[英]Casting to a generic class with interface
Updated with input from MarcinJuraszek 更新了MarcinJuraszek的输入
I have a feeling I'm bumping into a co / contra variance problem here, but I'm not sure I understand how to fix it. 我有一种感觉,我在这里碰到了一个共同/逆向差异问题,但我不确定我是否理解如何修复它。 I have a class like this: 我有一个这样的课:
public interface ISomeClass<TEnum, out S>
{
TEnum Dim { get; }
IEnumerable<S> Inc { get; }
}
public class SomeClass<TEnum, S> : ISomeClass<TEnum, S>
where TEnum : struct, IConvertible
where S : IMyInterface
{
public TEnum Dim { get; set; }
public IEnumerable<S> Inc { get; set; }
}
and I have a class that implements IMyInterface
我有一个实现IMyInterface
的类
public class MyImplementation : IMyInterface
{
}
And, of course, I have a class with a SomeClass
property: 当然,我有一个带有SomeClass
属性的类:
public class MyContainer<TEnum> where TEnum : struct, IConvertible
{
public SomeClass<TEnum, IMyInterface> MyProp { get; set; }
}
Now my problem is that I can't assign a SomeClass<MyEnum, MyImplementation>
to the MyProp
property because I get an InvalidCastException
at runtime complaining that it can't cast the SomeClass<MyEnum, MyImplementation>
to SomeClass<MyEnum, IMyInterface>
. 现在我的问题是我无法将SomeClass<MyEnum, MyImplementation>
分配给MyProp
属性,因为我在运行时收到InvalidCastException
,抱怨它无法将SomeClass<MyEnum, MyImplementation>
InvalidCastException
为SomeClass<MyEnum, MyImplementation>
SomeClass<MyEnum, IMyInterface>
。
How do I work around this? 我该如何解决这个问题?
Example, this doesn't compile: 例如,这不编译:
var c = new MyContainer<MyEnum>();
c.MyProp = new SomeClass<MyEnum, MyImplementation>();
Here's a dot net fiddle 这是一个小点网小提琴
You can make it work by having your generic type parameter invariant (either covariant or contravariant, depending on its members). 您可以通过使泛型类型参数不变(协变或逆变,取决于其成员)来使其工作。 However, in C# you can only declare generic parameters invariant on interface, so you'd have to declare another interface: 但是,在C#中,您只能声明接口上不变的泛型参数,因此您必须声明另一个接口:
public interface ISomeClass<TEnum, in S>
{
}
public class SomeClass<TEnum, S> : ISomeClass<TEnum, IMyInterface>
where TEnum : struct, IConvertible
where S : IMyInterface
{
}
public class MyContainer<TEnum> where TEnum : struct, IConvertible
{
public ISomeClass<TEnum, IMyInterface> MyProp { get; set; }
}
That would make following code compile: 这将使以下代码编译:
var container = new MyContainer<DayOfWeek>();
container.MyProp = new SomeClass<DayOfWeek, MyImplementation>();
Another possible solution would be to use another interface, where S
generic type parameter doesn't exist: 另一种可能的解决方案是使用另一个接口,其中S
泛型类型参数不存在:
public interface ISomeClass<TEnum>
where TEnum: struct, IConvertible
{
}
public class SomeClass<TEnum, S> : ISomeClass<TEnum>
where TEnum : struct, IConvertible
where S : IMyInterface
{
}
public class MyContainer<TEnum> where TEnum : struct, IConvertible
{
public ISomeClass<TEnum> MyProp { get; set; }
}
Bonus - as of why it doesn't work: 奖金 - 为什么它不起作用:
Let's imagine that your code compiles, and you can assign MyClass<T>
to MyClass<IT>
as long as T
implements IT
. 让我们假设您的代码编译,只要T
实现IT
,您就可以将MyClass<T>
分配给MyClass<IT>
。 You could have following class: 你可以有以下课程:
class MyClass<T>
{
public List<T> MyProp { get; set; }
}
And do 并做
MyClass<IMyInterface> instance = new MyClass<MyInterfaceImplementation>();
with that instance.MyProp
would be List<MyInterfaceImplementation>
but you had access to it as if it was List<IMyInterface>
so you could try adding element of MyOtherInterfaceImplementation
which would crash at runtime. 使用该instance.MyProp
List<MyInterfaceImplementation>
将是List<MyInterfaceImplementation>
但您可以访问它,就像它是List<IMyInterface>
因此您可以尝试添加MyOtherInterfaceImplementation
元素,这会在运行时崩溃。 Not fun. 不好玩。
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