[IronPython] ValueError: cannot assign to value types

[Count László de Almásy]

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
>> _______________________________________________
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>
>
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-- 
Cheers, László