static extern int GetBestInterface(UInt32 DestAddr, out UInt32 BestIfIndex);

Declare Function GetBestInterface Lib "iphlpapi.dll" (TODO) As TODO

    public static extern int GetBestInterface(UInt32 destAddr, out UInt32 bestIfIndex);

        int result = Win32ApiCalls.GetBestInterface(ipv4AddressAsUInt32, out interfaceindex);