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If you need to escape a string to use in a xml file (or stream), you have to escape those entities:

To achieve this result you could use the SecurityElement.Escape(string str) C# function, but it has a problem.

If your string has some entities already escaped, it escapes them again.
It happens to us testing our sitemaps generator when it finds URLs on a page that are already escaped.

So I’ve developed this function that tests every & character before to escape it.

public string EscapeXmlString(string URL)
{
//Avoid errors if the string is already escaped for xml use
    for (int i = 0; i < URL.Length-1; i++)
    {
        if (URL[i] == ‘&’)
        {
            switch (URL[i + 1])
            {
                case ‘a’:
                    if ((i + 5 < URL.Length) && (URL.Substring(i, 6) == “'”))
                    {
                        continue;
                    }
                    else
                    {
                        if ((i + 4 < URL.Length) && (URL.Substring(i, 5) == “&”))
                        {
                            continue;
                        }
                        else
                        {
                            //Escape it
                            URL = URL.Insert(i+1, “amp;”);
                        }
                    }
                break;
                case ‘q’:
                    if ((i + 5 < URL.Length) && (URL.Substring(i, 6) == “"”))
                    {
                        continue;
                    }
                    else
                    {
                        //Escape it
                        URL = URL.Insert(i+1, “amp;”);
                    }
                break;
                case ‘g’:
                    if ((i + 3 < URL.Length) && (URL.Substring(i, 4) == “>”))
                    {
                        continue;
                    }
                    else
                    {
                        //Escape it
                        URL = URL.Insert(i+1, “amp;”);
                    }
                break;
                case ‘l’:
                    if ((i + 3 < URL.Length) && (URL.Substring(i, 4) == “<”))
                    {
                        continue;
                    }
                    else
                    {
                        //Escape it
                        URL = URL.Insert(i+1, “amp;”);
                    }
                break;
                default://Escape it
                    URL = URL.Insert(i+1, “amp;”);
                    break;
            }
        }
    }

    URL = URL.Replace(“‘”, “'”);
    URL = URL.Replace(“\”“, “"”);
    URL = URL.Replace(“>”, “>”);
    URL = URL.Replace(“<”, “<”);

    return URL;
}