XPath and XmlNodeList, the horror, the horror!

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

Thanks all for the previous help btw.

What is going on here? It works yet it doesn't want to work in a
loop.

XmlNodeList nodeList = root.SelectNodes(String.Format("/cars/
mfg[@name='{0}'/model]", folderName));

This works:

nodeList[0].ChildNodes[0].Attributes["name"].Value.ToString()

nodeList[0].ChildNodes[1].Attributes["name"].Value.ToString());

but if I try and loop through them I get an error: Expression must
evaluate to a node-set.

my loop:

for (int x = 0; x < nodeList[0].ChildNodes.Count; x++)
{
XmlNode node = nodeList[0].ChildNodes[x];

url = new UrlPattern();

url.Name = node.Attributes["name"].Value.ToString();
url.RegexString = node.Attributes["regexString"].Value.ToString();
url.RealPath = node.Attributes["realPath"].Value.ToString();
url.Format = node.Attributes["format"].Value.ToString();

rgUrlPatterns.Add(url);
}

Archived from group: microsoft>public>dotnet>languages>csharp

Jon Skeet [C# MVP] · Joined: 08 Aug 2007 Posts: 266

DotNetNewbie wrote:

> but if I try and loop through them I get an error: Expression must
> evaluate to a node-set.

When? At compile-time, or execution time?

Where? Which line of code?

Could you post a short but complete program which demonstrates the
problem?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

--
Jon Skeet -
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

On Dec 12, 6:17 pm, Jon Skeet [C# MVP] wrote:
> DotNetNewbie wrote:
>
>
>
> > but if I try and loop through them I get an error: Expression must
> > evaluate to a node-set.
>
> When? At compile-time, or execution time?
>
> Where? Which line of code?
>
> Could you post a short but complete program which demonstrates the
> problem?
>
> Seehttp://www.pobox.com/~skeet/csharp/complete.htmlfor details of
> what I mean by that.
>
> --
> Jon Skeet - http://www.pobox.com/~skeet Blog:http://www.msmvps.com/jon.skeet
> World class .NET training in the UK:http://iterativetraining.co.uk

This line reports the error:

for (int x = 0; x < nodeList[0].ChildNodes.Count; x++)

I'll post the working code next....

Marc Gravell · Joined: 08 Aug 2007 Posts: 20

Well, what does the xpath mean?

XmlNodeList nodeList = root.SelectNodes(String.Format("/cars/
mfg[@name='{0}'/model]", folderName));

Support folderName is "Foo", then this gives xpath of

"/cars/mfg[@name='Foo'/model]"

I suggest that you simply need to lose the single quotes.

Marc

Marc Gravell · Joined: 08 Aug 2007 Posts: 20

typo: Support = Suppose

Stupid fingers, be less sloppy!

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

On Dec 12, 6:17 pm, Jon Skeet [C# MVP] wrote:
> DotNetNewbie wrote:
>
>
>
> > but if I try and loop through them I get an error: Expression must
> > evaluate to a node-set.
>
> When? At compile-time, or execution time?
>
> Where? Which line of code?
>
> Could you post a short but complete program which demonstrates the
> problem?
>
> Seehttp://www.pobox.com/~skeet/csharp/complete.htmlfor details of
> what I mean by that.
>
> --
> Jon Skeet - http://www.pobox.com/~skeet Blog:http://www.msmvps.com/jon.skeet
> World class .NET training in the UK:http://iterativetraining.co.uk

Jon,

My XML:

My codebehind:

protected void Page_Load(object sender, EventArgs e)
{

string pathToFile = String.Format("{0}/cars.xml",

HttpContext.Current.Request.PhysicalApplicationPath);

XmlDocument xmlDoc = new XmlDocument();

xmlDoc.Load(pathToFile);

XmlNode root = xmlDoc.DocumentElement;

string mfgName = "honda";

XmlNodeList nodeList = root.SelectNodes(String.Format("/
cars/car[@mfg='{0}'/model]", mfgName));

