Fun with Server.GetLastError() in classic ASP on Windows Server 2008

One of our sites, written many moons ago in classic ASP using JScript, uses a bunch of custom error pages to handle 404 errors, scripting errors, and so on.

Our error handling code looks like this:

var error=Server.GetLastError(); 
var errorMessage=""; 
errorMessage +=Server.HTMLEncode(error.Category); 
if (error.ASPCode) errorMessage +=Server.HTMLEncode(", " +  error.ASPCode); 
var errorNumber=error.number;
errorNumber = (errorNumber < 0 ? errorNumber + 0x100000000 : errorNumber).toString(16);
errorMessage +=" error 0x" + errorNumber + " (from " + Request.ServerVariables("SCRIPT_NAME").Item + ")\r\n\r\n";
if (error.ASPDescription) errorMessage += "ASPDescription: " + error.ASPDescription + "\r\n"; 
if (error.Description) errorMessage +="Description: " + error.Description + "\r\n"; 
// and then we can display/log the error message 

On our old server, this worked because the HTTP 500 error page was mapped to a custom URL, /errors/500.asp, which included the code above.

When we migrated our site onto IIS7 recently, this stopped working - the custom page was still executing, but Server.GetLastError() wasn’t returning any information about what had gone wrong.

There was a very similar known bug in Vista which was supposedly fixed in SP1, but it looks like the same fix isn’t part of Windows 2008 Server yet. There is a workaround, though - if you set the site’s default error property (under IIS settings > Error Pages > Edit Feature Settings…)to the custom page (see below), IIS will invoke this page whenever an error is not handled by an explicitly configured status-code handler (so your 404, etc. handlers will still work) - but for some reason, handling the error this way means Server.GetLastError() still works properly.