.net - What is the difference between member val and member this in F#? -

when created class containing generic, mutable .net stack in f# example below, stack ignores push onto it.

open system.collections.generic  type interp(code: int array) =     member val pc = 0 get, set     member this.stack: stack<int> = new stack<int>()      member this.code = code  let interp = interp([|1;2;3|]) interp.stack.push(1) printfn "%a" interp.stack // prints "seq[]" wat?! 

yet if make stack mutable via property:

open system.collections.generic  type interp(code: int array) =     member val pc = 0 get, set     member val stack: stack<int> = new stack<int>() get, set      member this.code = code  let interp = interp([|1;2;3|]) interp.stack.push(1) printfn "%a" interp.stack // prints "seq[1]" 

everything magically works i'd expect.

what on earth going on here? understanding of immutability previous languages (c# mostly) though stack in first example immutable member, immutablity should go far reference (aka shouldn't able reassign stack itself). should still able push values to/from it. missing, , if trying mutate stack wrong thing, why doesn't throw exception or compile error?

if try compile first version, , use e.g. reflector decompile c#, you'll see stack member defined this:

public class interp {     public stack<int> stack     {         { return new stack<int>(); }     }      // other members omitted clarity... } 

as can see, valid c# code, not want.

the second version cross-compiles this:

public class interp {     internal int[] code;     internal stack<int> stack@;      public interp(int[] code) : this()     {         this.code = code;         this.stack@ = new stack<int>();     }      public stack<int> stack     {         { return this.stack@; }         set { this.stack@ = value; }     }      // other members omitted clarity... } 

this seems more you'd want property do.