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How can I get value of element attribute in XML

Please how can I get value of attribute Value of element StatusCode in this XML:
<LogoutResponse
ID="_f525259e-7e91-4282-9dc3-a0da65a4a17a"
Version="2.0"
IssueInstant="2021-05-17T15:41:55Z"
InResponseTo="_5089729f-5cc0-4a66-a3c1-e710cde92897"
Destination="https://idp.xyz/logout.aspx"
xmlns="urn:oasis:names:tc:SAML:2.0:protocol">
<Issuer xmlns="urn:oasis:names:tc:SAML:2.0:assertion">https://abc.xyz/api</Issuer>
<Status>
<StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success" />
</Status>
</LogoutResponse>

Due to the namespaces, this gets a little messy:
var doc = new XmlDocument();
doc.LoadXml(#"<LogoutResponse
ID=""_f525259e-7e91-4282-9dc3-a0da65a4a17a""
Version=""2.0""
IssueInstant=""2021-05-17T15:41:55Z""
InResponseTo=""_5089729f-5cc0-4a66-a3c1-e710cde92897""
Destination=""https://idp.xyz/logout.aspx""
xmlns=""urn:oasis:names:tc:SAML:2.0:protocol"">
<Issuer xmlns=""urn:oasis:names:tc:SAML:2.0:assertion"">https://abc.xyz/api</Issuer>
<Status>
<StatusCode Value=""urn:oasis:names:tc:SAML:2.0:status:Success"" />
</Status>
</LogoutResponse>");
var ns = new XmlNamespaceManager(doc.NameTable);
ns.AddNamespace("oasis", "urn:oasis:names:tc:SAML:2.0:protocol");
var attr = (XmlAttribute)doc.SelectSingleNode(
"/oasis:LogoutResponse/oasis:Status/oasis:StatusCode/#Value", ns);
System.Console.WriteLine(attr.Value);

You can try with XmlDocument.
Note: since you have a name space (xmlns ...) all elements in your xml
by default in name space. hence you are getting object reference
error. I have updated the code and .netfiddle. please have a look
var xml = #"<LogoutResponse
ID=""_f525259e-7e91-4282-9dc3-a0da65a4a17a""
Version=""2.0""
IssueInstant=""2021-05-17T15:41:55Z""
InResponseTo=""_5089729f-5cc0-4a66-a3c1-e710cde92897""
Destination=""https://idp.xyz/logout.aspx""
xmlns=""urn:oasis:names:tc:SAML:2.0:protocol"">
<Issuer xmlns=""urn:oasis:names:tc:SAML:2.0:assertion"">https://abc.xyz/api</Issuer>
<Status>
<StatusCode Value=""urn:oasis:names:tc:SAML:2.0:status:Success"" />
</Status>
</LogoutResponse>";
var doc = new System.Xml.XmlDocument();
doc.LoadXml(xml);
var nsManager = new XmlNamespaceManager(doc.NameTable);
nsManager.AddNamespace("ns", "urn:oasis:names:tc:SAML:2.0:protocol");
var result = doc.SelectSingleNode("/ns:LogoutResponse/ns:Status/ns:StatusCode/#Value", nsManager).InnerText;
Console.WriteLine(result);

By using LINQ to XML API.
It is available in the .Net Framework since 2007.
c#
void Main()
{
XDocument xdoc = XDocument.Parse(#"<LogoutResponse xmlns='urn:oasis:names:tc:SAML:2.0:protocol'
Destination='https://idp.xyz/logout.aspx' ID='_f525259e-7e91-4282-9dc3-a0da65a4a17a'
InResponseTo='_5089729f-5cc0-4a66-a3c1-e710cde92897' IssueInstant='2021-05-17T15:41:55Z' Version='2.0'>
<Issuer xmlns='urn:oasis:names:tc:SAML:2.0:assertion'>https://abc.xyz/api</Issuer>
<Status>
<StatusCode Value='urn:oasis:names:tc:SAML:2.0:status:Success'></StatusCode>
</Status>
</LogoutResponse>");
XNamespace ns1 = "urn:oasis:names:tc:SAML:2.0:protocol";
string StatusCodeValue = xdoc.Descendants(ns1 + "StatusCode")
.Attributes("Value")
.FirstOrDefault()?.Value;
Console.WriteLine("StatusCodeValue='{0}'", StatusCodeValue);
}
Output
StatusCodeValue='urn:oasis:names:tc:SAML:2.0:status:Success'

XPathSelectElement always return null

string xml = #"<vehicle> <cars> <car>tet</car> </cars> </vehicle>";
XElement x = XElement.Parse(xml);
var node = x.XPathSelectElement("//vehicle/cars");
why is the node always null?

XPath should be
var node = x.XPathSelectElement("cars");
OUTPUT:
<cars>
<car>tet</car>
</cars>

Adding an element to this xml structure

<root>
<element1>innertext</element1>
<element2>innertext</element2>
<element3>
<child1>innertext</child1>
</element3>
</root>
I have an xml structure shown above.
I would like to "append" the xml file (it is already created) to add another "child" inside element3>, so that it will look like this:
<root>
<element1>innertext</element1>
<element2>innertext</element2>
<element3>
<child1>innertext</child1>
<child2>innertext</child2>
</element3>
</root>
Linq to xml and/or Xpath would be great
EDIT:
I have tried doing this:
XElement doc = XElement.Load(mainDirectory);
XElement newElem = doc.Elements("element3").First();
newElem.Add(new XElement("child2", "child2innertext"));
doc.Add(newElem);
doc.Save(mainDirectory);

XmlDocument xDoc = new XmlDocument();
xDoc.Load("filename.xml");
foreach (XmlNode xNode in xDoc.SelectNodes("/root/element3"))
{
XmlElement newElement = xDoc.CreateElement("Child2");
xNode.AppendChild(newElement);
xNode.InnerText = "myInnerText";
}

