Replace special characters from xml string

Question

Is that possible to replace special characters E.G "<" to "& lt;" etc... from string which i read from XML file by File.ReadAllText(path) method without replacing brackets from xml tags ? For example:

<fragment>
<set-header name="MyHeader">
    <value>Sample text < </value>
</set-header>
</fragment>

I need to replace "<" this symbol from tag without touching this "< value>" brackets.

Its important to do that without using E.G XmlDocument class becouse i can't read it becouse of this special character and it throws an exception.

Answer 1

The classes contained in the System.Xml.Linq Namespace escape/unescape automatically for you.

string xml = """
    <fragment>
        <set-header name="MyHeader">
            <value>Sample text &lt; </value>
        </set-header>
    </fragment>
    <fragment>
        <set-header name="MyHeader">
            <value>Sample text &gt; </value>
        </set-header>
    </fragment>
    """;

var doc = XDocument.Parse($"<root>{xml}</root>");
foreach (XElement element in doc.Descendants("value")) {
    Console.WriteLine(element.Value);
}

Prints:

Sample text <
Sample text >

Note that you must embed these fragments into a single root element, otherwise you will get an exception telling you that you have more than one root element.

---

The other way round works as well:

var doc = new XDocument(new XElement("test", "value = <x> "));
string xml = doc.ToString();
Console.WriteLine(xml);

Prints:

<test>value = &lt;x&gt; </test>

---

Attempt to fix the bad xml:

// Escape the extra <
string xml = Regex.Replace(malformedXml, @"<([^\w/])", @"&lt;$1");

// Escape the extra >
xml = Regex.Replace(xml, @"(\W)>", @"$1&gt;");

This works only if the < can be identified as not being part of an open or closing tag. This regex searches for a < not followed by either a word character or a / and replaces it by < and the following character (the group number 1 denoted as $1).

The second Replace replaces > not preceeded by a word character or a double quote.

Test:

string malformedXml = """
    <fragment>
        <set-header name="MyHeader">
            <value>Sample text < > </value>
        </set-header>
    </fragment>
            
    """;
string xml = Regex.Replace(malformedXml, @"<([^\w/])", @"&lt;$1");
xml = Regex.Replace(xml, @"([^\w""])>", @"$1&gt;");
Console.WriteLine(xml);

Prints:

<fragment>
    <set-header name="MyHeader">
        <value>Sample text &lt; &gt; </value>
    </set-header>
</fragment>

This works with this example of a malformed XML, but since we don't know all possible ways the XML could be malformed, we have no guarantee that this will always work.

The only good solution is to fix the problem at the source, i.e., by the provider of this XML.