将xml文档附加到C#中的xml节点？

string xmlString = "<root><child1>value1</child1></root>";
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml(xmlString);
XmlNode xmlNode = xmlDoc.DocumentElement;

string xmlString2 = "<root2<child2>value2</child2></root2>";
XmlDocument xmlDoc2 = new XmlDocument();
xmlDoc2.LoadXml(xmlString2);
XmlNode xmlNode2 = xmlDoc2.DocumentElement;

xmlNode.AppendChild(xmlNode2);

using System;
using System.Xml;

class Program
{
    static void Main(string[] args)
    {
        string xmlString = "<root><child1>value1</child1></root>";
        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.LoadXml(xmlString);
        XmlNode xmlNode = xmlDoc.DocumentElement;

        string xmlString2 = "<root2<child2>value2</child2></root2>";
        XmlDocument xmlDoc2 = new XmlDocument();
        xmlDoc2.LoadXml(xmlString2);
        XmlNode xmlNode2 = xmlDoc2.DocumentElement;

        xmlNode.AppendChild(xmlNode2);

        Console.WriteLine(xmlDoc.OuterXml);
    }
}

<root><child1>value1</child1><root2<child2>value2</child2></root2></root>