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

问题描述

在下面的一段code的我希望能暗中从元素投到 baseElements 因为 TBASE 隐式转换为 IBase的。

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;
    }
}

不过，我得到了在评论中提到的错误。

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

从规格报价：

A型 T＆LT; A1，...，一个＆GT; 是方差 - 转换为一个类型 T＆LT; B1，...，BN＆GT; 如果 T 是一个接口或变异类型参数 T＆LT; X1，...，Xn有害＆GT; ，并为每个变量类型参数曦以下情况之一成立：

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:

  

• 曦是协变的和隐式引用或标识转换从艾存在碧

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

曦是逆变和隐式引用或标识转换从碧存在艾

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

曦是不变和身份转换，从艾存在碧

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

检查我的code，这似乎是与规范是一致的：

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

• 的IEnumerable＆LT;出T＆GT; 是一个接口类型

的IEnumerable＆LT;出T＆GT; 声明与变异类型参数

T 是协变

隐式引用转换从 TBASE 存在 IBase的

所以 - 它是在C＃编译器4的错误

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

推荐答案

方差仅适用于引用类型（或出现的标识的转换）。它不知道， TBASE 是引用类型，除非添加：类：

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：

public struct Evil : IBase {}

这篇关于这是在C＃4协方差错误？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！