How can a struct inherit from a class in type parameters constrains?

Question

I saw the following line of code:

class Sample<T,U> where T:class where U: struct, T

In the case above, parameter U is value type, and it derives from reference type T.

How can that line be legal?
Also, if a value type inherits from a reference type, where is memory allocated: heap or stack?

Answer 1

Contrary to another answer, there are types beyond T=System.Object where this compiles:

class Samplewhere T:class where U:struct, T

The "T : class" constraint doesn't actually mean that T has to be a class. It means T has to be a reference type. That includes interfaces, and structs can implement interfaces. So, for example, T=IConvertible, U=System.Int32 works perfectly well.

I can't imagine this is a particularly common or useful constraint, but it's not quite as counterintuitive as it seems at first sight.

As to the more general point: as Obiwan Kenobi says, it all depends on your point of view. The CLI spec has quite a complicated explanation of this, where "derives from" and "inherits from" don't mean quite the same thing, IIRC. But no, you can't specify the base type of a value type - it's always either System.ValueType or System.Enum (which derives from System.ValueType) and that's picked on the basis of whether you're declaring a struct or an enum. It's somewhat confusing that both of these are, themselves, reference types...

Answer 2

All structs derive from the ValueType type implicitly. You cannot specify an explicit base type.
Refer to this MSDN tutorial on structs as posted by codemelt.

• When you instantiate a struct e.g. as a local variable, they are allocated on the stack (better performance)
• Classes may contain structs as members - in which case they are allocated on the heap.

Answer 3

MSDN says,

There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class object. A struct can implement interfaces, and it does that exactly as classes do.

Answer 4

Sasha Wrote:

If there is no inheritance allowed, then why is the following legal:

class Samplewhere T:class where U:struct, T

In the case above, parameter U is value type, and it derives from T -- reference type

Although that's legal from a generic contract standpoint, you'll never get any useful code that uses that class to compile, because you'll never have a type other than T=System.Object that fulfills the U constraint. You might consider that a very minor bug in the implementation of generics in C#.

Answer 5

Structs cannot inherit from anything other than System.ValueType or System.Enum. There is no way a struct can inherit from a normal reference type. So unfortunately this question cannot be answered.