C＃类型转换错误，尽管通用约束 [英] C# Type Conversion Error Despite Generic Constraint

问题描述

为什么，对类P的类型参数T必须继承自A的泛型约束，第一次调用成功，但第二次调用失败，类型转换错误详述在注释中：

Why, with a generic constraint on type parameter T of class P of "must inherit from A", does the first call succeed but the second call fail with the type conversion error detailed in the comment:

abstract class A { }

static class S
{
    public static void DoFirst(A argument) { }
    public static void DoSecond(ICollection<A> argument) { }
}

static class P<T>
    where T : A, new()
{
    static void Do()
    {
        S.DoFirst(new T());             // this call is OK

        S.DoSecond(new List<T>());      // this call won't compile with:

        /* cannot convert from 'System.Collections.Generic.List<T>'
           to 'System.Collections.Generic.ICollection<A>' */
    }
}

List< T> 是 ICollection< A> ？ b $ b

Shouldn't the generic constraint ensure that List<T> is indeed ICollection<A>?

推荐答案

这是C＃缺少 支持数组协方差）的rel =nofollow noreferrer>协方差。 C＃4将在接口类型上添加此功能，并且还将更新多个BCL接口类型以支持它。

This is an example of C#'s lack of covariance on generic types (C# does support array covariance). C# 4 will add this feature on interface types and also will update several BCL interface types to support it as well.

请参阅 C＃4.0：协方差和对比度：

在本文中，我将尝试覆盖一个
的C＃4.0创新。
新特性之一是协方差和
违反类型参数，
现在由通用委托
和通用接口支持。首先让我们
看看这些词是什么意思：）

In this article I’ll try to cover one of the C# 4.0 innovations. One of the new features is covariance and contravariance on type parameters that is now supported by generic delegates and generic interfaces. First let’s see what does these words mean :)

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