With XDocument you can achieve this as:
string xml = "<root><element1>innertext</element1><element2>innertext</element2><element3><child1>innertext</child1></element3></root>";
var doc = XDocument.Parse(xml); //use XDocument.Load("filepath"); in case if your xml is in a file.
var el3 = doc.Descendants("element3").FirstOrDefault();
el3.Add(new XElement("child2", "innertext"));

Please, try this LINQPAD example
void Main()
{
var xml =
#"<root>
<element1>innertext</element1>
<element2>innertext</element2>
<element3>
<child1>innertext</child1>
</element3>
</root>";
var doc = XDocument.Parse(xml);
doc.Root.Element("element3")
.Add(new XElement("child2", "innertext"));
doc.Dump();
}

Whats wrong with my XPath?

I try to parse this XML file (config file from Chirpy):
<?xml version="1.0" encoding="utf-8" ?>
<root xmlns="urn:ChirpyConfig"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ChirpyConfig http://www.weirdlover.com/chirpy/chirp.xsd">
<FileGroup Name="Built.debug.js" Minify="false">
<File Path="jquery/jquery-1.7.2.js"/>
<File Path="jquery.address/jquery.address-1.4.js" />
</FileGroup>
</root>
with this code:
var path = Server.MapPath("~/Scripts/ScriptfilesMashup.chirp.config");
var file = new XPathDocument(path);
var nav = file.CreateNavigator();
var nodes = nav.Select("/root/FileGroup/File");
but nodes is always empty, regardless of how I call the nav.Select method. I barely used XPath before so maybe I'm doing it wrong - but what? Only the selector * gives me the root node.
What would be the selector to get the Path Attribute of all File nodes?
EDIT: SOLUTION
Thanks to Kirill, the final solution looks like this:
var path = Server.MapPath("~/Scripts/ScriptfilesMashup.chirp.config");
var file = new XPathDocument(path);
var nav = file.CreateNavigator();
var ns = "urn:ChirpyConfig";
XmlNamespaceManager nsMgr = new XmlNamespaceManager(nav.NameTable);
nsMgr.AddNamespace("x", ns);
var nodes = nav.Select("/x:root/x:FileGroup/x:File/#Path", nsMgr);
while(nodes.MoveNext())
{
var path = nodes.Current.Value;
}

It is because elements root, FileGroup and File are defined in urn:ChirpyConfig namespace.
Use this:
XPathDocument xmldoc = new XPathDocument(xmlFile);
XPathNavigator nav = xmldoc.CreateNavigator();
XmlNamespaceManager nsMgr = new XmlNamespaceManager(nav.NameTable);
nsMgr.AddNamespace("x", "urn:ChirpyConfig");
XPathNavigator result = nav.SelectSingleNode("/x:root/x:FileGroup/x:File", nsMgr);

Using C# to parse a SOAP Response

I am trying to get the values for faultcode, faultstring, and OrderNumber from the SOAP below
<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP:Body>
<faultcode>1234</faultcode>
<faultstring>SaveOrder:SetrsOrderMain:Cannot change OrderDate if GLPeriod is closed, new OrderDate is 3/2/2010:Ln:1053</faultstring>
<detail>
<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP:Body UserGUID="test">
<m:SaveOrder xmlns:m="http://www.test.com/software/schema/" UserGUID="test">
<Order OrderNumber="1234-1234-123" Caller="" OrderStatus="A" xmlns="http://www.test.com/software/schema/">
Here is my code in C#
XDocument doc = XDocument.Load(HttpContext.Current.Server.MapPath("XMLexample.xml"));
var errorDetail = new EcourierErrorDetail
{
FaultCode = from fc in doc.Descendants("faultcode")
select fc.Value,
FaultString = from fs in c.Descendants("faultstring")
select fs.Value,
OrderNumber = from o in
doc.Descendants("detail").Elements("Order").Attributes("OrderNumber")
select o.Value
};
return errorDetail;
I am able to get the values for both faultcode and faultstring but not the OrderNumber. I am getting "Enumeration yielded no results." Can anyone help? Thanks.

Yes, you're ignoring the XML namespace when selecting:
<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
.....
<m:SaveOrder xmlns:m="http://www.test.com/software/schema/" UserGUID="test">
<Order OrderNumber="1234-1234-123" Caller="" OrderStatus="A" xmlns="http://www.test.com/software/schema/">
The <Order> tag is inside the <m:SaveOrder> tag which uses the XML namespace prefixed by the m: prefix.
Also, you're trying to select the "detail" and then you skip to the "Order" node directly (using .Elements()) - you missed the <m:SaveOrder> node in between.
You need to take that into account when selecting:
XDocument doc = XDocument.Load(HttpContext.Current.Server.MapPath("XMLexample.xml"));
XNamespace xmlns = "http://www.test.com/software/schema/";
var orderNode = doc.Descendants(xmlns + "SaveOrder").Elements(xmlns + "Order");
var value = from o in orderNode.Attributes("OrderNumber")
select o.Value;
Does that give you a result??

A clean solution could be using System.Xml.XPath. Here is where I found the example
This is how to use it in this case:
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml(result);
XmlNamespaceManager xmlnsManager = new XmlNamespaceManager(xmlDoc.NameTable);
xmlnsManager.AddNamespace("soap", "http://schemas.xmlsoap.org/soap/envelope/");
xmlnsManager.AddNamespace("m", "http://www.test.com/software/schema/");
var orderNode = xmlDoc.SelectSingleNode("//m:Order", xmlnsManager);
var value = orderNode.Attributes["OrderNumber"].Value;