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How can I fill in missing XML elements with default values?

I'm developing web applications for in-house use and designed to interact with Microsoft's Customer Relationship Management system. The way the CRM API works with respect to retrieving data is that you call the Retrieve method on a proxy object for the type of object you are trying to retrieve, and specify the guid of the actual instance of that object you are interested in.

For example (pseudocode) :

strOutput = oInvoice.Retrieve( user_authorization, invoiceGuid, elements_you_would_like_returned)

would return an XML string describing the specific invoice who's guid is 'invoiceGuid'. If you leave 'elements_you_would_like_returned', it returns the entire invoice.

HOWEVER (this is the root of my problem), if fields in the invoice object are blank (quite a common occurance), the XML element containing that column's information is missing.

An example:
Typically once the XML string is returned, it is loaded into an XmlDocument. If I wish to display the 'Bill To' address, for instance--

xmlDoc.LoadXML(strOutput) 'output from above
textbox1.Text = xmlDoc("invoice").Item("billto_city").InnerText

-- but if the billto_city field was left blank, this throws an 'Object reference not set to an instance of object' exception.

It is not really feasable to put this inside a Try-Catch block, as I would need to do this for every column in the Invoice object.

I DO have an xsd which describes the CRM Invoice, however, having done some research, I found at http://www.w3.org/TR/xmlschema-0/ that:
 "Default attribute values apply when attributes are missing, and default element values apply when elements are empty." That seems disheartening.

So my question is this: Is there any way to put missing XML elements into the returned string so the application wouldn't throw that exception when I try and print out all the values?

Thanks in advance for any help, and for reading that long and convoluted post.
-adlibdev
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adlibdev
Asked:
adlibdev
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1 Solution
 
rdcproCommented:
One should *always* test for the presence of a node before attempting to access it.  For example, any method that returns a node should be tested to see if it returned null, before trying to look at the InnerText property.  Here's a javascript example (.NET isn't much different, though):

var oNode = xmlDoc.selectSingleNode("foo/bar/snafu");
sMyText = (oNode != null) ? oNode.text : "";

In this case, sMyText equals the value of the snafu element, if snafu exists, and is an empty string if it doesn't.

If you try to combine the two together, like:

sMyText = xmlDoc.selectSingleNode("foo/bar/snafu").text;

you're asking for trouble!  ;^)

But don't try to use try-catch blocks for this, because the overhead is high.

Regards,
Mike Sharp
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adlibdevAuthor Commented:
That's not a bad way of doing it. Still a bit tedious, but I guess you can't have everything. Thanks for your help.
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rdcproCommented:
Well, that's one of the reasons I like XSLT...

Regards,
Mike Sharp
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