//Response.Write("nodeList: " + (null == nodeList));
//Response.Write("nodelist2: " +
nodeList[0].ChildNodes[0].Attributes["name"].Value.ToString());
//Response.Write("nodelist2: " +
nodeList[0].ChildNodes[1].Attributes["name"].Value.ToString());
//Response.End();

for (int x = 0; x < nodeList[0].ChildNodes.Count; x++)
{
XmlNode node = nodeList[0].ChildNodes[x];

Response.Write("forloop name = " +
node.Attributes["name"].Value.ToString());

}

}

Hans Kesting · Joined: 02 Oct 2007 Posts: 3

DotNetNewbie submitted this idea :
> Thanks all for the previous help btw.
>
> What is going on here? It works yet it doesn't want to work in a
> loop.
>
>
>
> XmlNodeList nodeList = root.SelectNodes(String.Format("/cars/
> mfg[@name='{0}'/model]", folderName));
>

Shouldn't that XPath be
"/cars/mfg[@name='{0}']/model"
(moved the ] bracket)
or is that a typo in the post?

Hans Kesting

Martin Honnen · Joined: 08 Aug 2007 Posts: 70

DotNetNewbie wrote:

>
>
>
>
>
>
>
>
>
>
>
>
>
> My codebehind:
>
> protected void Page_Load(object sender, EventArgs e)
> {
>
>
> string pathToFile = String.Format("{0}/cars.xml",
>
> HttpContext.Current.Request.PhysicalApplicationPath);
>
>
> XmlDocument xmlDoc = new XmlDocument();
>
> xmlDoc.Load(pathToFile);
>
> XmlNode root = xmlDoc.DocumentElement;
>
> string mfgName = "honda";
>
> XmlNodeList nodeList = root.SelectNodes(String.Format("/
> cars/car[@mfg='{0}'/model]", mfgName));

Looks to me like you want
XmlNodeList modelList = xmlDoc.SelectNodes(string.Format(
"/cars/car[@mfg = '{0}']/model", mfgName));
and then simply
foreach (XmlElement model in modelList)
{
Response.Write(model.GetAttribute("name") + "");
}

--

Martin Honnen --- MVP XML
http://JavaScript.FAQTs.com/

Jon Skeet [C# MVP] · Joined: 08 Aug 2007 Posts: 266

DotNetNewbie wrote:

> My codebehind:

Now would be a very good time to learn how to write a short but
complete program, rather than presenting codebehind snippets.

See http://pobox.com/~skeet/csharp/incomplete.html

You'll find it a *lot* quicker to debug problems like this (which have
nothing to do with ASP.NET per se) when you've got them in a small test
harness.

--
Jon Skeet -
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

On Dec 13, 2:17 pm, Jon Skeet [C# MVP] wrote:
> DotNetNewbie wrote:
>
>
>
> > My codebehind:
>
> Now would be a very good time to learn how to write a short but
> complete program, rather than presenting codebehind snippets.
>
> Seehttp://pobox.com/~skeet/csharp/incomplete.html
>
> You'll find it a *lot* quicker to debug problems like this (which have
> nothing to do with ASP.NET per se) when you've got them in a small test
> harness.
>
> --
> Jon Skeet - http://www.pobox.com/~skeet Blog:http://www.msmvps.com/jon.skeet
> World class .NET training in the UK:http://iterativetraining.co.uk

Ok the code works fine in a Console application, but on the web I am
getting an error.

My console code (xml file: http://pastebin.com/m550db746 )

using System;
using System.Xml;

class Program
{
static void Main(string[] args)
{

XmlDocument xmlDoc = new XmlDocument();

xmlDoc.Load(@"E:\Test\cars.xml");
string mfgName = "honda";

XmlNodeList modelList = xmlDoc.SelectNodes(string.Format("/
cars/car[@mfg = '{0}']/model", mfgName));

foreach (XmlElement model in modelList)
{
Console.WriteLine(model.GetAttribute("name"));
}

Console.ReadLine();

}
}

I used the exact same code for my web page, but I replaced the
console.writes with response.writes, and loading the XML by URL.

