedException();
}
}
我需要的是能够使用上述这样的类:
What I would need is to be able to use the above class like this:
IRepository库;
IRepository repository;
或
RepositoryBase库;
RepositoryBase repository;
然后我希望能够分配是这样的:
Then I'd like to be able to assign something like this:
资料库=新RepositoryDerived1();
repository = new RepositoryDerived1();
但它给人的IRepository类编译时错误
But it gives compile time error on the IRepository class.
如果。我删除从IRepository类的OUT的关键字,它给了我一个错误
If I remove the "out" keyword from the IRepository class, it gives me another error that
RepositoryDerived1不能被转换为IRepository。
"RepositoryDerived1" can not be converted to "IRepository".
为什么,以及如何解决它?
Why and how to fix it?
感谢
推荐答案
的IList< T>
不是协变的。如果你改变了的IList< T>
到的IEnumerable< T>
,并删除:
IRepository<新的()
约束> IList<T>
is not covariant. If you change the IList<T>
to IEnumerable<T>
, and remove the : new()
constraint from IRepository<out T>
(as the abstract base class doesn't satisfy that) it'll work:
public interface IRepository<out T> where T : BaseClass
{
IEnumerable<T> GetAll();
}
public abstract class RepositoryBase<T> : IRepository<T> where T : BaseClass, new()
{
public abstract IEnumerable<T> GetAll();
}
public class RepositoryDerived1<T> : RepositoryBase<T> where T : BaseClass, new()
{
public override IEnumerable<T> GetAll()
{
throw new NotImplementedException();
}
}
这篇关于如何解决这个问题?无效的方差:类型参数'T'必须是有效目不暇接的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!