NativeMethods is broken when running x64 / AnyCPU programs using DockPanel Suite. It doesn't crash, but leads to a bug that prevents windows to be undocked and docked again, as seen here:
https://github.com/AdamsLair/duality/issues/41
I have manually fixed NativeMethods according to pinvoke.net docs and it fixes the problem on my machine. Please verify and integrate the fix in your master branch.
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