My code is here:

protected void Page_Load(object sender, EventArgs e)
{
XmlDocument xmlDoc = new XmlDocument();

xmlDoc.Load(Server.MapPath("/cars.xml"));
string mfgName = "honda";

XmlNodeList modelList = xmlDoc.SelectNodes(string.Format("/
cars/car[@mfg = '{0}']/model", mfgName));

foreach (XmlElement model in modelList)
{
Response.Write("Name: " +
model.GetAttribute("name"));
}
}

(same XML file).

Again the error on the webpage is:

Exception Details: System.Xml.XPath.XPathException: Expression must
evaluate to a node-set.
Source Error:
foreach (XmlElement page in nodeList)

Jon Skeet [C# MVP] · Joined: 08 Aug 2007 Posts: 266

DotNetNewbie wrote:

> Again the error on the webpage is:
>
> Exception Details: System.Xml.XPath.XPathException: Expression must
> evaluate to a node-set.
> Source Error:
> foreach (XmlElement page in nodeList)

Has it definitely load the file correctly? What does it look like in
the debugger?

--
Jon Skeet -
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

Martin Honnen · Joined: 08 Aug 2007 Posts: 70

DotNetNewbie wrote:

> I used the exact same code for my web page, but I replaced the
> console.writes with response.writes, and loading the XML by URL.
>
> My code is here:
>
> protected void Page_Load(object sender, EventArgs e)
> {
> XmlDocument xmlDoc = new XmlDocument();
>
> xmlDoc.Load(Server.MapPath("/cars.xml"));
> string mfgName = "honda";
>
> XmlNodeList modelList = xmlDoc.SelectNodes(string.Format("/
> cars/car[@mfg = '{0}']/model", mfgName));
>
> foreach (XmlElement model in modelList)
> {
> Response.Write("Name: " +
> model.GetAttribute("name"));
> }
> }
>
> (same XML file).
>
> Again the error on the webpage is:
>
> Exception Details: System.Xml.XPath.XPathException: Expression must
> evaluate to a node-set.
> Source Error:
> foreach (XmlElement page in nodeList)

But your code above has
foreach (XmlElement model in modelList)
so why would the error message say
foreach (XmlElement page in nodeList)
? That does not make sense to me. It looks like the error is elsewhere,
not in the code you provided.

--

Martin Honnen --- MVP XML
http://JavaScript.FAQTs.com/

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

On Dec 13, 6:12 pm, Jon Skeet [C# MVP] wrote:
> DotNetNewbie wrote:
>
>
>
> > Again the error on the webpage is:
>
> > Exception Details: System.Xml.XPath.XPathException: Expression must
> > evaluate to a node-set.
> > Source Error:
> > foreach (XmlElement page in nodeList)
>
> Has it definitely load the file correctly? What does it look like in
> the debugger?
>
> --
> Jon Skeet - http://www.pobox.com/~skeet Blog:http://www.msmvps.com/jon.skeet
> World class .NET training in the UK:http://iterativetraining.co.uk

yes the file is loaded correctly, I put debug tag when it loads the
file, and during the loop.

After it loads the file and hits the forloop, in vs.net I can view
that the cars.xml file has loaded just fine.

DotNetNewbie · Joined: 11 Dec 2007 Posts: 7

On Dec 14, 11:43 am, DotNetNewbie wrote:
> On Dec 13, 6:12 pm, Jon Skeet [C# MVP] wrote:
>
>
>
> > DotNetNewbie wrote:
>
> >
>
> > > Again the error on the webpage is:
>
> > > Exception Details: System.Xml.XPath.XPathException: Expression must
> > > evaluate to a node-set.
> > > Source Error:
> > > foreach (XmlElement page in nodeList)
>
> > Has it definitely load the file correctly? What does it look like in
> > the debugger?
>
> > --
> > Jon Skeet - http://www.pobox.com/~skeet Blog:http://www.msmvps.com/jon.skeet
> > World class .NET training in the UK:http://iterativetraining.co.uk
>
> yes the file is loaded correctly, I put debug tag when it loads the
> file, and during the loop.
>
> After it loads the file and hits the forloop, in vs.net I can view
> that the cars.xml file has loaded just fine.

Ok got it to work, I was referencing the child nodes incorrectly!
thanks for all your replies!

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