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Encode specific part of string and load it to xml

When I load string xml into XmlDocument object, it throws
'<', hexadecimal value 0x3C, is an invalid attribute character.
string xml= Request.Form["webformfield"] + string.Empty;
// it will read the input from webform in encoded format
e.g:
<Models><Model ModelID="F2434" ModelName="FTest 1 & Income MP" />
try around:
//decoded the whole string
StringWriter sw = new StringWriter();
Server.HtmlDecode(models, sw); // this is an internal method of the framework.
models = sw.ToString();
after decoding the string, the string will be stored below
//string xml = "<Models><Model ModelId=\"124\" ModelNameWithSpecialCHars=\"Test1 <> & \"' characters \"/><Model ModelId=\"124\" ModelNameWithSpecialCHars=\"Test2 <> & \"' characters \"/></Models>";
XmlDocument xmlDocument = new XmlDocument();
xmlDocument.LoadXml(xml);
Console.WriteLine(xmlDocument.OuterXml);
I have changed xml string manually at runtime and it worked. Changed value of ModelNameWithSpecialCHars attribute from the string.
added string image, because when I written the encoded specials characters, it was showing it in decoded format. find the below code.
Changed string:
XmlDocument xmlDocument = new XmlDocument();
xmlDocument.LoadXml(xml);
Console.WriteLine(xmlDocument.OuterXml);
Console.ReadLine();
Is there any way I can encode only specific part of string. e.g
string xml = "<Models><Model ModelId=\"124\" ModelNameWithSpecialCHars=\"Tes <> & \"' characters \"/></Models>";
in the above string, I need to encode only value of ModelNameWithSpecialCHars attribute. ("Tes <> & "')

Sure - just put the part that needs encoding in a separate variable and use the WebUtility.HtmlEncode method (found in System.Net):
string bad_xml = "Tes <> & \"' characters ";
bad_xml = WebUtility.HtmlEncode(bad_xml);
string xml = "<Models><Model ModelId=\"124\" ModelNameWithSpecialCHars=\"" + bad_xml + "\"/Model></Models>"
although you might find it cleaner to add elements and attributes using the XmlDocument class since you want to have an XmlDocument in the end anyway, and it will encode text for you into valid XML, which has slightly different escaping requirements (although for your test string they are equivalent).

{"'\u0004', hexadecimal value 0x04, is an invalid character

I am trying to convert a file to XML format that contains some special characters but it's not getting converted because of that special characters in the data.
I have already this regex code still it's not working for me please help.
The code what I have tried:
string filedata = #"D:\readwrite\test11.txt";
string input = ReadForFile(filedata);
string re1 = #"[^\u0000-\u007F]+";
string re5 = #"\p{Cs}";
data = Regex.Replace(input, re1, "");
data = Regex.Replace(input, re5, "");
XmlDocument xmlDocument = new XmlDocument();
try
{
xmlDocument = (XmlDocument)JsonConvert.DeserializeXmlNode(data);
var Xdoc = XDocument.Parse(xmlDocument.OuterXml);
}
catch (Exception ex)
{
Console.WriteLine(ex);
}

0x04 is a transmission control character and cannot appear in a text string. XmlDocument is right to reject it if it really does appear in your data. This does suggest that the regex you have doesn't do what you think it does, if I'm right that regex will find the first instance of one or more of those invalid characters at the beginning of a line and replace it, but not all of them. The real question for me is why this non-text 'character' appears in data intended as XML in the first place.
I have other questions. I've never seen JsonConvert.DeserializeXmlNode before - I had to look up what it does. Why are you using a JSON function against the root of a document which presumably therefore contains no JSON? Why are you then taking that document, converting it back to a string, and then creating an XDocument from it? Why not just create an XDocument to start with?

How Can I Encode My New Xml Document?

