[英]Casting generic type with interface constraint
I have the following classes and interfaces 我有以下类和接口
public interface IFoo {}
public class Foo : IFoo {}
public interface IWrapper<T> where T : IFoo {}
public class Wrapper<Foo> : IWrapper<Foo> {}
How can I cast Wrapper<Foo>
to IWrapper<IFoo>
? 如何将
Wrapper<Foo>
投射到IWrapper<IFoo>
? An exception is raised when using Cast (InvalidCastException) as I get null when using as. 使用Cast(InvalidCastException)时引发异常,因为使用as时为null。
Thanks for the help! 谢谢您的帮助!
UPDATE UPDATE
Here is a more concrete example: 这是一个更具体的示例:
public interface IUser {}
public class User : IUser {}
public interface IUserRepository<T> where T : IUser {}
public class UserRepository : IUserRepository<User> {}
Now I need to be able to do something like this: 现在,我需要能够执行以下操作:
UserRepository up = new UserRepository();
IUserRepository<IUser> iup = up as IUserRepository<IUser>;
I'm using .net 4.5. 我正在使用.net 4.5。 Hope this helps.
希望这可以帮助。
From your edit, you actually want: 通过编辑,您实际上想要:
public interface IUserRepository<out T> where T : IUser {}
public class UserRepository : IUserRepository<User> {}
then you can do: 那么您可以执行以下操作:
IUserRepository<IUser> iup = new UserRepository();
note you can only add add the out
modifier to the type parameter T
if it appears in the output position everywhere in the definition of IUserRepository
eg 注意,只有在
IUserRepository
定义中的输出位置中都出现out
修饰符时,才可以将out
修饰符添加到类型参数T
中,例如
public interface IUserRepository<out T> where T : IUser
{
List<T> GetAll();
T FindById(int userId);
}
if it appears anywhere in the input position, such as a method parameter or property setter it will fail to compile: 如果它出现在输入位置的任何地方,例如方法参数或属性设置器,它将无法编译:
public interface IUserRepository<out T> where T : IUser
{
void Add(T user); //fails to compile
}
Wrapper<Foo>
needs to be Wrapper<IFoo>
. Wrapper<Foo>
必须是Wrapper<IFoo>
。 Then you should be able to cast it. 然后,您应该可以投射它。 And it needs to implement the interface too.
并且它也需要实现接口。
The cast below works... I don't think you can cast an objects generic type parameter to a different type (ie IWrapper<Foo>
to IWrapper<IFoo>
). 下面的转换有效...我认为您不能将对象的泛型类型参数转换为其他类型(即
IWrapper<Foo>
到IWrapper<IFoo>
)。
void Main()
{
var foo = new Wrapper();
var t = foo as IWrapper<IFoo>;
t.Dump();
}
public interface IFoo {}
public class Foo : IFoo {}
public interface IWrapper<T> where T : IFoo {}
public class Wrapper : IWrapper<IFoo> {}
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