.NET or Mono functions in PG

I am investigating the possibility of having stored procedures
and functions written in .NET language flavors. I remember a long time
ago there was a gborg project wanting to implement it, but that is long 
gone I guess. anyway....

I think there are two possibilities:

a) Use of MS.NET core and implement it only for Windows. 
b) Use of Mono core and have the possibility to run it also on *nix systems.
c) Or perhaps a hybrid of the above would be possible.

Any implementation would require to invoke/call an external runtime (Mono or
MS.CRL).
I wonder what the performance would be.

I would like to have your opinion about this idea. 

Regards,
Gevik Babakhani

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