这是 C# 4 中的协方差错误吗? [英] Is this a covariance bug in C# 4?

问题描述

在下面的一段代码中，我希望能够从 elements 隐式转换为 baseElements 因为 TBase 可以隐式转换为 IBase.

In the following piece of code I expected to be able to implicitly cast from elements to baseElements because TBase is implicitly convertible to IBase.

public interface IBase { }
public interface IDerived : IBase { }
public class VarianceBug
{
    public void Foo<TBase>() where TBase : IBase
    {
        IEnumerable<TBase> elements = null;
        IEnumerable<IDerived> derivedElements = null;
        IEnumerable<IBase> baseElements;

        // works fine
        baseElements = derivedElements;

        // error CS0266: Cannot implicitly convert type 
        //   'System.Collections.Generic.IEnumerable<TBase>' to 
        //   'System.Collections.Generic.IEnumerable<IBase>'. 
        //   An explicit conversion exists (are you missing a cast?)
        baseElements = elements;
    }
}

但是，我收到了评论中提到的错误.

However, I get the error that is mentioned in the comment.

引用规范:

A 类型 T 可以方差转换为类型 T 如果 T> 是使用变体类型参数 T<X1, …, Xn> 声明的接口或委托类型，并且对于每个变体类型参数 Xi，以下条件之一成立:

A type T<A1, …, An> is variance-convertible to a type T<B1, …, Bn> if T is either an interface or a delegate type declared with the variant type parameters T<X1, …, Xn>, and for each variant type parameter Xi one of the following holds:

• Xi 是协变的，并且存在从 Ai 到 Bi

• Xi is covariant and an implicit reference or identity conversion exists from Ai to Bi

Xi 是逆变的，并且存在从 Bi 到 Ai

Xi is contravariant and an implicit reference or identity conversion exists from Bi to Ai

Xi 是不变的，存在从 Ai 到 Bi

Xi is invariant and an identity conversion exists from Ai to Bi

检查我的代码，它似乎与规范一致:

Checking my code, it appears to be consistent with the spec:

• IEnumerable 是一种接口类型

IEnumerable 是用变体类型参数声明的

IEnumerable<out T> is declared with variant type parameters

T 是协变的

存在从 TBase 到 IBase

那么 - 这是 C# 4 编译器中的错误吗?

So - is it a bug in the C# 4 compiler?

推荐答案

Variance 仅适用于引用类型(或存在 identity 转换).不知道TBase是引用类型，除非你加上:class:

Variance only works for reference-types (or there is an identity conversion). It is not known that TBase is reference type, unless you add : class:

 public void Foo<TBase>() where TBase : class, IBase

因为我可以写一个:

public struct Evil : IBase {}

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