.Net Null Reference Exception in DocumentXPathNavigator - asp.net

Question

I've run into an interesting exception. On one of my production servers, occasionally I'll get the following exception.
System.NullReferenceException: Object reference not set to an instance of an object.
at System.Xml.DocumentXPathNavigator.get_NameTable()
at System.Xml.Xsl.Runtime.XmlQueryContext.get_DefaultNameTable()
at System.Xml.Xsl.Runtime.XmlQueryRuntime..ctor(XmlQueryStaticData data, Object defaultDataSource, XmlResolver dataSources, XsltArgumentList argList, XmlSequenceWriter seqWrt)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer, Boolean closeWriter)
at System.Xml.Xsl.XmlILCommand.Execute(IXPathNavigable contextDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter results)
at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, TextWriter results)
at sftControls.SearchListLandingPage.getHtml(IXPathNavigable document, String Xsl, String headerLinks)
Opening up the IL and looking through each method suggests that the DocumentXPathNavigator => this.document is null. What's strange is that I can in no way duplicate this issue locally or on our test servers.
A little background, and this might be the key...
This is a web application(running .Net 3.5) that caches the XmlDocument that gets passed into XslCompiledTransform.Transform(). Our caching is serviced through a distributed cache server. The XmlDocument is serialized and cached so a reference to the specific object should not continue to exist. That leads me to believe that either something is explicitly preventing this.document from ever being set when the implicit XmlDocument -> DocumentXPathNavigator happens or some reference somewhere is removed while the Transform() is running. Either way I am perplexed.
So, I guess my questions are:
Has anyone else seen this before?
Does anyone know what could be causing it or how to deal with it?
Thanks
EDIT:
After further thinking about this, the only thing that would make sense is that it's a race condition. The caching appliance is somehow expiring the object while the application is using the object.
EDIT/EDIT:
.Net 3.5, the assembilies being referenced above are 2.0

Answer 1

In case anyone has this same exception.
We found that this exception occurs when xmldocument.HasChildNodes is false or DocumentElement is null. Verifying that the document has elements, BEFORE querying on it, resolves this.

Answer 2

I experienced a very similar exception inside an XslCompiledTransform:
System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at MS.Internal.Xml.Cache.XPathDocumentNavigator.get_NodeType()
at System.Xml.XPath.XPathNavigator.MoveToNonDescendant()
at System.Xml.Xsl.Runtime.DescendantMergeIterator.MoveNext(XPathNavigator input)
at <xsl:template name="SystemPartIncluded">(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, IList`1 PartNumber, XPathNavigator Description, IList`1 DescriptionOverride, IList`1 Quantity, IList`1 ExtendedPrice, String LabelWhiteSpace)
at <xsl:template name="compiler:generated"> (30)(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current, XPathNavigator isBundledWithCanvas, XPathNavigator SoftwareExtendedPrice, XPathNavigator SoftwareUnitPrice, IList`1 PixelNet, IList`1 Revision_Date, IList`1 Catalyst)
at <xsl:template name="compiler:generated"> (45)(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current, XPathNavigator SoftwareUnitPrice, XPathNavigator HardwareExtendedPrice, XPathNavigator HardwareUnitPrice, XPathNavigator PixelNetUnitPrice, XPathNavigator FusionUnitPrice)
at <xsl:template match="Quote">(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current, Double {urn:schemas-microsoft-com:xslt-debug}position, Double {urn:schemas-microsoft-com:xslt-debug}last)
at <xsl:template match="/">(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current)
at Root(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer)
at MyApp.clsXML.XslTransformationToHtml(IQEntities DC, Int32 loggedOnUserID, IXPathNavigable input, String xslStylesheetFilePath, XsltArgumentList xslArgs, String XmlRepositoryVersion) in MyApp\App_Data\Static Classes\xml_routines.cs:line 510
at MyApp.clsQuote.RenderOneQuote(IQEntities DC, Int32 loggedOnUserID, Int32 quoteID, Boolean booPrintFormat, Boolean booDiscount_Prices, Boolean booIs_Super_Administrator, Boolean isHostRep, String strOrganization_Table_Caption, Boolean showBottomLineOnly) in MyApp\App_Data\Static Classes\quote_routines.cs:line 476
at MyApp.Pages.AJAX.BrowseQuotes.RenderSelectedQuote(Int32 quoteID) in MyApp\Pages\AJAX\BrowseQuotes.aspx.cs:line 66
at MyApp.Pages.AJAX.BrowseQuotes.ucQuotePicker1_OnQuoteSelectedEvent(Object sender, Int32 quoteID) in b:\DotNet\IQ\WinMetrics.IQ\Pages\AJAX\BrowseQuotes.aspx.cs:line 56
at System.Web.UI.WebControls.GridView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Like you, we were passing an XPathDocument to the Xsl transform and the XPathDocument was being retrieved from distributed cache. Unfortunately, an XPathDocument is not serializable and therefore unusable after it is retrieved from the distributed cache. We fixed the problem by using an XmlDocument instead of an XPathDocument and using SerializedXmlDocument to serialize and deserialize it as follows:
public static XmlDocument CachedXmlDoc
{
get
{
lock (_locker)
{
// maintain the Part XmlDocument in the distributed cache so that it can be reset from any production instance
SerializedXmlDocument serializedXmlDoc = Reliable.Application[Global.CACHED_XML_DOC] as SerializedXmlDocument;
XmlDocument cachedDoc = (serializedXmlDoc == null) ? null : serializedXmlDoc.XmlDocument;
if (cachedDoc == null)
{
cachedDoc = myController.CreateXmlDoc();
Reliable.Application[Global.CACHED_XML_DOC] = new SerializedXmlDocument(cachedDoc);
}
return cachedDoc;
}
}
set
{
lock (_locker)
{
if (value == null)
{
Reliable.Application[Global.CACHED_XML_DOC] = null;
}
else
{
throw new Exception("Tried to set the cached XmlDocument to something other than null.");
}
}
}
}