T的通用约束同时是引用类型和值类型？

Question

我对理解泛型约束如何工作有一个问题.我想我在这里缺少一些重要的东西.我在评论中附上了我的问题,并希望提供一些解释.

//1st example:

class C <T, U>
    where T : class
    where U : struct, T
{
}
//Above code compiles well, 
//On first sight it looks like U might be reference type and value type
//at the same time. The only reason I can think of, is that T may be an 
//interface which struct can implement, Am I correct?

//2nd example

class CC<T, U>
    where T : class, new ()
    where U : struct, T
{
}

//I added also a reguirement for parameterless constructor
//and, much to my surprise, it still compiles what is
//a bit inexplicable for me.
//What 'U' would meet the requirement to be 
//value type, reference type and have a contructor at the same time?

Answer 1

这没什么不对.让我们看一下类型参数约束的定义:

• T : class - 类型参数T必须是引用类型,包括任何类,接口,委托或数组类型.
• U : struct - 类型参数U必须是值类型.
• U : T - 类型参数U必须是或来自类T.

所以你需要做的就是找到一个从引用类型派生的值类型.起初可能听起来不可能,但是如果你觉得有点难,你会记得所有的结构派生自这个类object,所以这适用于你的两个例子:

new C<object, int>();

但是如果换成struct和class那么它将无法编译:

// Error - Type parameter 'T' has the 'struct' constraint so 'T'
//         cannot be used as a constraint for 'U'
class C<T, U>
    where T : struct
    where U : class, T
{
}