Xml 循环引用

Question

我正在尝试使用下面的类结构生成以下 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>

执行下面的代码时，出现异常：

System.ArgumentException: 'Cannot insert a node or any ancestor of that node as a child of itself.'

。类本身没有任何循环引用，所以我一定做错了什么。

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;
    }
}

如有任何指点，我们将不胜感激。

Answer 1

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;
    }
}

您正在返回

elementParent

，而不是

elementDesktop

。

您错误地识别了引发异常的行。其实是：

elementDesktops.AppendChild(desktop.ToXmlElement(document, elementDesktops));

所以，

desktop.ToXmlElement

中的某些东西就是原因。快速的试错表明，注释掉

VirtualDesktop.ToXmlElement

中的所有内容并不能解决问题，因此它介于

var elementDesktop = document.CreateElement("Desktop")

和

return elementParent;

之间...等等...

Answer 2

除了 canton7 的答案之外，看看如何处理（某些）

ToXmlElement

方法的返回节点。

在这些

ToXmlElement

方法中，您添加要返回到给定父元素的节点。然后，在各个

foreach

循环中的这些方法之外，再次将该节点添加到父元素。