WPF .Net 4 - OneWayToSource binding to write-only property works on some machines! How?

Question

I recently added a OneWayToSource binding in our code to a read-only property in a view model. At the time I did not know about the change in .Net 4, that this will cause a

Answer 1

A bit of guessing.

There's plausible that after this change in net 4.0 they decided to make it more consistent in .net 4.5: if getter exists property is reread back to UI, otherwise nothing terrible happens and no exception is thrown.

If so and you have .net 4.5 installed you can get this behavior instead of the expected one.
We've met cases when updates to 4.5 propagated to 4.0 silently.