T的通用约束同时是引用类型和值类型？

Question

I have a problem with understanding how generic constraints work. I think I am missing something important here. I have enclosed my questions in the comments and would be grateful for providing some explanation.

//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

There's nothing wrong with that. Let's look at the definition of the constraints on the type parameters :

• T : class - The type argument T must be a reference type, including any class, interface, delegate, or array type.
• U : struct - The type argument U must be a value type.
• U : T - The type argument U must be or derive from the class T.

So all you need to do is find a value type that derives from a reference type. At first that might sound impossible, but if you think a bit harder you will remember that all structs derive from the class object , so this works fine for both your examples:

new C<object, int>();

However if you swap struct and class then it won't compile:

// 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
{
}