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find if a code exists in xml, using c#

I am starting to work with xml and I am trying to know if there is a way to search a code in this.
Here you are my xml
<?xml version="1.0" encoding="UTF-8"?>
<doctors_hospital>
<doctor>
<code>1757D</code>
<name>one</name>
</doctor>
<doctor>
<code>1169L</code>
<name>two</name>
</doctor>
... continues xml
</doctors_hospital>
I want to look for the code "aab" using c#, and this is my code..
var document =new XmlDocument();
document.Load("O:\\test\\doctor.xml");
XmlNode doctor;
XmlNode root = document.DocumentElement;
doctor = root.SelectSingleNode("/doctors_hospital/doctor/code='aab'");
I can not do this. any suggestion? thanks

Assuming SelectingSingleNode takes a standard XPath expression, what you want to use is
/doctors_hospital/doctor[code='aab']
This will select the entire doctor node with the matching code value.

I agree with Jim, alternatively you could also use Linq to Xml and do this.
XDocument doc = XDocument.Load(filepath);
var codeExist = doc.Descendants("code").Any(x=>(string)x.Value == "1169L");
Check this Demo

C# Skip anything to next tag

I have a log file in xml format like
<log> // skip this node
<?xml version="1.0" encoding="UTF-8"?>
<qbean logger="main-logger">
</qbean>
</log>
<log> // go to this node
</log>
Now ReadToNextSibling("log") throw an exception an I need to skip content of first "log" tag and move to next "log" tag without throwing exception.
Is there a way?

Hint:
Your XML is invalid since the <?xml version="1.0" encoding="UTF-8"?> has to be before the root element. You can search for it and remove it if that fixes your problem. You can use yourXml.Repalce("<?xml version=\"1.0\" encoding=\"UTF-8\"?>", "")
You have to create a root element for your XML to be valid for parsing.
Then, you can use the XmlDocument class to parse the XML data that you have and skip anything you want. You would need something like this:
var document = new XmlDocument();
document.LoadXml(yourXml);
document.DocumentElement.ChildNodes[1]

Error in Xml parsing with C# XmlDocument class

When I add xmls attribute to my root element this code through a exception at third line " Object reference not set to an instance of an object" but after removing xmls attribute from root element it it works fine.
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load("file.xml");
MessageBox.Show(xmlDoc.SelectSingleNode("person/name").InnerText);
here is my xmlfile
<?xml version="1.0" encoding="utf-8"?>
<person xmlns="namespace path">
<name>myname</name>
</person>
I want to know why it does not works after adding xmlns attribute to my root element. Do I have to use another method for parsing ?.

You need to add namespace messenger to resolve namespaces to your xml file.
Consider this example
XML File
<?xml version="1.0" encoding="utf-8"?>
<person xmlns="http://www.findpersonName.com"> // Could be any namespace
<name>myname</name>
</person>
and in your code
XmlDocument doc = new XmlDocument();
doc.Load("file.xml");
//Create an XmlNamespaceManager for resolving namespaces.
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("ab", "http://www.findpersonName.com");
MessageBox.Show(doc.SelectSingleNode("//ab:name", nsmgr).InnerText);

Note
If the XPath expression does not include a prefix, it is assumed
that the namespace URI is the empty namespace. If your XML includes a
default namespace, you must still add a prefix and namespace URI to
the XmlNamespaceManager; otherwise, you will not get a node selected.
For more information, see Select Nodes Using XPath Navigation.
XmlNamespaceManager ns = new XmlNamespaceManager(xmldoc.NameTable);
ns.AddNamespace("something", "http://or.other.com/init");
XmlNode node = xmldoc.SelectSingleNode("something:person/name", ns);

You may want to consider using XDocument and Linq to process your XML document.
The following example provides a rough example:
XDocument xDoc = XDocument.Load("file.xml");
var personNames = (from x in xDoc.Descendants("person").Descendants("name") select x).FirstOrDefault();
How to Get XML Node from XDocument

How to change the data within elements in a XML file using C#?

I'm kind of new to XML files in C# ASP.NET. I have a XML in the below format:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Installation>
<ServerIP>192.168.20.110</ServerIP>
<DB_Name>USTCKT1</DB_Name>
<Username>jorame</Username>
<Password>Cru$%e20</Password>
<Table_PreFix>TCK</Table_PreFix>
</Installation>
I need to change the values within each element. For example, when an user clicks I should be able to replace 192.168.20.110 with 192.168.1.12.
How can I accomplish this? Any help will be really appreciated.

You should look at using the methods in the XDocument class. http://msdn.microsoft.com/en-us/library/bb301598.aspx
Specifically look at the methods: Load(string) - to load an XML file, Element() - to access a specific element and Save(string) - to save the XML document. The page on Element() has some sample code which can help.
http://msdn.microsoft.com/en-us/library/system.xml.linq.xcontainer.element.aspx

You can do something like this using the XDocument class:
XDocument doc = XDocument.Load(file.xml);
doc.Element("Installation").Element("ServerIP").Value = "192.168.1.12";
//Update the rest of the elements
doc.Save(file.xml);
More Details
If you run into namespace issues when selecting your elements you will need to include the xml namespace in the XElement selectors eg doc.Element(namspace + "Installation")

In general, you can do it in the following steps:
Create a new XmlDocument object and load the content. The content might be a file or string.
Find the element that you want to modify. If the structure of your xml file is too complex, you can use xpath you find what you want.
Apply your modification to that element.
Update your xml file.
Here is a simple demo:
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load("file.xml"); // use LoadXml(string xml) to load xml string
string path = "/Installation/ServerIP";
XmlNode node = xmlDoc.SelectSingleNode(path); // use xpath to find a node
node.InnerText = "192.168.1.12"; // update node, replace the inner text
xmlDoc.Save("file.xml"); // save updated content
Hope it's helpful.

How can I correct the code to read a xml file?

This code works as it is but when I reference an external xml file in doc.Loadxml, it stops working. How can I get it to work? I don't quite understand.
I use this to call GetXmlData and provide source for the gridview :GridView1.ItemsSource = GetXmlData();
private static object GetXmlData()
{
XmlDocument doc = new XmlDocument();
doc.LoadXml(#"<?xml version=""1.0"" encoding=""utf-8"" ?>
<Products>
<Product>
<ID>1</ID>
<Name>ASP.NET</Name>
</Product>
</Products>
");
XmlDataProvider provider = new XmlDataProvider();
provider.IsAsynchronous = false;
provider.Document = doc;
provider.XPath = "Products/Product";
return new ObservableCollection<XmlNode>((IEnumerable<XmlNode>)provider.Data);
}

You need
doc.Load(fileName);
instead of
doc.LoadXml(xml);

XMLDocument has several Load methods, see them with their description:
Load(Stream) Loads the XML document from the specified stream.
Load(String) Loads the XML document from the specified URL.
Load(TextReader) Loads the XML document from the specified TextReader.
Load(XmlReader) Loads the XML document from the specified XmlReader.
LoadXml(string) Loads the XML document from the specified string.
You're using the last one which is as described used to load XML from a string.
Since you need to load the XML from a file, so you've to use to Load method, as opposed to LoadXml. I think second method is better suited for your situation. You can pass the fullpath of the XML file.

This should help you:
XmlDocument doc = new XmlDocument();
doc.Load(file_path);
The method you are calling only loads xml from a string. You need to read it from a file which requires a different method.