泛型如何实现结构？

Question

I was thinking about this. classes are obviously passed around by ptr. I suspect structs are passed around by copying it but i don't know for sure. (it seems like a waste for an int array to have every element a ptr. and passing ptrs for ints)

But thinking about it, List<MyStruct> can not know the size of my struct. What happens when i do this? Are there multiple copies of "List`1" and every time i use it with a storage size it does not have it creates a new implementation? (adjusting for the new offsets of T and such).

That could make sense since the source would be in the CIL inside of a DLL. But i am completely guessing, how is it done? Perhaps a reference or page # to the ECMA standards?

Answer 1

Generics use the concept of open and closed generic types: A parametrized generic class definition (ie List<T> ) is an open generic type of which the runtime generates a closed generic type for each different use you have in your code, ie a different type is created for List<int> and for List<MyStruct> - for each closed generic type the size and type of T is known at run-time.

Clarification from MSDN :

When a generic type or method is compiled into Microsoft intermediate language (MSIL), it contains metadata that identifies it as having type parameters. How the MSIL for a generic type is used differs based on whether the supplied type parameter is a value type or reference type.

When a generic type is first constructed with a value type as a parameter, the runtime creates a specialized generic type with the supplied parameter or parameters substituted in the appropriate locations in the MSIL. Specialized generic types are created one time for each unique value type that is used as a parameter.

Generics work somewhat differently for reference types. The first time a generic type is constructed with any reference type, the runtime creates a specialized generic type with object references substituted for the parameters in the MSIL. Then, every time that a constructed type is instantiated with a reference type as its parameter, regardless of what type it is, the runtime reuses the previously created specialized version of the generic type. This is possible because all references are the same size.

Answer 2

The CLR compiles 1 version of the generic class and uses it for all reference types. It also compiles 1 version for every value type usage to optimize for performance.