调用AddAfterSelf触发InvalidOperationException：XML文档结构非法

背景 #

我编写了两段C#代码用于生成XML结构，但组合调用时出现错误。

生成基础XML的代码 #

private static XNode CreateXmlFolderSchema()
{
    XDocument document = new XDocument(new XElement("configurations"));
    document.Root!.Add(new XElement("config", new XAttribute("name", "RootFolder")));
    return document.FirstNode!;
}

这段代码生成的XML：

<configurations>
  <config name="RootFolder" />
</configurations>

生成文件夹节点的代码 #

private static XNode MapFolderToXmlDocument(Folder rootFolder, XNode parentNode)
{
    XElement folder = new XElement("folder",
    new XAttribute("name", rootFolder.Name),
    new XAttribute("root", rootFolder.Root),
    new XAttribute("default", rootFolder.Default),
    new XAttribute("permission", rootFolder.Permission),
    new XAttribute("expiry", rootFolder.Expiry),
    CreateActionElement(rootFolder));
    parentNode.AddAfterSelf(folder);
    return folder;
}

这段代码单独生成的XML：

<folder name="/"
        root="false"
        default="false"
        permission="read, write"
        expiry="0">
    <action bUploadChangePermssions="false"
            eUploadNewPermissions=""
            sUploadRename=""
            bUploadSubmit="false"
            sUploadDelete=""
            eDownloadIndicator=""
            sDownloadIndicator=""
            sDownloadWriteEvent="false"
            bChangePermissions="false"
            eDownloadNewPermissions=""
            bUploadSessionEnd="false"
            bDeleteOnDownload="false" />
</folder>

期望的最终XML结构 #

<configurations>
    <config name="RootFolder" />
    <folder name="/"
        root="false"
        default="false"
        permission="read, write"
        expiry="0">
    <action bUploadChangePermssions="false"
            eUploadNewPermissions=""
            sUploadRename=""
            bUploadSubmit="false"
            sUploadDelete=""
            eDownloadIndicator=""
            sDownloadIndicator=""
            sDownloadWriteEvent="false"
            bChangePermissions="false"
            eDownloadNewPermissions=""
            bUploadSessionEnd="false"
            bDeleteOnDownload="false" />
    </folder>
</configurations>

遇到的错误 #

执行MapFolderToXmlDocument(someFolderObject, CreateXmlFolderSchema())时，在parentNode.AddAfterSelf(folder)处触发InvalidOperationException，错误信息为："This operation would create an incorrectly structured document"。经在线XML验证器确认，期望的XML结构是合法的。

问题原因 #

CreateXmlFolderSchema方法返回的是XML文档的根节点<configurations>。当调用AddAfterSelf时，尝试在根节点后面添加新的<folder>节点，这会导致XML文档出现多个根节点，违反了XML文档只能有一个根节点的规则，因此触发错误。

解决方案 #

方案一：修改CreateXmlFolderSchema返回子节点 #

让方法返回<config>节点而非根节点，这样AddAfterSelf会将<folder>节点添加到<config>的同级位置，也就是根节点内部：

private static XNode CreateXmlFolderSchema()
{
    XDocument document = new XDocument(new XElement("configurations"));
    var configNode = new XElement("config", new XAttribute("name", "RootFolder"));
    document.Root!.Add(configNode);
    return configNode; // 返回<config>节点
}

方案二：修改MapFolderToXmlDocument的节点添加逻辑 #

判断传入的父节点是否为根节点，如果是，直接将<folder>节点添加到根节点的子集合中：

private static XNode MapFolderToXmlDocument(Folder rootFolder, XNode parentNode)
{
    XElement folder = new XElement("folder",
    new XAttribute("name", rootFolder.Name),
    new XAttribute("root", rootFolder.Root),
    new XAttribute("default", rootFolder.Default),
    new XAttribute("permission", rootFolder.Permission),
    new XAttribute("expiry", rootFolder.Expiry),
    CreateActionElement(rootFolder));
    
    if (parentNode is XElement parentElement && parentElement.Parent == null)
    {
        // 传入的是根节点，直接添加到其内部
        parentElement.Add(folder);
    }
    else
    {
        parentNode.AddAfterSelf(folder);
    }
    return folder;
}

内容的提问来源于stack exchange，提问作者Vocaloidas