Рисование примитивов в c#

Только лишь для затравки...

Начнем с абстракции

public enum FigureType
{
    Circle,
    Line,
    Rectangle,
    Sector,
    Text,
    Ellipse,
    ShadedSector
}
interface IFigure
{
    FigureType Type { get; }
    string Name { get; }
    Point FirstPoint { get; set; }
    Point SecondPoint { get; set; }
}

Далее реализуем интерфейс так

class Figure : IFigure
{
    //ctor
    public Figure(FigureType figureType, string name)
    {
        if (string.IsNullOrEmpty(name)) throw new ArgumentException(nameof(name));
        Type = figureType;
        Name = name;
    }
    public FigureType Type { get; private set; }
    public string Name { get; private set; }
    public Point FirstPoint { get; set; }
    public Point SecondPoint { get; set; }
}

Сама форма такая

public partial class Form1 : Form
{
    private IFigure _selectedFigure;
    private bool _IsDrawing;
    public Form1()
    {
        InitializeComponent();
        SetDataToListBox();
        _listBoxFigures.SelectedIndexChanged += ListBoxFigures_SelectedChanged;
        _panelCanvas.MouseDown += PanelCanvas_MouseDown;
        _panelCanvas.MouseUp += PanelCanvas_MouseUp;
        _panelCanvas.Paint += PanelCanvas_Paint;
        this.CenterToScreen();
        this.Text = "Примерчик";
    }
    /// <summary>
    /// Заполнение списка
    /// </summary>
    private void SetDataToListBox()
    {
        _listBoxFigures.DataSource = new List<IFigure>
        {
            new Figure(FigureType.Line, "Отрезок"),
            new Figure(FigureType.Rectangle, "Прямоугольник"),
            new Figure(FigureType.Circle, "Окружность"),
        };
        _listBoxFigures.DisplayMember = nameof(IFigure.Name);
    }
    /// <summary>
    /// Обработка события выбора в списке
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void ListBoxFigures_SelectedChanged(object sender, EventArgs e)
    {
        if (_listBoxFigures.SelectedItem == null) return;
        _selectedFigure = _listBoxFigures.SelectedItem as IFigure;
    }
    /// <summary>
    /// Обработка события нажатия левой кнопки мыши на Панели
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void PanelCanvas_MouseDown(object sender, MouseEventArgs e)
    {
        if (_selectedFigure == null) return;
        _selectedFigure.FirstPoint = new Point(e.X, e.Y);
    }
    /// <summary>
    /// Обработка события отпускания кнопки мыши на Панели
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void PanelCanvas_MouseUp(object sender, MouseEventArgs e)
    {
        if (_selectedFigure == null) return;
        _selectedFigure.SecondPoint = new Point(e.X, e.Y);
        _IsDrawing = true;
        _panelCanvas.Refresh();
    }
    /// <summary>
    /// Обработка события перерисовки Панели
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void PanelCanvas_Paint(object sender, PaintEventArgs e)
    {
        if (!_IsDrawing) return;
        switch (_selectedFigure.Type)
        {
            case FigureType.Circle:
                throw new NotImplementedException();
            case FigureType.Line:
                DrawLine(e.Graphics);
                break;
            case FigureType.Rectangle:
                DrawRectangle(e.Graphics);
                break;
            case FigureType.Sector:
                throw new NotImplementedException();
            case FigureType.Text:
                throw new NotImplementedException();
            case FigureType.Ellipse:
                throw new NotImplementedException();
            case FigureType.ShadedSector:
                throw new NotImplementedException();
        }
        _IsDrawing = false;
    }
    /// <summary>
    /// Рисование Прямоугольника
    /// </summary>
    /// <param name="graphics"></param>
    private void DrawRectangle(Graphics graphics)
    {
        var width = _selectedFigure.SecondPoint.X - _selectedFigure.FirstPoint.X;
        var height = _selectedFigure.SecondPoint.Y - _selectedFigure.FirstPoint.Y;
        var rectangle =
            new Rectangle(_selectedFigure.FirstPoint.X, _selectedFigure.FirstPoint.Y, width, height);
        Pen blackPen = new Pen(Color.Black, 5);
        graphics.DrawRectangle(blackPen, rectangle);
        blackPen.Dispose();
    }
    /// <summary>
    /// Рисование Линии
    /// </summary>
    /// <param name="graphics"></param>
    private void DrawLine(Graphics graphics)
    {
        Pen blackPen = new Pen(Color.Black, 5);
        graphics.DrawLine(blackPen, _selectedFigure.FirstPoint, _selectedFigure.SecondPoint);
        blackPen.Dispose();
    }
}

Работает это так

Весь пример можно скачать здесь