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Due to many questions regarding the msXML parser bug that causes
VFP clients to crash, this page has been made available to provide
information on the bug and a patch file that Microsoft has made
available.
To fix these problems it's highly recommended that
you install the MSXML3 (or later) update of the parser which brings
the Microsoft XML parser up to the latest version and fixes this
particular problem. You can download this update from:
http://msdn.microsoft.com/xml/general/xmlparser.asp
A redistributable which you can distribute with your
apps is also available from:
http://msdn.microsoft.com/xml/articles/msxmlcabfile.asp
The bug is described in the following knowledgebase
article at http://msdn.microsoft.com.
Search on the following ID:
Q255953
LoadXML via Invoke returns S_FALSE when loading
invalid xml where 5.0 always returned S_OK.
This page describes a patch file that can be ordered
from Product Support Services. However, this patch can be superseded
by the MSXML 3 update.
The issue:
There is a bug in the MSXML.DLL file distributed
by Microsoft with Internet Explorer 5.01 and also the shipping
version of Windows 2000. This bug causes an unhandled COM exception
(OLE Error 1) whenever a non-existant node, attribute, collection or
element is accessed. These operations should normally return NULL,
but instead the Error 1 is raised and no value is returned. This
error manifests itself in any COM client other than VB/VBScript.
VB/VBScript apparently ignores COM Error 1 as part of the language.
This problem will show in Visual FoxPro applications with a
number of code scenarios. A simple repro scenario is the following:
lcXML = [<?xml version="1.0"?><root><data>somedata</data></root>]
loXML = CREATEOBJECT("Microsoft.XMLDOM")
loXML.loadXML(lcXML)
? loXML.parseError.reason && Bombs
Trying to access the parseError object which has not been
initialized results in a COM exception. The proper result for this
type of operation would be a blank string. Note that any properties
of the object will cause this failure.
More importantly this problem occurs when you look for nodes in
the XML content that aren't there. Assuming the above XML:
loDocRoot = loXML.documentElement
lcValue = loDocRoot.selectSingleNode("data") && returns 'somedata'
lcValue = loDocRoot.selectSingleNode("notthere") && Bombs
The result of the last line of code should be a NULL value stored
in lcValue, but instead the COM exception occurs and lcValue is left
unchanged. Along the same lines digging into childnode collections
that don't exist cause these same type of errors - basically any
operation that normally should return NULL will bomb. This makes it
very difficult to parse documents that have dynamic structure with
nodes that may or may not be there.
Working around this bug
This bug may not be a problem for you if you always deal with
documents that have fixed structure and you never access nodes that
aren't there. Also, you can't access essentially uninitialized
values such as the parseError object. Otherwise you can work around
this problem by wrapping all of your XML access into error handlers.
This is messy at best and relies on the assumption your code is
otherwise solid and doesn't error.
However, I wouldn't recommend this. Microsoft has acknowledged
this bug and at the time of this writing has fixed it (see
Knowledgebase article above).
The West Wind Team
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