方法1：通过XmlDocument的OuterXml属性，见XmlDocument类
该方法获得的xml字符串是不带格式的，可读性差

方法2：利用XmlWriterSettings控制格式等一系列参数，见XmlWriterSettings类
例子：

using System.IO;
using System.Text;
using System.Xml;
...
XmlDocument doc = new XmlDocument();
doc.LoadXml("<item><name>wrench</name></item>");
// 使用XmlWriterSettings来控制格式化输出
XmlWriterSettings settings = new XmlWriterSettings
{
	Encoding = Encoding.UTF8,
    Indent = true,
    IndentChars = "  ", // 设置缩进字符和大小
    NewLineChars = "\r\n" // 设置换行符
};
using (StringWriter textWriter = new StringWriter())
{
    using (XmlWriter xmlWriter = XmlWriter.Create(textWriter, settings))
    {
        doc.Save(xmlWriter); // 将XML写入StringWriter
    }
    string xmlString = textWriter.ToString(); // 获取字符串
    Console.WriteLine(xmlString);
}

通过StringWriter作为输出流有一个弊端，它默认按照UTF16编码，无法更改，譬如上面的XmlWriterSettings指定了UTF8但实际得到的字符串仍然是UTF16的
改用MemoryStream作为流可以自定义编码，如下：

using (MemoryStream ms = new MemoryStream())  
{
     using (XmlWriter xmlWriter = XmlWriter.Create(ms, settings))
     {
         doc.Save(xmlWriter); // 将XML写入MemoryStream
     }
     string xmlString = Encoding.UTF8.GetString(ms.ToArray());  // 获取字符串
     Console.WriteLine(xmlString);
 }