The GetExtensionVersion function is the first entry-point function IIS. This function allows your ISAPI extension to register its version information with IIS.
BOOL WINAPI GetExtensionVersion(
HSE_VERSION_INFO* pVer
);
GetExtensionVersion must return TRUE for IIS to be able to use your ISAPI DLL. If this function returns FALSE, IIS will not use the extension.
IIS calls this function when the DLL is initially loaded, and the call is made in the system’s security context.