How can one use GetHeader api of PHTTP_FILTER_PREPROC_HEADERS to retrieve fields, such as "url" and others, in unicode? I could not find any documentation on this topic.
Thanks a lot,
It is not possible to retrieve a value in Unicode using GetHeader.
ISAPI Filter is an ANSI API - all values you can get/set using the API must be ANSI. Yes, I know this is shocking; after all, it is 2006 and everything nowadays are in Unicode... but remember that this API originated more than a decade ago when barely anything was 32bit, much less Unicode. Also, remember that the HTTP protocol which ISAPI directly manipulates is in ANSI and not Unicode.
Now, one should note that GetServerVariable() of ISAPI Filter on IIS6, like ISAPI Extension on IIS6, is able to retrieve server variable values in Unicode, using the UNICODE_ prefix in front of server variable names. Prior versions of IIS do not support nor retrieve any values in Unicode in ISAPI.
However, this does NOT apply to request headers retrieved via the HEADER_ or HTTP_ prefix (i.e. you cannot use UNICODE_HTTP_ACCEPT_ENCODING to retrieve Unicode value for the Accept-Encoding HTTP request header). It also does not make sense, either, because HTTP headers are not transported in Unicode. You might as well make the MultiByteToWideChar() call yourself if you want the value in Unicode.
The usual caveat with GetServerVariable() and ISAPI Filters is that not all server variables are valid in any given ISAPI Filter event (i.e. you cannot retrieve UNICODE_URL in SF_NOTIFY_PREPROC_HEADERS because it is not yet valid, but you can retrieve it in a later event like SF_NOTIFY_AUTHENTICATE).
In other words, for fields like URL that you would retrieve with GetHeader, it is simply not possible for you to retrieve them in Unicode during the SF_NOTIFY_PREPROC_HEADERS filter event.