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static extern int ProgIDFromCLSID([In] ref Guid clsid,

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static extern string ProgIDFromCLSID([In()]ref Guid clsid);

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    static extern int ProgIDFromCLSID([In()]ref Guid clsid, [MarshalAs(UnmanagedType.LPWStr)]out string lplpszProgID);

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       int    result = ProgIDFromCLSID(ref g, out progId);

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    static extern string ProgIDFromCLSID([In()]ref Guid clsid);

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       string progId = ProgIDFromCLSID(ref g);

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Documentation

[ProgIDFromCLSID] on MSDN