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how to change xml string value on fly

I have an xml response string and I want to change a value inside and log it.
<xml>
<ns2:abcd>
<password>sample</password>
</ns2:abcd>
I want to change the password value into encrypted version.
I am have tried using XmlDocument.SelectSingleNode but was thinking is there any better way than this?

Btw you need ns2 namespace to be declared, otherwise your xml will not be valid. After adding namespace definition, you can parse and modify your xml with Linq to Xml:
XDocument xdoc = XDocument.Parse(xml);
var passwordElement = xdoc.XPathSelectElement("//password");
passwordElement.Value = Encrypt((string)passwordElement);
xdoc.Save(path_to_xml);

No - there is no better way than using proper XML classes.
XmlDocument or XDocument would be perfectly fine for this task. If you XML is very large you may want to look into streaming with XmlReader (unlikely necessary in your case).

You might also consider looking into xsd.exe. With xsd.exe, you can deserialize your xml into a type-safe object model. From there, it's easy to manipulate the data.

C# how to fill xml file with string value

For example, my string is:
<Here>
<Hey>smth</Hey>
<Hi>else</Hi>
</Here>
I want my document x.xml to have that content. I tried
xmlDoc.InnerXml = thatString;
but it throws an exception.

Try,
XElement here = XElement.Parse("<Here><Hey>smth</Hey><Hi>else</Hi></Here>");
Then to save it to a file.
here.Save("filePath");

XmlDocument.LoadXml
http://msdn.microsoft.com/en-us/library/system.xml.xmldocument.loadxml.aspx
even better - use linq to xml. i prefer the XDocument class of XmlDocument

I beleive your code is for reading only. You have to use XmlTextWriter instead.

Loading string into XML Data

I am developing an application where I am reading a file, converting the contents into string and then loading the string in XML. But the issue that I am facing is that while loading the string data into XML I am getting an exception of invalid characters. I am using the following piece of code. Could any one help me to resolve the issue. Thank you in advance.
ZipFileEntry objContactXML;
String xmlData = ASCIIEncoding.UTF8.GetString(objContactXML.FileData);
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml(xmlData);
Regards,
Sanchaita

Firstly, this is a nasty bit of code:
ASCIIEncoding.UTF8
Please use just Encoding.UTF8 - it's UTF-8, not ASCII.
Now, you can create a StringReader around your XML text data - but you'd actually be better off not turning it into string data at all. It may be encoded in something other than UTF-8 - and the XML parser knows how to deal with that. It's entirely possible that this is why you're running into problems with your current approach. Leave the data in binary and parse that:
using (MemoryStream stream = new MemoryStream(objContactXML.FileData))
{
document.Load(stream);
}
As an aside, if you're using .NET 3.5 or higher, I would strongly advise you to use LINQ to XML (XDocument etc) instead of the old DOM API. LINQ to XML is a much nicer API.
In LINQ to XML, you'd use:
XDocument document;
using (MemoryStream stream = new MemoryStream(objContactXML.FileData))
{
document = XDocument.Load(stream);
}

What is the easiest way of handling xml files with C#?

I'm developing a windows app using C#. I chose xml for data storage.
It is required to read xml file, make small changes, and then write it back to hard disk.
Now, what is the easiest way of doing this?

XLinq is much comfortable than the ordinary Xml, because is much more object oriented, supports linq, has lots of implicit casts and serializes to the standard ISO format.

The best way is to use XML Serialization where it loads the XML into a class (with various classes representing all the elements/attributes). You can then change the values in code and then serialize back to XML.
To create the classes, the best thing to do is to use xsd.exe which will generate the c# classes for you from an existing XML document.

I think the easiest way of doing it - it is using XmlDocument class:
var doc = new XmlDocument();
doc.Load("filename or stream or streamwriter or XmlReader");
//do something
doc.Save("filename or stream or streamwriter or XmlWriter");

I think I found the easiest way, check out this Project in Codeproject. It is easy to use as XML elements are accessed similarly to array elements using name strings as indexes.
Code sample to write bool property to XML:
Xmlconfig xcfg = new Xmlconfig("config.xml", true);
xcfg.Settings[this.Name]["AddDateStamp"]["bool"].boolValue = checkBoxAddStamp.Checked;
xcfg.Save("config.xml");
Sample to read the property:
Xmlconfig xcfg = new Xmlconfig("config.xml", true);
checkBoxAddStamp.Checked = xcfg.Settings[this.Name]["AddDateStamp"]["bool"].boolValue;
To write string use .Value, for int .intValue.

You can use LINQ to read XML Files as described here...
LINQ to read XML

Check out linq to XML

C#, XML Query Question

I have tons of XML files all containing a the same XML Document, but with different values. But the structure is the same for each file.
Inside this file I have a datetime field.
What is the best, most efficient way to query these XML files? So I can retrieve for example... All files where the datetime field = today's date?
I'm using C# and .net v2. Should I be using XML objects to achieve this or text in file search routines?
Some code examples would be great... or just the general theory, anything would help, thanks...

This depends on the size of those files, and how complex the data actually is. As far as I understand the question, for this kind of XML data, using an XPath query and going through all the files might be the best approach, possibly caching the files in order to lessen the parsing overhead.
Have a look at:
XPathDocument, XmlDocument classes and XPath queries
http://support.microsoft.com/kb/317069
Something like this should do (not tested though):
XmlNamespaceManager nsmgr = new XmlNamespaceManager(new NameTable());
// if required, add your namespace prefixes here to nsmgr
XPathExpression expression = XPathExpression.Compile("//element[#date='20090101']", nsmgr); // your query as XPath
foreach (string fileName in Directory.GetFiles("PathToXmlFiles", "*.xml")) {
XPathDocument doc;
using (XmlTextReader reader = new XmlTextReader(fileName, nsmgr.NameTable)) {
doc = new XPathDocument(reader);
}
if (doc.CreateNavigator().SelectSingleNode(expression) != null) {
// matching document found
}
}
Note: while you can also load a XPathDocument directly from a URI/path, using the reader makes sure that the same nametable is being used as the one used to compile the XPath query. If a different nametable was being used, you'd not get results from the query.

You might look into running XSL queries. See also XSLT Tutorial, XML transformation using Xslt in C#, How to query XML with an XPath expression by using Visual C#.
This question also relates to another on Stack Overflow: Parse multiple XML files with ASP.NET (C#) and return those with particular element. The accepted answer there, though, suggests using Linq.

If it is at all possible to move to C# 3.0 / .NET 3.5, LINQ-to-XML would be by far the easiest option.
With .NET 2.0, you're stuck with either XML objects or XSL.