[IronPython] ValueError: cannot assign to value types

[Dino Viehland]

If you want an immediate work around I think:

type(props).Name.Info.SetValue(props, value)

might work in 2.6.  You'll need to do "import clr" for "Info" to be available
on the reflected field object.

> -----Original Message-----
> From: users-bounces at lists.ironpython.com [mailto:users-
> bounces at lists.ironpython.com] On Behalf Of Count László de Almásy
> Sent: Monday, August 10, 2009 6:07 PM
> To: Discussion of IronPython
> Subject: Re: [IronPython] ValueError: cannot assign to value types
>
> i'd feek ok about any method that at least allowed me to work around
> the current situaiton. a warning seems fine to me. even better, an
> explicit way for us to enable the behavior without a warning.
>
> On Mon, Aug 10, 2009 at 6:27 PM, Dino Viehland<dinov at microsoft.com>
> wrote:
> > Just out of curiosity - how would you feel if we enabled the behavior
> > but issued a warning when it occurred?  In v1.0 when we made this
> > decision we had no warning support and it could serve as a mitigating
> > factor in enabling it.  But it could also be too noisy.
> >
> > It's funny, I just got had to fix a bug in IronPython where they
> allow
> > this but it doesn't really work.
> >
> > Given:
> >
> > List<SomeValueType> x;
> > X[0].Bar = 42;
> >
> > fails to update the list.
> >
> >
> >> -----Original Message-----
> >> From: users-bounces at lists.ironpython.com [mailto:users-
> >> bounces at lists.ironpython.com] On Behalf Of Count László de Almásy
> >> Sent: Monday, August 10, 2009 5:20 PM
> >> To: Discussion of IronPython
> >> Subject: Re: [IronPython] ValueError: cannot assign to value types
> >>
> >> i dunno. i think this behavior needs to be re-thought for Ipy 2.6. i
> >> understand the reasons behind the decision, but the bottom line is
> >> that i'm unable to work with a perfectly valid, legal .NET assembly.
> >> it's one thing to do this as the default behavior, but to not allow
> >> the programmer to override it if that is his choice is not correct
> in
> >> my opinion.
> >>
> >> On Fri, Aug 7, 2009 at 11:04 PM, Curt
> Hagenlocher<curt at hagenlocher.org>
> >> wrote:
> >> > __setattr__ it's not a *CLR*-level instance method of the type.
> >> IronPython
> >> > just makes it look like it is.
> >> > One possibility is to simply create an entirely new Parcel from
> >> scratch,
> >> > assigning values as appropriate. For example, given the following
> C#
> >> > classes:
> >> >
> >> > public struct Point {
> >> >     public int x, y;
> >> > }
> >> > public class Line {
> >> >     public Point to, from;
> >> > }
> >> >
> >> > You can do this from Python:
> >> >
> >> > F:\tcwb\libs>ipy.exe
> >> > IronPython 2.6 Beta 2 (2.6.0.20) on .NET 2.0.50727.4927
> >> > Type "help", "copyright", "credits" or "license" for more
> information.
> >> >>>> import clr
> >> >>>> clr.AddReference('x')
> >> >>>> import Point, Line
> >> >>>> l = Line()
> >> >>>> l.to.x
> >> > 0
> >> >>>> l.to.x = 1
> >> > Traceback (most recent call last):
> >> >   File "<stdin>", line 1, in <module>
> >> > ValueError: cannot assign to value types
> >> >>>> l.to = Point(x = 1, y = 2)
> >> >>>> l.to.x
> >> > 1
> >> >>>>
> >> >
> >> > On Fri, Aug 7, 2009 at 8:40 PM, Count László de Almásy
> >> <calmasy at gmail.com>
> >> > wrote:
> >> >>
> >> >> i'm really stuck trying to change values for an object of type
> >> >> 'Parcel', which comes from the openmetaverse library i'm using.
> how
> >> >> this library is implemented i don't have control over, but the
> given
> >> >> interface is to make local changes to these Parcel objects, then
> use
> >> >> its Update() method to commit the changes. but it seems in
> >> IronPython
> >> >> i can't change these objects.
> >> >>
> >> >> >>> type(props)
> >> >> <type 'Parcel'>
> >> >>
> >> >> >>> props.Name = "Yes"
> >> >> Traceback (most recent call last):
> >> >>  File "<stdin>", line 1, in <module>
> >> >> ValueError: cannot assign to value types
> >> >>
> >> >> after hours of struggling, i found
> >> >> http://ironpython.codeplex.com/Wiki/View.aspx?title=Value%20Types
> >> >> which seems to explain why i'm being bit here. bit it also says
> >> >> "updates are still possible via instance methods on the value
> type
> >> >> itself." so isn't "__setattr__" an instance method? seems i'm not
> >> able
> >> >> to use that either:
> >> >>
> >> >> >>> props.__setattr__("Name", "Yes")
> >> >> Traceback (most recent call last):
> >> >>  File "<stdin>", line 1, in <module>
> >> >> ValueError: Attempt to update field 'Name' on value type
> 'Parcel';
> >> >> value type fields cannot be directly modified
> >> >>
> >> >> ok, so what are my options? how do i get around this? i *have* to
> be
> >> >> able to update objects of this type somehow.
> >> >>
> >> >> i can create a new object of that type, but the constructor
> doesn't
> >> >> allow me to pre-stock it with values ( since i guess it assumes
> i'll
> >> >> be able to change them later), and then of course once it's
> created
> >> i
> >> >> can't change anything.
> >> >>
> >> >> --
> >> >> Cheers, László
> >> >> _______________________________________________
> >> >> Users mailing list
> >> >> Users at lists.ironpython.com
> >> >> http://lists.ironpython.com/listinfo.cgi/users-ironpython.com
> >> >
> >> >
> >> > _______________________________________________
> >> > Users mailing list
> >> > Users at lists.ironpython.com
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> >> >
> >> >
> >>
> >>
> >>
> >> --
> >> Cheers, László
> >> _______________________________________________
> >> Users mailing list
> >> Users at lists.ironpython.com
> >> http://lists.ironpython.com/listinfo.cgi/users-ironpython.com
> >
> > _______________________________________________
> > Users mailing list
> > Users at lists.ironpython.com
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> >
>
>
>
> --
> Cheers, László
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