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For some reason, the Content property of NSTreeController is typed as NSTreeController. From apple's docs, it looks like it should just be NSObject. As a result, if you try to use this property, you get an InvalidCastException.
I could not come up with a simple sample to reproduce the error, but if needed, I could make a small project and attach it to the bug.
The most recent stable installers (as of June 18, 2014) still have this problem for NSTreeController (at least in MonoMac).
As an easy workaround, cast to NSObjectController, then access its Content property instead.