String escape into XML

SecurityElement.Escape(string s)

public static string XmlEscape(string unescaped)
{
    XmlDocument doc = new XmlDocument();
    XmlNode node = doc.CreateElement("root");
    node.InnerText = unescaped;
    return node.InnerXml;
}

public static string XmlUnescape(string escaped)
{
    XmlDocument doc = new XmlDocument();
    XmlNode node = doc.CreateElement("root");
    node.InnerXml = escaped;
    return node.InnerText;
}

EDIT: You say "I am concatenating simple and short XML file and I do not use serialization, so I need to explicitly escape XML character by hand".

I would strongly advise you not to do it by hand. Use the XML APIs to do it all for you - read in the original files, merge the two into a single document however you need to (you probably want to use XmlDocument.ImportNode), and then write it out again. You don't want to write your own XML parsers/formatters. Serialization is somewhat irrelevant here.

If you can give us a short but complete example of exactly what you're trying to do, we can probably help you to avoid having to worry about escaping in the first place.

---

Original answer

It's not entirely clear what you mean, but normally XML APIs do this for you. You set the text in a node, and it will automatically escape anything it needs to. For example:

LINQ to XML example:

using System;
using System.Xml.Linq;

class Test
{
    static void Main()
    {
        XElement element = new XElement("tag",
                                        "Brackets & stuff <>");

        Console.WriteLine(element);
    }
}

DOM example:

using System;
using System.Xml;

class Test
{
    static void Main()
    {
        XmlDocument doc = new XmlDocument();
        XmlElement element = doc.CreateElement("tag");
        element.InnerText = "Brackets & stuff <>";
        Console.WriteLine(element.OuterXml);
    }
}

Output from both examples:

<tag>Brackets &amp; stuff &lt;&gt;</tag>

That's assuming you want XML escaping, of course. If you're not, please post more details.