Why do I have to copy “this” when using LINQ in a struct (and is it OK if I do)?

Question

The code belows contains a simple LINQ query inside an immutable struct.

struct Point
{
   static readonly List NeighborIndexes;
   //and ot         


        

Answer 1

Instance methods on structs are called with a reference to this – a hidden ref parameter.
This is why struct methods are able to mutate the structs they're called on.

When you use this (or any other local variable / parameter) inside a lambda expression or LINQ query, the compiler turns it into a field on a compiler-generate closure class.

The CLR does not support ref fields, so it would be impossible for the captured this to work the same way as a regular this. (this is also the reason that you can't use ref parameters inside lambdas)

Iterator methods have the same issue – they are compiled into a hidden enumerator class, and all variables or parameters become fields in the class (this is why iterators cannot take ref parameters).
However, for iterators, C# made the opposite decision. Inside an iterator, you can use this, but it will be copied to a field on the enumerator class.
This means that if you mutate a struct inside an iterator, the mutations will not happen to the caller's copy.