生成指定XML时触发System.ArgumentException异常求助

尝试通过给定类结构生成目标XML文档，但执行代码时抛出异常：System.ArgumentException: 'Cannot insert a node or any ancestor of that node as a child of itself.'，类本身无循环引用，求解决思路。

目标XML #

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<VirtualDesktopManager>
  <Categories>
    <Category Name="Category 1">
      <Desktops>
        <Desktop Name="Desktop 1">
          <Applications Name="Application 1" />
          <Applications Name="Application 2" />
        </Desktop>
      </Desktops>
    </Category>
  </Categories>
</VirtualDesktopManager>

相关代码 #

private static void Main ()
{
    var database = new Database();
    var category = new VirtualDesktopCategory();
    var desktop = new VirtualDesktop();
    var application = new VirtualDesktopApplication();

    category = new VirtualDesktopCategory() { Name = "Cat 1", };
    database.Categories.Add(category);
    desktop = new VirtualDesktop() { Name = "Desktop 1", };
    category.Desktops.Add(desktop);
    application = new VirtualDesktopApplication() { Name = "Application 1", };
    desktop.Applications.Add(application);
    application = new VirtualDesktopApplication() { Name = "Application 2", };
    desktop.Applications.Add(application);

    database.ToXmlDocument().InnerText.Dump();
}

public class Database
{
    public string Name { get; set; } = "";
    public List<VirtualDesktopCategory> Categories { get; private set; } = new();

    public XmlDocument ToXmlDocument()
    {
        var document = new XmlDocument();
        var xml = @"<?xml version=""1.0"" encoding=""UTF-8"" standalone=""yes"" ?><VirtualDesktopManager></VirtualDesktopManager>";

        document.LoadXml(xml);
        document.DocumentElement?.AppendChild(this.ToXmlElement(document, document.DocumentElement));

        return document;
    }

    public XmlElement ToXmlElement(XmlDocument document, XmlElement elementParent)
    {
        var elementCategories = document.CreateElement("Categories");

        elementParent.AppendChild(elementCategories);

        foreach (var category in this.Categories)
        {
            // System.ArgumentException: Cannot insert a node or any ancestor of that node as a child of itself.
            elementCategories.AppendChild(category.ToXmlElement(document, elementCategories));
        }

        return elementCategories;
    }
}

public class VirtualDesktopCategory
{
    public string Name { get; set; } = "";
    public List<VirtualDesktop> Desktops { get; private set; } = new();

    public XmlElement ToXmlElement(XmlDocument document, XmlElement elementParent)
    {
        var elementCategory = document.CreateElement("Category");
        var elementDesktops = document.CreateElement("Desktops");

        elementCategory.AppendAttribute(document, nameof(this.Name), this.Name);

        elementParent.AppendChild(elementCategory);
        elementCategory.AppendChild(elementDesktops);

        foreach (var desktop in this.Desktops)
        {
            elementDesktops.AppendChild(desktop.ToXmlElement(document, elementDesktops));
        }

        return elementCategory;
    }
}

public class VirtualDesktop
{
    public string Name { get; set; } = "";
    public List<VirtualDesktopApplication> Applications { get; private set; } = new();

    public XmlElement ToXmlElement(XmlDocument document, XmlElement elementParent)
    {
        var elementDesktop = document.CreateElement("Desktop");
        var elementApplications = document.CreateElement("Applications");

        elementDesktop.AppendAttribute(document, nameof(this.Name), this.Name);

        elementParent.AppendChild(elementDesktop);
        elementDesktop.AppendChild(elementApplications);

        foreach (var application in this.Applications)
        {
            elementApplications.AppendChild(application.ToXmlElement(document, elementApplications));
        }

        return elementParent;
    }
}

public class VirtualDesktopApplication
{
    public string Name { get; set; } = "";

    public XmlElement ToXmlElement(XmlDocument document, XmlElement elementParent)
    {
        var elementApplication = document.CreateElement("Application");

        elementApplication.AppendAttribute(document, nameof(this.Name), this.Name);

        elementParent.AppendChild(elementApplication);

        return elementApplication;
    }
}

public static class Extensions
{
    public static XmlAttribute AppendAttribute(this XmlElement element, XmlDocument document, string name, string value)
    {
        var attribute = document.CreateAttribute(name);

        attribute.Value = value;
        element.Attributes.Append(attribute);

        return attribute;
    }
}

解决思路 #

错误根源 #

问题出在VirtualDesktop类的ToXmlElement方法返回值上：
其他类（如VirtualDesktopCategory、VirtualDesktopApplication）的ToXmlElement方法都返回自己创建的节点（比如elementCategory、elementApplication），但VirtualDesktop返回的是传入的elementParent（也就是父节点elementDesktops）。

当VirtualDesktopCategory遍历桌面列表时，执行elementDesktops.AppendChild(desktop.ToXmlElement(document, elementDesktops))，相当于尝试把elementDesktops节点作为子节点插入到自身中，直接触发了"无法将节点或其祖先作为自身子节点插入"的异常。

修复代码 #

修改VirtualDesktop类的ToXmlElement方法，让它返回自己创建的elementDesktop节点，而非elementParent：

public class VirtualDesktop
{
    public string Name { get; set; } = "";
    public List<VirtualDesktopApplication> Applications { get; private set; } = new();

    public XmlElement ToXmlElement(XmlDocument document, XmlElement elementParent)
    {
        var elementDesktop = document.CreateElement("Desktop");
        var elementApplications = document.CreateElement("Applications");

        elementDesktop.AppendAttribute(document, nameof(this.Name), this.Name);

        elementParent.AppendChild(elementDesktop);
        elementDesktop.AppendChild(elementApplications);

        foreach (var application in this.Applications)
        {
            elementApplications.AppendChild(application.ToXmlElement(document, elementApplications));
        }

        // 改为返回自身创建的节点
        return elementDesktop;
    }
}

额外优化点 #

1. 目标XML中应用节点是<Applications>，但代码里创建的是<Application>，若要完全匹配目标格式，需把VirtualDesktopApplication类里的CreateElement("Application")改成CreateElement("Applications")。
2. Database类的ToXmlDocument方法中原XML字符串存在多余引号转义，已在上述代码中修正为正确格式。

内容的提问来源于stack exchange，提问作者Raheel Khan