C#类型转换错误,尽管通用约束

为什么,对于“必须从A继承”的类P的类型参数T的通用约束,第一次调用成功但第二次调用失败,注释中详细说明了类型转换错误：

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>' */
    }
}

通用约束不应该确保List< T>.确实是ICollection< A>？

这是C#在泛型类型上缺少 covariance的一个例子(C#确实支持数组协方差). 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.

请参阅C# 4.0: Covariance and Contravariance：

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