尝试从一个节点创建一个XML文档，并得到“无效的XML文档。文档没有根元素”。

Question

我正在尝试从一些XML中提取单个节点，并将该节点及其所有子内容写入一个新的XML文档，然后将其保存到磁盘。在某些情况下(例如我的测试用例)，有多个节点符合条件，并且所有这些节点都必须写入单独的文件。

到目前为止，我的函数中的所有内容都按预期工作，直到我将文件写入磁盘。抛出的异常是：“异常抛出：'System.Xml.XmlException‘in System.Xml.dll”，附加的消息是“无效的XML document。文档没有根元素”。我该如何解决这个问题？

我添加了很多控制台输出，以显示每个阶段都按预期工作。我的函数如下：

private void ExtractQIF()
        {
            var httpClient = new HttpClient();
            int QIFFound = 0;
            try
            {
                string assetsTarget = httpAuthority + "/asset";                
                var XMLRaw = httpClient.GetStringAsync(assetsTarget);
                XmlDocument XML = new XmlDocument();
                XML.LoadXml(XMLRaw.Result);                                             
                XmlNodeList QIFDocuments = XML.GetElementsByTagName("QIFDocument"); 
                Console.WriteLine("QIF Documents found: " + QIFDocuments.Count);        // WE ARE SUCCESSFULLY FINDING QIFDOCS IN THE ASSETS REQ
                foreach (XmlNode QIFDoc in QIFDocuments)
                {
                    QIFFound++;
                    try
                    {
                        Console.WriteLine(QIFDoc.InnerXml); // Debug shizzle
                        Console.WriteLine($"Document #{QIFFound} is being saved");
                        XmlDocument OutputQIFFile = new XmlDocument();
                        OutputQIFFile.ImportNode(QIFDoc, true);
                        Console.WriteLine("Node Imported to Document"); // Debug Shizzle
                        OutputQIFFile.Save(QIFSaveLocation); // This is the bastard line that isn't working
                        Console.WriteLine($"Document {QIFFound} Saved to Disk"); // Debug Shizzle
                        //Finally we should change the file extension to .qif
                    }
                    catch (Exception balls)
                    {
                        Console.WriteLine("COULD NOT EXTRACT QIF: Failed at document " + QIFFound);
                        Console.WriteLine(balls.Message);
                    }

                }
                Console.WriteLine("All QIF written to disk");                
            }
            catch
            { MessageBox.Show("COULD NOT EXTRACT ALL QIF");}

        }

控制台相关信息如下：

QIF Documents found: 2
<QPId xmlns="http://qifstandards.org/xsd/qif3">ec871a54-5bb5-470b-9ff7-de757d7726a1</QPId><Version xmlns="http://qifstandards.org/xsd/qif3"><TimeCreated>2019-01-22T15:51:20.874+00:00</TimeCreated></Version><Header xmlns="http://qifstandards.org/xsd/qif3">.... Blah Blah Blah, a shit tonne of XML
Document #1 is being saved
Node Imported to Document
Exception thrown: 'System.Xml.XmlException' in System.Xml.dll
COULD NOT EXTRACT QIF: Failed at document 1
Invalid XML document. The document does not have a root element.
<QPId xmlns="http://qifstandards.org/xsd/qif3">27400291-ea3d-41de-afee-69d186c4288f</QPId><Version xmlns="http://qifstandards.org/xsd/qif3"><TimeCreated>2019-01-22T15:50:16.827+00:00</TimeCreated></Version><Header xmlns="http://qifstandards.org/xsd/qif3"><Application> .... Blah Blah Bla, a shit tonne of XML
Document #2 is being saved
Node Imported to Document
Exception thrown: 'System.Xml.XmlException' in System.Xml.dll
COULD NOT EXTRACT QIF: Failed at document 2
Invalid XML document. The document does not have a root element.
All QIF written to disk

我还尝试将节点附加到文档本身，但围绕它们的错误和建议似乎将我带回了上面试图实现它的方式。

在我看来，该节点已正确填充。有什么想法吗？

Answer 1

根据the docs，ImportNode只复制指定的节点并返回它，实际上并不将其插入到文档中。您可以在给出的示例中看到需要添加的节点。

因此，要使其工作，您需要进行更改以执行以下操作：

var outputQIFFile = new XmlDocument();

var newQifNode = outputQIFFile.ImportNode(QIFDoc, true);

outputQIFFile.AppendChild(newQifNode);

outputQIFFile.Save(QIFSaveLocation);

然而，正如评论中所建议的，我强烈建议你看看LINQ to XML (XDocument和朋友)。这是一个更加现代和用户友好的API。