C# to COM define interface

Question

I have a function in a COM dll that returns a string and takes a string.

I tried this interface, while it compiles, when called it doesn't work. Not sure how to define it?

[return: MarshalAs(UnmanagedType.HString, SafeArraySubType = VarEnum.VT_VARIANT)]
        [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime), DispId(16)]
        object GetNextOTMOption([In] ref object[] param);