[IronPython] Array Access Problem

[Jim Hugunin]

Drew Moore wrote:
> Jim Hugunin wrote:
> >
> >Because of the way that value types work, there's no difference
between
> >the results of
> >  apt[0].X = 0    and   apt[0] = Point(0, apt[0].Y)
> >
> >The big question is how important it is that both of the following
are
> >equivalent.
> >  apt[0].X = 0    and    tmp = apt[0]; tmp.X = 0
> >
> Just curious, maybe I'm off base...
> Is concurrency a concern here?

The answer to this question is almost always yes no matter what the
context ;-)

> Can instances of value types be shared between threads? (Perhaps
> different languages?)

There aren't really instances of value types.  That's part of the
problem.  Value types are parts of other instances - either items in an
array or inlined fields in classes.  The things that hold value types
can be shared between threads and languages.

> If so, what if another thread is concurrently modifying a different
> member of the underlying struct?

This is a good point.  In that case there is a potential difference
between my two examples above.  The one on the left can only change the
X part of the point, but the one on the right could in theory overwrite
another thread's changes to the Y part of the point.

> If the client code 'just changes one attribute', but in actuality the
> entire structure gets written back, it may make for some very
> hard-to-pin-down bugs.
> (if this might happen, maybe immutable value types aren't so bad.)

I don't see this as too much of a problem.  If IronPython supports
apt[0].X=0, then that will only be implemented in such a way that just
the X part is modified.  If we decided to require
apt[0]=Point(0,apt[0].Y) then it should be pretty clear to the
programmer that they're replacing both fields of the point
simultaneously.

I just tried this in J# to see if there was a clever idea to steal.  J#
behaves just like the current IronPython does with value types:

		Form f = new Form();
		System.out.println(f.get_ClientSize());

		f.get_ClientSize().set_Height(10);
		System.out.println(f.get_ClientSize());

		f.set_ClientSize(new Size(100, 100));
		System.out.println(f.get_ClientSize());

{Width=292, Height=266} <-- starting size
{Width=292, Height=266} <-- height unchanged by set_Height
{Width=100, Height=100} <-- changed by setting to new Size object

-Jim