XmlElementsAndAttributes.cs

void Main()
{
	var doc = new XmlDocument();
	doc.LoadXml("<d><t>This is &amp; a \"test\" and a 'tested' test</t></d>");	
	doc.OuterXml.Dump();
	
	var node = doc.CreateElement("d2");
	node.InnerText = "this & that <doc> and \"test\" and 'tested'";
	doc.DocumentElement.AppendChild(node);
	var attr = doc.CreateAttribute("note","this & that <doc> and \"test\" and 'tested'");
	node.Attributes.Append(attr);
	
  // <d><t>This is &amp; a "test" and a 'tested' test</t><d2 d2p1:note="" xmlns:d2p1="this &amp; that &lt;doc&gt; and &quot;test&quot; and 'tested'">this &amp; that &lt;doc&gt; and "test" and 'tested'</d2></d>
	doc.OuterXml.Dump();
}

XmlString.cs

void Main()
{
	var s = "Characters: < > \" ' &" + ((char) 10);
	for (int i = 0; i < 5; i++)
	{
		s = s + s;
	}
	s.Dump();
	
	var sw = new Stopwatch();
	sw.Start();
	
	for (int i = 0; i < 300000; i++)
	{
		XmlStringEx(s);
	}
	sw.Stop();
	sw.ElapsedMilliseconds.Dump();
	("XElement: " + XmlStringEx(s)).Dump();

	sw.Reset();
	sw.Start();

	for (int i = 0; i < 300000; i++)
	{
		XmlString(s);
	}
	sw.Stop();
	sw.ElapsedMilliseconds.Dump();
	("String: " + XmlString(s)).Dump();

	sw.Reset();
	sw.Start();
	
	for (int i = 0; i < 300000; i++)
	{
		XmlStringEscaped(s);
	}
	sw.Stop();
	sw.ElapsedMilliseconds.Dump();
	("Escaped: " + XmlStringEscaped(s)).Dump();


	sw.Reset();
	sw.Start();

	for (int i = 0; i < 300000; i++)
	{
		XmlStringDoc(s);
	}
	sw.Stop();
	sw.ElapsedMilliseconds.Dump();
	("XmlDoc: " + XmlStringDoc(s)).Dump();

	
}

// Define other methods and classes here

public static string XmlString(string text, bool isAttribute = false)
{
	if (string.IsNullOrEmpty(text))
		return text;
		
	if (!isAttribute)
		return new XElement("t", text).LastNode.ToString();

	return new XAttribute("__n",text)
			     .ToString().Substring(5).TrimEnd('\"');	
}

private static XmlDocument _xmlDoc;

public string XmlStringDoc(string text)
{
	_xmlDoc = _xmlDoc ?? new XmlDocument();
	var el = _xmlDoc.CreateElement("t");
	el.InnerText = text;
	return el.InnerXml;
}

public static string XmlString(string text)
{	
	var sb = new StringBuilder(text.Length);
	
	foreach(var chr in text)
	{
		switch(chr)
		{		
			case '<':
				sb.Append("&lt;");
				break;
			case '>':
				sb.Append("&gt;");
				break;
//			case '\"':
//				sb.Append("&quot;");
//				break;
			case '&':
				sb.Append("&amp;");
				break;

			// legal control characters
			case '\n':
				sb.Append("\n");
				break;
			case '\r':
				sb.Append("\r");
				break;
			case '\t':
				sb.Append("\t");
				break;			
			
			default:
				if (chr < 32)
					throw new InvalidOperationException("Invalid character in Xml String. Chr " + Convert.ToInt16(chr) + " is illegal.");
				else
					sb.Append(chr);
				break;
		}								
	}
	
	return sb.ToString();
}

public static string XmlStringEscaped(string input)
{
	return  System.Security.SecurityElement.Escape(input);
}