Getting the last error in a custom error page

When you provide a custom error handler page, it is useful to be able to log what caused the problem, so that you can fix it. Normally, this is just a case of using

        Exception lastError = Server.GetLastError();
        if (lastError == null)
            {
            ErrorLog("An error occurred, but no error information is available");
            }
        else
            {
            ErrorLog(lastError.ToString());
            }

This fails however, because the redirect to the error page loses the error information. You can handle a Page Error event and pass the error through in a session variable, but that is a bit clunky, and means coding in your Master page and / or each actual page, plus the session variable itself. Alternatively, there is a redirect mode as part of the custom error setup which redirects without re-loading. This preserves error information. Change your customErrors declaration from:

    <customErrors defaultRedirect="~/Whoops.aspx" mode="On"/>

To

    <customErrors defaultRedirect="~/Whoops.aspx" mode="On" redirectMode="ResponseRewrite"/>

Then GetLastError will work fine.