C# XmlDocument.LoadXML根级别数据无效：移除XML隐藏起始字符

问题场景 #

调用外部API返回的XML字符串被外层双引号包裹，且字符串开头存在隐藏字符，无法直接通过XmlDocument.LoadXml()正常解析，始终抛出"根级别数据无效"的异常。

返回的原始XML字符串（日志显示）：

"<?xml version='1.0' encoding='UTF-8'?>
<PDB>
    <TRANSACTION_TYPE>
        <TYPE>ERROR</TYPE>
    </TRANSACTION_TYPE>
    <ERROR>
        <DESCRIPTION>Bad Input. Please check.</DESCRIPTION>
    </ERROR>
</PDB>"

环境与尝试结果 #

基于.NET Framework 4.5.2的C#开发，尝试过多种针对XML解析"根级别数据无效"问题的常见解决方案均无效。执行string responseData = reader.ReadToEnd();获取到上述字符串后，调用XmlDocument.LoadXML()抛出以下异常：

System.Xml.XmlException: Data at the root level is invalid. Line 1, position 201.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace()
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at System.Xml.XmlLoader.LoadDocSequence(XmlDocument parentDoc)
at System.Xml.XmlDocument.Load(XmlReader reader)
at System.Xml.XmlDocument.LoadXml(String xml)

原尝试代码：

using (HttpWebResponse response = (HttpWebResponse)httpWebRequest.GetResponse())
{
    if (response.StatusCode == HttpStatusCode.Created)
    {
        using (var respStream = response.GetResponseStream())
        {
            using (var reader = new System.IO.StreamReader(respStream, true))
            {
                string responseData = reader.ReadToEnd(); 
                if (responseData.Contains(@"<TYPE>ERROR</TYPE>"))
                {
                    string _byteOrderMarkUtf8 = Encoding.UTF8.GetString(Encoding.UTF8.GetPreamble());
                    if (responseData.StartsWith(_byteOrderMarkUtf8, StringComparison.Ordinal))
                    {
                        log.DebugFormat("responseData starts with HIDDEN : {0}", _byteOrderMarkUtf8); // 日志中显示为?字符
                        // 尝试删除首字符时会删掉外层双引号，无法只移除隐藏字符
                    }                
                    XmlDocument doc = new XmlDocument();
                    doc.LoadXml(responseData);
                }
            }
        }
    }
}

有效解决方案 #

通过截取XML根节点<PDB>到</PDB>之间的核心内容，直接传入LoadXml()进行解析，避开外层引号与开头隐藏字符的干扰：

using (var reader = new System.IO.StreamReader(respStream, true))
{  
    string responseData = reader.ReadToEnd(); 
    if (responseData.Contains(@"<TYPE>ERROR</TYPE>"))
    {
        int pFrom = responseData.IndexOf("<PDB>");
        int pTo = responseData.LastIndexOf("</PDB>");

        string subresponseData = responseData.Substring(pFrom, (pTo - pFrom) + "</PDB>".Length);
        XmlDocument doc = new XmlDocument();
        doc.LoadXml(subresponseData);
    }
}

内容的提问来源于stack exchange，提问作者Ziggler