Как парсить xml?

Для такого файла

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<graph>
<title>Dijkstra</title> 
<points>
    <point id="1" y="3" x="1"/>
    <point id="2" y="30" x="10"/>
    <point id="3" y="300" x="100"/>
</points>
<lines>
    <line id="1" weight="10" to="2" from="1"/>
    <line id="2" weight="100" to="20" from="10"/>
    <line id="3" weight="1000" to="200" from="100"/>
</lines>

получилось так

Идем сюда и получаем такой набор классов

namespace WinFormsAppXmlParse
{
    [XmlRoot(ElementName = "point")]
    public class Point
    {
        [XmlAttribute(AttributeName = "id")]
        public string Id { get; set; }
        [XmlAttribute(AttributeName = "y")]
        public string Y { get; set; }
        [XmlAttribute(AttributeName = "x")]
        public string X { get; set; }
    }
    [XmlRoot(ElementName = "points")]
    public class Points
    {
        [XmlElement(ElementName = "point")]
        public List<Point> Point { get; set; }
    }
    [XmlRoot(ElementName = "line")]
    public class Line
    {
        [XmlAttribute(AttributeName = "id")]
        public string Id { get; set; }
        [XmlAttribute(AttributeName = "weight")]
        public string Weight { get; set; }
        [XmlAttribute(AttributeName = "to")]
        public string To { get; set; }
        [XmlAttribute(AttributeName = "from")]
        public string From { get; set; }
    }
    [XmlRoot(ElementName = "lines")]
    public class Lines
    {
        [XmlElement(ElementName = "line")]
        public List<Line> Line { get; set; }
    }
    [XmlRoot(ElementName = "graph")]
    public class Graph
    {
        [XmlElement(ElementName = "title")]
        public string Title { get; set; }
        [XmlElement(ElementName = "points")]
        public Points Points { get; set; }
        [XmlElement(ElementName = "lines")]
        public Lines Lines { get; set; }
    }
}

Будем работать в духе MVP, вот интерфейс и кодбихайнд формы, который она реализует

public interface IMainForm
{
    string Output { get; set; }
    event EventHandler<string> SelectedFile;
    event EventHandler<string> SelectedNode;
}
public partial class MainForm : Form, IMainForm
{
    public MainForm()
    {
        InitializeComponent();
    }
    public string Output
    {
        get => textBoxOutput.Text;
        set => textBoxOutput.Text = value;
    }
    //событие выбора файла
    public event EventHandler<string> SelectedFile;
    //событие выбора в listbox
    public event EventHandler<string> SelectedNode;
    //выбор файла
    private void buttonSelectFile_Click(object sender, EventArgs e)
    {
        OpenFileDialog openFileDialog = new OpenFileDialog();
        System.IO.FileInfo example = new System.IO.FileInfo("file.xml");
        openFileDialog.CheckFileExists = true;
        openFileDialog.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyComputer);
        openFileDialog.Filter = string.Format("{0} файлы ({1})|*{1}|Все файлы (*.*)|*.*",
                                example.Extension.Substring(1).ToUpper(),
                                example.Extension);
        var result = openFileDialog.ShowDialog();
        if (result == DialogResult.OK)
        {
            textBoxFile.Text = openFileDialog.FileName;
            textBoxOutput.Text = String.Empty;
            //вызываем событие выбора файла
            SelectedFile?.Invoke(this, openFileDialog.FileName);
        }
        else
        {
            textBoxFile.Text = String.Empty;
            textBoxOutput.Text = String.Empty;
        }
    }
    //выбор раздела
    private void listBoxSelectNode_SelectedValueChanged(object sender, EventArgs e)
    {
        var item = listBoxSelectNode.SelectedItem.ToString();
        //вызываем событие выбора в listbox
        SelectedNode?.Invoke(this, item);
    }
}

Вот класс Presenterа, в котором поработаем с xml

public class Presenter
{
    private readonly IMainForm _mainForm;
    private Graph _graph;
    //ctor
    public Presenter(IMainForm mainForm)
    {
        _mainForm = mainForm ?? throw new ArgumentNullException(nameof(mainForm));
        _mainForm.SelectedFile += _mainForm_SelectedFile;
        _mainForm.SelectedNode += _mainForm_SelectedNode;
    }

    /// <summary>
    /// Выбран файл
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void _mainForm_SelectedFile(object sender, string file)
    {
        if (String.IsNullOrEmpty(file)) throw new ArgumentNullException(file);
        XmlSerializer serializer = new XmlSerializer(typeof(Graph));
        // читаем файл и десериализуем
        using (StreamReader sr = new StreamReader(file))
        {
            _graph = (Graph)serializer.Deserialize(sr);
        }
    }
    /// <summary>
    /// Выбрана нода
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="item"></param>
    private void _mainForm_SelectedNode(object sender, string item)
    {
        if (String.IsNullOrEmpty(item)) throw new ArgumentNullException(item);
        if (_graph == null) return;
        switch (item)
        {
            case "title":
                _mainForm.Output = _graph.Title;
            break;
            case "points":
                _mainForm.Output = ShowPoints(_graph.Points);
                break;
            case "lines":
                _mainForm.Output = ShowLines(_graph.Lines);
                break;
            default:
                throw new ArgumentException(nameof(item));
        }
    }
    /// <summary>
    /// Формирование вывода Точек
    /// </summary>
    /// <param name="points"></param>
    /// <returns></returns>
    private string ShowPoints(Points points)
    {
        StringBuilder sb = new StringBuilder();
        foreach (var point in points.Point)
        {
            var str = $"Точка:{point.Id} - X:{point.X}, Y:{point.Y}";
            sb.AppendLine(str);
        }
        return sb.ToString();
    }
    /// <summary>
    /// Формирование вывода Линий
    /// </summary>
    /// <param name="lines"></param>
    /// <returns></returns>
    private string ShowLines(Lines lines)
    {
        StringBuilder sb = new StringBuilder();
        foreach (var line in lines.Line)
        {
            var str = $"Линия:{line.Id} - начало:{line.From}, конец:{line.To}, толщина:{line.Weight}";
            sb.AppendLine(str);
        }
        return sb.ToString();
    }
}

Presenter и View(форму) свяжем между собой в Program.cs так

[STAThread]
static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    MainForm form = new MainForm();
    Presenter presenter = new Presenter(form);
    Application.Run(form);
}