I have very appreciated your interest in my question and I'm very sorry that I didn't reply to you for long time.
I don't know exactly what happens.
I tried to reproduce the problem from scratch and I didn't succed. [I created two simple classes, one an inherited interface from the other and declared the global variables in the two mentioned ways and no problem in either ways never arise.]
So I don't know what to think.
But more then one time I saw that just rebuild the project, importing again all the modules save from "unexplicable" problems.
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