Sunday, March 07, 2010

IronPython and callback delegates with 'out' or 'ref' parameters -- the obvious way doesn't work

The actual how-to is given here.

The only on-line documentation I could find that mentioned this has a

TODO - Delegates with out/ref parameters

about halfway down.

So, experiment time. Build callback.dll containing (or the same with 'out'):

and call it from IronPython 2.6:

which yields

Array[object]((<System.IntPtr object at 0x000000000000002B [42]>, False))

In sum -- it fails silently; the direct return value is fine, but mutating the actually by-value parameter does nothing.

Replacing the bool with int, and using non-default values shows that the value of a ref parameter is passed in correctly, it just can't be passed out with direct assignment.

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