This is driving me nuts.  I have an XML file for a site:
  

<CWC_click_nav Created="1 August 2008">
	<Level1>
		<NavOption Label="Home">/</NavOption>
	</Level1>
	<Level1>
		<NavOption Label="Conferences">/conference/</NavOption>
		<Level2>
			<NavOption Label="About CWC Conferences">/conference/about.aspx</NavOption>
			<NavOption Label="Conference Team">/conference/team.aspx</NavOption>
		</Level2>
	</Level1>
	<Level1>
		<NavOption Label="Exhibitions">/exhibitions/</NavOption>
		<Level2>
			<NavOption Label="About CWC Exhibitions">/ex/about.aspx</NavOption>
			<NavOption Label="Exhibition Team">/exhib/team.aspx</NavOption>
		</Level2>
	</Level1>
</CWC_click_nav>

 In a Master file, I'm loading it up, checking the current directory and displaying the appropriate subnav.  This worked a treat using this code (sorry about the code view, the post editor didn't want to display it properly): 

void Page_Load()
    {
        if (!IsPostBack)
        {
            //Get the xml file
            XmlTextReader xtr = new XmlTextReader(Server.MapPath("/CWC_click_nav.xml"));
            xtr.WhitespaceHandling = WhitespaceHandling.None;
            XmlDocument xDoc = new XmlDocument();
            xDoc.Load(xtr);

            //This is the list of top level nav options
            XmlNodeList xNavList = xDoc.SelectNodes("CWC_click_nav/Level1");

            //Start the nav list
            litNav.Text += "<ul class=\"orange\">" + Environment.NewLine;

            foreach (XmlNode xNavNode in xNavList)
            {   

                //Process XML file, works a treat
            }  

            //Finish the nav list
            litNav.Text += "</ul>" + Environment.NewLine;
        }
    }

OK, so thats all good. I then decide to cache the XML file for perfomance reasons naturally. So my code changes to this:  

void Page_Load()
{
    if (!IsPostBack)
    {
        //HttpContext.Current.Cache.Remove("SiteNav");

        if (HttpContext.Current.Cache.Get("SiteNav") == null)
        {
            litNavLoad.Text += "no cache.";
            XmlTextReader xtrCache = new XmlTextReader(Server.MapPath("/CWC_click_nav.xml"));
            HttpContext.Current.Cache.Insert("SiteNav", xtrCache, new CacheDependency(Server.MapPath("/CWC_click_nav.xml")));
            litNavLoad.Text += "Data loaded from cache at " + DateTime.Now.ToString() + ".";
        }

        XmlTextReader xtr = (XmlTextReader)HttpContext.Current.Cache.Get("SiteNav");
        xtr.WhitespaceHandling = WhitespaceHandling.None;
        XmlDocument xDoc = new XmlDocument();
        xDoc.Load(xtr);

        //This is the list of top level nav options
        XmlNodeList xNavList = xDoc.SelectNodes("CWC_click_nav/Level1");

        //Start the nav list
        litNav.Text += "<ul class=\"orange\">" + Environment.NewLine;

        foreach (XmlNode xNavNode in xNavList)
        {
            //None of this happens!!!
        }  

</script>

It works whenever I uncomment the  

HttpContext.Current.Cache.Remove("SiteNav");

 line, in which case it doesn't find the Cache object "SiteNav", recreates it, then loads the Cache object into the XmlTextReader.  But if I then comment out that line and run the page again, I end up with an empty nav, just the opening and closing <ul> tags and no errors.  I tried testing to see if the Cache was actually working with a string object, and thats fine, I could load it and read it again on multiple page visits.  Its just this XML file which isn't working.

 
Any ideas? 

 UPDATE:

I am now getting an error, which was being hidden by another .NET/IIS issue.

On line

xtr.WhitespaceHandling = WhitespaceHandling.None;

I am now getting this error:

Operation is not valid due to the current state of the object.

[InvalidOperationException: Operation is not valid due to the current state of the object.]
   System.Xml.XmlTextReaderImpl.set_WhitespaceHandling(WhitespaceHandling value) +1959009
   System.Xml.XmlTextReader.set_WhitespaceHandling(WhitespaceHandling value) +11
   ASP.cwc_click_master.Page_Load() in d:\Shared\WEBSERVER\CWC_click\CWC_click.master:39
   System.Web.Util.CalliHelper.ArglessFunctionCaller(IntPtr fp, Object o) +5
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +858603
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   System.Web.UI.Control.LoadRecursive() +47
   System.Web.UI.Control.LoadRecursive() +131
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1436

 I presume this is because it hasn't been able to load the Cache object, so I put in a check to see if xtr was null:

            XmlTextReader xtr = (XmlTextReader)HttpContext.Current.Cache.Get("SiteNav");

             if (xtr == null)
                litNav.Text = "nothing";
            else
                xtr.WhitespaceHandling = WhitespaceHandling.None;

But its still erroring out.

 I am still stuck on this.  I have streamlined the code and found out exactly where the error is happening.  See below:

1    XmlTextReader xtr;
2    
3    //HttpContext.Current.Cache.Remove("SiteNav");
4    
5    if (HttpContext.Current.Cache.Get("SiteNav") == null)
6    {
7    	XmlTextReader xtrCache = new XmlTextReader(Server.MapPath("/CWC_click_nav.xml"));
8    	HttpContext.Current.Cache.Insert("SiteNav", xtrCache, new CacheDependency(Server.MapPath("/CWC_click_nav.xml")));
9    	litNavLoad.Text += "Data loaded from cache at " + DateTime.Now.ToString() + ".";
10   	xtr = (XmlTextReader)HttpContext.Current.Cache.Get("SiteNav");
11   }
12   else
13   {
14   	litNavLoad.Text += "Cached nav.";
15   	xtr = (XmlTextReader)HttpContext.Current.Cache.Get("SiteNav");
16   }
17   
18   xtr.WhitespaceHandling = WhitespaceHandling.None;
19   

  The first time the page loads, HttpContext.Current.Cache.Get("SiteNav") is null, so it loads the xml file into cache, reads it back into the XmlTextReader and carries on processing past line 18.  However, the next time the page loads, HttpContext.Current.Cache.Get("SiteNav") is not null, line 15 loads the cache into the XmlTextReader exactly the same way as before (lines 10 and 15), but it error out immediately on line 18.

I have tried loading the SiteNav cache object into an integer, just to see if it was being recognised as an XML file, and it gave me an incorrect type error, so it definitely thinks its an XML file in the cache.  Its just that after the first time the master page is loaded, the cache object exists but the content does not.

Please, any ideas? 

Hi kitster ,

kitster:

HttpContext.Current.Cache.Insert("SiteNav", xtrCache,

Do not cache a XmlTextReader object , try to load it using XmlDocument then cache the XmlDocument object.

That was it.  New code as follows:

//Get the xml file
XmlDocument xDoc = new XmlDocument();

if (HttpContext.Current.Cache.Get("SiteNav") == null)
{
	xDoc.Load(Server.MapPath("/CWC_click_nav.xml"));
	HttpContext.Current.Cache.Insert("SiteNav", xDoc, new CacheDependency(Server.MapPath("/CWC_click_nav.xml")));
	litNavLoad.Text += "Data loaded from cache at " + DateTime.Now.ToString() + ".";
}
else
{
	litNavLoad.Text += "Cached nav.";
	xDoc = (XmlDocument)HttpContext.Current.Cache.Get("SiteNav");
}