格式化XML：输出有缩进效果的XML字符串

1. 一般情况下使用以下代码即可将XML字符串重新格式化：
        private string FormatXml(string source)
        {
            StringBuilder sb = new StringBuilder();
            XmlTextWriter writer = null;
            
            try
            {
                XmlDocument doc = new XmlDocument();
                doc.LoadXml(source);
                writer = new XmlTextWriter(new StringWriter(sb));
                writer.Formatting = Formatting.Indented;
                
                doc.WriteTo(writer);
            }
            finally
            {
                if (writer != null) writer.Close();
            }