C# 使用接口的隐式运算符
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implicit operator using interfaces
提问by Michael Meadows
I have a generic class that I'm trying to implement implicit type casting for. While it mostly works, it won't work for interface casting. Upon further investigation, I found that there is a compiler error: "User-defined conversion from interface" that applies. While I understand that this should be enforced in some cases, what I'm trying to do does seem like a legitimate case.
我有一个通用类，我正在尝试为其实现隐式类型转换。虽然它主要工作，但它不适用于界面转换。经过进一步调查，我发现有一个编译器错误：“用户定义的接口转换”适用。虽然我知道这应该在某些情况下强制执行，但我正在尝试做的事情似乎是合法的。
Here's an example:
下面是一个例子：
public class Foo<T> where T : IBar
{
private readonly T instance;
public Foo(T instance)
{
this.instance = instance;
}
public T Instance
{
get { return instance; }
}
public static implicit operator Foo<T>(T instance)
{
return new Foo<T>(instance);
}
}
Code to use it:
使用它的代码：
var concreteReferenceToBar = new ConcreteBar();
IBar intefaceReferenceToBar = concreteReferenceToBar;
Foo<ConcreteBar> concreteFooFromConcreteBar = concreteReferenceToBar;
Foo<IBar> fooFromConcreteBar = concreteReferenceToBar;
Foo<IBar> fooFromInterfaceBar = intefaceReferenceToBar; // doesn't work
Does anyone know a workaround, or can anyone explain in a satisfactory way why I shuouldn't be able to cast interfaceReferenceToBarimplicitly to Foo<IBar>, since in my case it is not being converted, but only contained within Foo?
有谁知道解决方法，或者谁能以令人满意的方式解释为什么我不能interfaceReferenceToBar隐式转换为Foo<IBar>，因为在我的情况下它没有被转换，而只包含在 Foo 中？
EDIT:It looks like covariance might offer salvation. Let's hope the C# 4.0 specification allows for implicit casting of interface types using covariance.
编辑：看起来协方差可能会带来救赎。让我们希望 C# 4.0 规范允许使用协方差隐式转换接口类型。
采纳答案by Adam Hughes
The reason you can't do this is because it is specifically forbidden in the C# language specification:
你不能这样做的原因是因为它在 C# 语言规范中是明确禁止的：
A class or struct is permitted to declare a conversion from a source type S to a target type T provided all of the following are true:
- ...
- Neither S nor T is
objector an interface-type.
如果满足以下所有条件，则允许类或结构声明从源类型 S 到目标类型 T 的转换：
- ...
- S 和 T 都不是
object或接口类型。
and
和
User-defined conversions are not allowed to convert from or to interface-types. In particular, this restriction ensures that no user-defined transformations occur when converting to an interface-type, and that a conversion to an interface-typesucceeds only if the object being converted actually implements the specified interface-type.
不允许用户定义的转换从或转换为 interface-types。特别是，此限制确保在转换为interface-type时不会发生用户定义的转换，并且仅当被转换的对象实际实现了指定的 interface-type 时，才能成功转换为 interface-type。