I have a string with a XML text and i want to save it like XML. I encoded string (to "utf-8") but when i want to make XML from that - my cyrillic symbols in Value don't displayed right . What i need to do to encode my XML document ?
part of my xml :
<rev:Code>Мои данные</rev:Code>
my code:
string send = Encoding.GetEncoding("utf8").GetString(Encoding.GetEncoding("utf-8").GetBytes(send));
XmlDocument docsec = new XmlDocument();
docsec.LoadXml(send);
docsec.Save("C:\\XmlNEW.xml");
Original text :Мои данные
I see it after creating XML :Мои данные

I worked with russian textfiles before to convert them to rtf and used "Encoding.GetEncoding(1251)" for that purpose.

Problem was in Save Method because it use xml encoding, i take my answer from this : Answer
XmlDocument docsec = new XmlDocument();
docsec.LoadXml(send);
using (TextWriter writer = new StreamWriter("C:\XmlNEW.xml", false, Encoding.UTF8))
docsec.Save(writer);

Escaping ONLY contents of Node in XML

I have a part of code mentioned like below.
//Reading from a file and assign to the variable named "s"
string s = "<item><name> Foo </name></item>";
XmlDocument doc = new XmlDocument();
doc.LoadXml(s);
But, it stops working if the contents has characters something like "<", ">"..etc.
string s = "<item><name> Foo > Bar </name></item>";
I know, I have to escape those characters before loading but, if I do like
doc.LoadXml(System.Security.SecurityElement.Escape(s));
, the tags (< , >) are also escaped and as a result, the error occurs.
How can I solve this problem?

a tricky solution:
string s = "<item><name> Foo > Bar </name></item>";
s = Regex.Replace(s, #"<[^>]+?>", m => HttpUtility.HtmlEncode(m.Value)).Replace("<","ojlovecd").Replace(">","cdloveoj");
s = HttpUtility.HtmlDecode(s).Replace("ojlovecd", ">").Replace("cdloveoj", "<");
XmlDocument doc = new XmlDocument();
doc.LoadXml(s);

Assuming your content will never contain the characters "]]>", you can use CDATA.
string s = "<item><name><![CDATA[ Foo > Bar ]]></name></item>";
Otherwise, you'll need to html encode your special characters, and decode them before you use/display them (unless it's in a browser).
string s = "<item><name> Foo > Bar </name></item>";

Assign the content of string to the InnerXml property of node.
var node = doc.CreateElement("root");
node.InnerXml = s;
Take a look at - Different ways how to escape an XML string in C#

It looks like the strings that you have generated are strings, and not valid XML. You can either get the strings generated as valid XML OR if you know that the strings are always going to be the name, then don't include the XML <item> and <name> tags in the data.
Then when you create the XMLDocument. do a CreateElement and assign your string before resaving the results.
XmlDocument doc = new XmlDocument();
XmlElement root = doc.CreateElement("item");
doc.AppendChild(root);
XmlElement name = doc.CreateElement("name");
name.InnerText = "the contents from your file";
root.AppendChild(name);

XmlReader read document with unescaped &s

I am trying to parse an XMl document that i received into a string from a web service call.
String content = ...;//long xml document
using(TextReader reader = new StringReader(content))
using(XmlReader xml_reader = XmlReader.Create(reader, settings))
{
XML = new XPathDocument(xml_reader);
}
however i get an exception :
An error occurred while parsing EntityName. Line 1, position 1721.
i looked through the document around that character and it was in the middle of a random tag, however about 20-30 chars earlier i noticed that there were unescaped ampersands (& characters), so im thinking that that is the problem.
running:
content.Substring(1700, 100);//results in the following text
"alue>1 time per day& with^honey~&water\\\\</Value></Frequency></Direction> </Directions> "
^unescaped & char 1721 is the 'w'
how can i successful read this document as xml?

verify that your xml encoding matches theirs (the top of the document, something like <?xml version="1.0" encoding="ISO-8859-9"?>). Substitute the value from the webservice xml document for webserviceEncoding below
using(XmlReader r = XmlReader.Create(new StreamReader(fileName, Encoding.GetEncoding(webserviceEncoding)))) {
XML = new XPathDocument( r );
// ...
}
If that doesn't work
Replace it in the string prior to loading it into an xml parser
Notify the webservice vendor