To convert a derived class to a base class, you can use "upcast" (note that this cannot fail!):
1 2 3 4
> let rnd = new System.Random();; val rnd : System.Random > let o = (rnd :> obj);; val o : obj
To do it the other way, you can use "downcast" (which can fail!):
1 2 3 4 5 6 7 8
> let rndAgain = (o :?> System.Random);; val rndAgain : System.Random > let rndAgain = (o :?> System.DateTime);; // this fails val rndAgain : System.DateTime System.InvalidCastException: Specified cast is not valid. at <StartupCode$FSI_0010>.$FSI_0010._main() stopped due to error
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