Are structs 'pass-by-value'?

Question

I\'ve recently tried to create a property for a Vector2 field, just to realize that it doesn\'t work as intended.

public Vector2 Position { get;         


        

Answer 1

Every storage location of a structure type holds all the fields, private and public, of that struct. Passing a parameter of a structure type entails allocating space for that structure on the stack and copying all of the fields from the structure to the stack.

With regard to working with structures stored within a collection, using mutable structs with the existing collections generally requires reading out a struct to a local copy, mutating that copy, and the storing it back. Assuming MyList is a List, and one wants to add some local variable z to MyList[3].X:

  Point temp = MyList[3];
  temp.X += Z;
  MyList[3] = temp;

This is mildly annoying, but is often cleaner, safer, and more efficient than using immutable structs, way more efficient than immutable class objects, and way safer than using mutable class objects. I'd really like to see compiler support for a better way for collections to expose value-type elements. There are ways of writing efficient code to handle such exposure with good semantics (e.g. a collection object could react when elements were updated, without requiring those elements to have any link to the collection) but the code reads horribly. Adding compiler support in a manner conceptually similar to closures would allow efficient code to also be readable.

Note that contrary to what some people claim, a structure is fundamentally different from a class-type object, but for every structure type there is a corresponding type, sometimes referred to as a "boxed structure", which derives from Object (see the CLI (Common Language Infrastructure) specification, sections 8.2.4 and 8.9.7). Although the compiler will implicitly convert any struct into its corresponding boxed type when necessary to pass it to code that expects a reference to a class-type object, will allow references to boxed structs to have their contents copied into real structs, and will sometimes allow code to work with boxed structs directly, boxed structs behave like class objects, because that's what they are.